Classic 911 engine tweak

Simon y

Full member
Points
61
Location
Johannesburg South Africa
Car
modified 911 (2.4T)
Hi Guys,

this is my first post, I'm looking for anyone who has gone through a 911 engine rebuild, built in some hot parts and actually made it work. I have gone through a string of experts and apart from spending more on the car, than I originally paid for it, I still haven't got a result.

I started with a 2.7 Ltr engine from 1975 911, ran the car for a while and loved it, but decided it was time for a rebuild, and started to look around for someone to do the work. After a chat we got started, and decided to up the output a bit with a few mods.

1. 2.7 Liter engine bored to 2.8
2. J.E. pistons to give 10.5:1 compression
3. RSR camshafts
4. Twin spark conversion
5. Lose the Zenith carbs and fit fuel injection

The first attempt used a 996 turbo inlet plenum, but with the long duration cams, it was awful. We dropped the RSR cam and fitted one from a GT2 Evo (porsche super cup)

I drove it for a while and it was low on power, so after a couple of years we had another go.

1. found that the heads were from a 2 liter engine, so I got a set from a 2.7 and put them on
2. re-installed the RSR cam.
3. Fitted high rate valve springs and titanium retainers

It was really bad again so we put the GT2 cam back in.

I drove it for a while (6 months) and then decided on yet another attempt

1. installed individual throttle bodies (Jenvey from the U.K.)
2. Installed a DC40 cam from Dougherty in the USA. immediately got 20% more power , but still a huge flat spot in exactly the same place.

I noticed that the inlet manifold was 33mm and the inlet port was 36mm so I decided to sort that out

Bored the inlet manifolds to 36mm and blended everything with a die grinder so no steps or rough surfaces.

Had it re-mapped again and still can't get power, and there's a huge mid range flat spot. It makes plenty of torque at about 3000 revs, then nothing until about 5000 revs, and both peaks are about the same.

Is there not a tried and tested formula for tweaking an engine like a 2.7L? There must be thousands of these out there, and even Porsche made the 2.7RS and then based the 2.8RSR on the 2.7 so there seems to be plenty of examples to follow.

Has anybody got any idea why I can't get more than 120 Kw at the back axle (on a chassis dynamometer) I am testing at 5800 feet altitude (Johannesburg) so allow 18% for that, and about 12% losses for the drivetrain. This means I should get around 158 Kw at the crank, if I was at sea level. The standard 2.7 could do that back in the day. With all the hot cams, fuel injection, high compression, twin spark stuff I ought to see a lot more, and that big flat spot is killing me. If anyone has "been there, got the T shirt" and has any ideas why a straightforward "old school" set of modifications like this, no turbo, no nitrous, just elementary stuff which ought to be a piece of cake, won't work, please repond.

Cheers guys,

Simon.
 
One idea you could try.
But first, with the dyno and when it was fuel injected, did they know what they were doing. I went to a place once and when I came back I couldn't even get a straight answer or even pay for the job as the "owner" wasn't there at the time to take the money. No readout of the test either. Needless to say I left and didn't go back.

That aside, although this is refering to a different manufacturer the principles are the same. With the ECU programming for tunes there could be options to look at through doing your own tuning. It would take a bit of reading and learning. But if you can get the hang if it you could possibly set up to tune "on the fly". Check this forum out
www.delcohacking.net
they know heaps about ecu and memcal tuning.
Mostly they deal with the AC Delco / GMH ECU's but they have played with all sorts.
Their getting started guide is very informative. I have purchased a USB communications port and NVRAM and installed it to tune my setup on a laptop when finished.

Another angle, If you are back on carbs, there is a simple way to check what the fuel mix is like. Especially when you hit the dead spot.
Once at the dead spot cut the engine and bring the car to a stop (this would require some safer quieter area to drive). Once stopped pull the spark plugs out and see if they're Black, white on the tip, or rusty on the tip.
My geuss is they wouldn't have a rusty tip due to the flat spot but a rusty tip would be the optimum burn and mix for the best performance. Do the same technique at the different revs where it does run better aswell to get and broader idea of the burn.
I always used this technique for tuning motor bikes the "backyard way".
Mates would always be baffled and comment that their bikes had never had so much power. Thats usually because they have their carby's tuned too rich.
Your's sounds more like to lean to me but I can only guess.
Your jets might need changing or if you have something similar to delorto's they may have an adjustable needle.
 
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I have rebuilt a 1973 911

The engines are very good but you cant just add bits from another model and expect results

The heads are completely different on a lot of the models . by this I mean that not only are the heads different on different size engines but they are on different models as well.
compression ratio varies a lot This means high lift cams are a waste of time on 911s without a decent head.

You need a 911 specialist cos i dont think you have been going to one
 
When I started with the job my first inclination was to buy a bigger engine and just drop it in. Job done, more power, no hassle. I spoke to a few potential suppliers, and was convinced that a rebuild incorporating the right parts was the way to go. My engine rebuilder was a company with decades of experience on Porkers (it's all they do), based at the Kyalami race track North of Johannesburg.

The guy who did my inlet and ECU has years of experience on high performance cars of all makes and types, so far he has not been wrong, he told me straight away that the cam was too wild, and he was right.

The car then went to a shop that runs a successful Porsche race team, and has built many 911s for the track with excellent results. This is where the heads were changed, Dougherty cams installed, remapping etc.

My point is that in every case, I have gone to a Porsche specialist for any internal work and followed their advice. It's not like I shopped around on ebay and collected a load of random parts, then expected them to work together. So far nothing that has been done has made a great deal of difference. I have had 2 different sets of cylinder heads, two engine mangement systems, two different intake systems, 3 different camshaft designs, it has been mapped in 2 separate shops, one on a rolling road, and the other on a dynapac chassis dynamometer. So far, the only thing that made a difference is the dougherty cam which gave me an immediate 20% more power, but still left the mid range flat spot. There has to be something we have overlooked.

I am thinking about installing a Lambda probe in the exhaust as the map is set up on the dyno and that's that. The only inputs are throttle position, air temp, engine temp and the crank angle encoder to run the ignition map. The cheap ECU I have on there now will not do it, but I am going to reinstall the Autronic SM4 so we can hook one up. The only other idea in my head is that my exhaust headers may be too long. When I play with the numbers in my exhaust program, it looks like the standard exhaust header is tuned for just over 5000 revs, whereas the cam peaks at 6800-7200 and the inlet probably for about the same range. My feeling is that I am getting some sort of reversion from the exhaust pushing burned gas back into the cylinder during the mid range and this is preventing me from getting a full charge into the cylinder.

I am just looking for ideas, that's all, as I can't just keep throwing money at this project. With a bit of luck, somebody who reads this will know somebody who went through a similar experience and maybe they will remember what it was that fixed the problem at the end of the day. Believe me, no amount of remapping has done the trick, nor any of the changes listed above. If anyone has a design for a successful exhaust system, I would like to hear about it and compare it with what I have on there.

Thanks for your input,

Simon.
 
Thanks for the response. I wasn't expecting anything so soon. I like the "old school approach" reading plugs is a skill a lot of people seem to have forgotten these days. I also cut my teeth on british bikes back in the day and some Jap 2 strokes afer that. I moved up to 4 wheels after a couple of close encounters with the tarmac.

Anyway, back to business. The car is running fuel injection at the moment. The first set of modifications should have worked, but I think it went wrong when the guy doing the inlet didn't talk to the guy who had done the engine internals and wasn't aware of the very wild RSR cam that was in there. I made things worse by saying that maybe I would look at turbocharging the engine as a phase 2. Because of this we fitted the inlet plenum, but with that cam, carbs or individual throttle bodies would have been the way to go.

I now know that you must decide from the beginning what you are going to do, and if you want to build a turbo engine, then you must do it from the start, if you are going to go "old school" with high compression and hot cams, then you need to do it from the first nut and bolt. I probably made things worse with my turbo idea, because that's why we opted for the inlet plenum, rather than carbs or ITBs.

The thing now, is that I have had the car in 3 different shops, I have had 2 different sets of cylinder heads, two different inlet systems, 3 different cams, 2 engine management systems and I am sure if I think about it, plenty more things besides, these are just the major items. Although each of these changes has made some small difference, the limit on peak power, and the mid range flat spot has always been there. You would have thought that when I bored out the inlet manifold to match the ports on my new 2.7 Ltr heads, I would have got more power once it was remapped to put more fuel in with the extra air. On the dyno I got nothing, not one extra Kw over and above the run that was done with the manifold at 33mm and the port at 36mm. The runs were done in separate shops, on different types of dynamometer by different tuning guys. The one is based up at the Swartkops raceway North of Kyalami, and the other is the guy that did my first inlet system a bit closer to home. Both these guys are well experienced and do good work. I did a lot of research before I used them, and would not have gone there if I wasn't happy that they know their stuff. The guy up at Swartkops does plenty of 911s for the track, and he could do no better than the other guy.

This is telling me that the cylinder heads, inlet system, camshafts, and engine management system are not the problem. There is something else acting as a restriction and we just haven't found it yet.

I am going to reinstall the better Autronic SM4 ECU, but as it was taken out with a side cutter and not much else, I am waiting for a new loom to come from Australia. When I have that, I am planning to fit Lambda sensors to the exhaust system. At the moment the only inputs are throttle position, engine temperature, air temperature and the crank angle encoder to run the ignition map. If I hook up a Lambda sensor we can use it to fine tune the air fuel ratio which can't do any harm, and if it turns out to be a waste of time, I can just put a plug in the hole.

There is one other area where I have had some ideas, and I wonder if anyone else has any comments. I still have the standard Porsche exhaust headers on there which measure about 925mm from the face of the head to the start of the 3 into 1 collector. When I run the numbers through my exhaust program I think that the header is tuned for best effect at just over 5000 revs. My cam peaks at 6800-7200 and the inlet is also designed for performance, so further up the rev range is probably where it's at. My thought are that the headers are too short and just maybe I am getting some sort of reversion between 3000 and 5000 revs that is pushing burned gas back into the cylinder which prevents me from getting a full charge in there. This would seriously affect the power output, but I am not sure whether the tuned length of the exhaust could have such a profound effect. It is not a big job to shorten the headers and carry out modifications to the pipe, so maybe when I get the Lambda sensor adaptors installed, we will shorten the headers, test again, and see what we get.

My pipe is 3 into 1 from each side, with the 2 outlets going to a 2 into 1 collector, then to a small silencer and out. I have thought about building a twin system without the 2 into 1, and fitting 2 tailpipes but I am not sure whether there any advantages to this. It seems that Porsche have used 6 - 2 - 1, as well as 6 - 2 systems on the 911 so it looks like both work.

Any ideas?

Regards,

Simon.
 
It seems to me there could be something in what your saying about the exhaust system
Did you still have the 10.5:1 compression setup. That's alot of pressure that needs to get out once your revs are up.
In regards to the Lambda's, Didn't they have them setup up with the tuning to read you O2 while tuning. I get the impression they put in a set tune "bin" (binary code)
To suit the certain mods on your car.??
I have an interesting Idea for you to think about and that's to put in a wideband O2 sensor (LAMBDA) system which can then allow a simple adjustment dial in the vehicle to adjust the fuel mix. You would definately want the ECU to go in by the sounds of it.
For the setup you have I would definately want to run an ECU you can interact with. That probably isn't the correct slang for what I'm saying) But hope it makes sense.
I'll look up the link for you to check out the cost for the wideband LAMBDA/ O2 sensor and digital device and post it. I think they were around $200.AUD
 
I will make this short

Getting an engine to run right isnt difficult but the more you move away from known set ups the more it requires specialist knowledge

Its all very well saying this is ok and that is ok but it clearly isnt

keeping things simple
There are possibly two issues - first the engine has been built with parts that dont go together .
for example put 2.4t heads on an 2.4 s engine and it has no chance of running anywhere near right
This includes the fuel system
If the above is right then a decent ECU and proffessional map should solve the problem

IMHO you need to get a proper fully programable ecu (motec or link G4 are good) and get it set up right by a specialist

Then worry about exhaust systems etc

BTW the huge flat spot could simply be wrong fuelling as in wrong system which a map cant cure.
 
Thanks for coming back, it sounds like you have seen this sort of thing before.

The original engine rebuild and modifications wer recommended by, and carried out by a leading Porsche specialist of some 30 years. The things that were done over and above a standard rebuild were.
1. Increasing displacement from 2.7 to 2.8
2. RSR cams
3. J.E. high compression pistons 10.5:1
4. Twin spark.

The car then went elsewhere to have the induction system and ECU fitted. I have recently contacted other Porsche specialists and asked "what if we were to start over?" and they recommended exactly the same modifications that were done to my car, but for some reason mine didn't work out. I think that this was because of the wrong cylinder heads being fitted which is why I spent a lot of time tracking down a set from a 2.7Rs, having them refurbished and converted for twin spark.

You would think that changing from entirely the wrong cylinder head, to the right one would have made at least a small difference, but it didn't. You would also think that changing from the turbo inlet plenum to the ITBs would have made a difference, likewise, no noticeable improvement.

The car has been mapped in two separate shops, both with plenty of experience on 911s and both got virtually the same results, not even 1Kw difference in peak power, and only a minor improvement in the mid range flat spot betwen one shop and the other.

I think it would be wise to change one thing, test again, then try something else and test some more, even though each trip to the dyno is half a day off work and a few bucks out of my pocket. Saying that, I think it would be best to reinstall my top notch ECU (Autronic SM4) and the Lambda probe as soon as the replacement loom arrives from Australia and leave any tweaks to the exhaust until later.

Thanks for your reply, I know some of my posts are quite long, but I have had 5 years of trying to get this car right, and I am trying to put down all the facts and as much of the history as I can.

Regards, and thanks again,

simon.
 
Ive just read that your car was a 1972 911t in another post . If so it couldnt have had 2 litre heads because they dont fit .
It did however have the heads I was talking about which are often mistaken for 2 litre heads because they are cast iron but they are specific to this model . I know because I have rebuilt one and needed specific parts for the head.
This engine is one of the few 911 motors that doesnt tune well.
A quote from a porsche website about this model

The 911T had the same detail improvements as made to the A series 911S and 911L. However, to reduce costs, Porsche fitted the 911T with an engine that, while based on that of the other 911s, produced just 110bhp at a low 4200rpm. This reduced power meant that the engine could be fitted with cheaper cast-iron cylinder heads (as opposed to the Biral aluminium/iron items which gave more efficient cooling) and a simpler crankshaft design. Furthermore, the compression ratio was dropped to 8.6:1 by using different pistons, and the camshafts had less lift. Carburettors, meanwhile, were smaller Weber 40IDT3C items.

All this makes sense - you fitted a different fuel system which had much bigger inlet size etc etc
 
Sleeper,

Well spotted, however the engine in there is not the engine originally installed in the car. A previous owner did the wide body, 930 turbo look conversion and obviously wanted a bit more go, so he picked up and installed a 2.7 engine from 1975 (engine number confirms this, and was checked by Porsche center in Johannesburg). I know for sure that the engine started out as a 1975 2.7S. Where the 2 liter heads came from I have no idea, but they were aluminium, I still have them on a shelf in my garage. I suspect that the 2.7 engine may have been damaged, and the owner fixed it with whatever he could lay his hands on at the time. This is why I have spent a lot of time and cash getting all the unsuitable stuff replaced, so I have a known quantity to work with.

I suppose this is one of the perils of buying a car with no known history, I was told it was a 2.4T when I bought it, so it came as quite a surprise when the engine numbers confirmed that it was a 2.7.

As previously posted, I handed it over to Porsche specialists, because my knowledge of the Porker was a bit skimpy at the time, boy has that changed over the last couple of years. The only discrepancy from the standard engine was the heads (of unknown origin) that have now been changed for the right ones. I was assured hat the 2.7 was an excellent engine to work with, and that with a basket of fairly straightforward modifications, it could be made to go like the clappers. My only criticism of the original rebuild is that I was told that they would be able to carry out engineering work on the heads which would overcome the problems. What they should have told me right there and then, was that I would have to find a set of heads before the rebuild could be completed. Then the RSR cams would have worked, as long as I either fitted the carbs back on, or went with ITBs from the beginning.

This is where they made a major booboo. It is obvious that they couldn't get enough compression with these heads, so they lowered the height of the cylinders from the bottom in an attempt to correct it. When I had it stripped to change the heads, there was evidence that the pistons had been tapping the underside of the heads. It wouldn't have taken much more for there to have been serious damage. As it was, it looks like we got away with it, and the pistons were not seriously damaged.

Regards,

Simon.
 
WOW, Interesting approach in my books, to deck the block for a quick fix from a stuff up on the compression. You have too much confidence in this lot that have been working on your car. 30 yrs experience means nothing if they are doing stuff like this to your engine. (that's assuming that it was that company that made those changes).
Have you done an actual compression test yourself to see what the compression is.
Not having thought much about it but changing to twin spark? That would mean a complete different ignition system giving the spark. Is there possibly any conflicts happening with that? ie; timing set to what it needs to be according to the engine but also the different cam setup. If your spark is igniting to early/or to late. The rest would be self explanitory.

I definately agree that the ECU want's to be finalized and lambda sensor fitted.

I would have expected the company that made those HUGE stuff ups should have given a warranty on that. I hope they have!!
If you are able to get another block I would be inclined to insist that company replace it for you. Even if it's a bare block then refit it with the pistons and gear the right way for the NASP setup you decided on. I'm interested to see how your ordeal turns out.
 
i tend to agree with cc

If you take a car to a specialist too tune an engine and they choose the upgrades but cant make it run right might I humbly suggest that they are maybe not quite the specialist they think they are

And the fact that they have been playing about with the block and compression further suggests that ( to me anyway). They should be paying to fix this IMHO if your facts are correct

Can you get bigger metal headgasket ( you can with nissans' rb26 )

And just one more thought....

do you have bosch K tronic injection because it is known to run really poorly with hot cams
 
The company that did the original rebuild was started and run by a guy with an impeccable record “back in the day”. What I didn’t realize at the time was that he had retired some time ago, and the company was being run by others, with no input from him. The new owner has since taken a position in S.A. Motorsport, and had to dispose of his interest for conflict of interest reasons. Consequently there are now more new owners, and even though they occupy the same premises and employ the same staff, have no business ties to the previous owners, and my chances of any kind of compensation, or corrective action for free, or even on the cheap, are pretty dismal.
In any case, the only remaining issue from the original rebuild was that I had the wrong cylinder heads, and that they had employed a somewhat dodgy “fix” to try and finish the job without having to find a new set of heads. This has since been corrected, at my expense, but hey, when you have spent this kind of money, what’s another few grand? Since the cylinder heads were swapped, I have now, by a somewhat circuitous route, got a 1975 2.7S with no inherent defects that I know about, and the modifications listed in previous posts. Although I am still short of the power I should be getting (by about 20%) and have this monster flat spot, I still get 158 Kw (211 Hp) at the crank (allowing for altitude compensation and drive train losses). Therefore my engine cannot be one of those 110 Hp “can’t be tuned under any circumstances” Porsche engines, or if it is, boy am I good!

The ignition system could have been handled a number of ways. There is a monster distributor available with 12 plug leads, and a variation with two distributor heads driven from a common drive shaft having 6 leads from each. The other way is to mount a second distributor at the front of one of the camshafts. This is not too difficult to do, although it does mean you have a bit of a rats nest of H.T. leads running all over the show. I had seen several 911s at track days which used, in my mind a far superior system. This is what we built into my car. There is a sensor that picks up the crank position by means of triggers welded onto the crank pulley, 3 of them, and we retained the original distributor, but all it does is tell the ECU which stroke we are on so it fires the right plug at the right time. The ECU then drives 2 x 3 channel ignitors, one for each bank. The ignitors in turn fire 6 x double ended ignition coils, one per cylinder, and the coil fires both the top and bottom plugs simultaneously. This means each coil doesn’t have that much to do. My original set up still ran on contact breaker points, and unless they were adjusted perfectly, my engine would run out of sparks at high revs. I think this has something to do with the recovery time, how long it takes the coil to charge up for the next spark.
I was a bit concerned that there were only three triggers on the crank pulley as some guys use 60 or so, with one missing to indicate the TDC position. Anyway, I contacted Autronic and was told that this is perfectly adequate, they run very high output, high revving powerboat engines and track cars with the three trigger set up and the ECU incorporates predictive software that knows exactly where the crank is at any time. If this wasn’t confirmed by the manufacturer I might have thought about it a bit, but they insist it’s fine.
I get to drive this car every day to work and back, so I know the roads really well, and more importantly, where the speed cameras are. Then at the weekend when there is no traffic I go to pick up the gardener from the station and get to explore the performance possibilities. I am not too sure how the gardener feels about it, but he hasn’t said anything so far. I don’t just try to go flat out, well not all the time anyway, but try various throttle settings and different gears so the engine runs at different speeds under different load conditions. I have accumulated quite a bit of this “seat of the pants” data capture and have a feel for what works and what doesn’t. There is a rough spot on pull away, say 1500 revs, then my monster flat spot starts at about 3000 and power falls away at a tad over 6000. Funny how each one occurs at twice the revs of the one before.
I also sat in on the dyno sessions and watched the proceedings. It sounds like you guys know a thing or two about all this, but just in case, let me tell you what I have seen. The ignition map comes on screen like an Excel spreadsheet with rows and columns. The engine revs are on the vertical axis and go up in steps of 500 revs. The load is on the horizontal axis and goes up in increments of 10%. This gives up 60 points to map. If the revs are between one mapped point and another, then the ECU extrapolates the mapped figures and finds an appropriate setting for the spark based on the fixed point at slightly higher, and slightly lower revs. This also happens when the engine is working between two mapped load points. A similar map is used for the air/fuel ratio.
When tuning my car, it is not possible to get the A/F ratio right where we get the flat spot, there is a big dip. We can increase the fuel, decrease the fuel, advance the spark, retard the spark and do what you will, we cannot improve on what I have got so far. This is what makes me think that it must be a hardware issue, rather than mapping. If you think about the faults I have found, that have now been corrected, there has to be something we have missed. The modifications that were carried out by my rebuilder, were also suggested by other specialists when I asked the “what if we start again from scratch” question, this was confirmed. Some of this stuff is virtually an industry standard. J.E. pistons running 10.5:1 compression is very commonplace as is my 2.8 Ltr conversion. Dougherty DC40 cams should be fine and are in any case very close to the Porsche S profile. The Jenvey ITBs are used on a lot of 911s and the kit I purchased was one of their standard assemblies that they have supplied to 911 owners all over the place. The ignition system might be if anything a bit over the top for a street car, but it would be hard to improve on it, even for a full race engine.

This leaves the exhaust system as the only bit that hasn’t either been fixed because some Herbert made a hash of it previously, or modified in line with the performance tweaks. If you drive this car you get a feeling that something is holding it back, and when trying to imagine what it might be I keep coming back to some sort of reversion causing burned gas to flow back into the cylinder displacing some of the fresh charge, and with less fuel/air in the cylinder, I get less bang. I have been chatting to another well respected (U.K. based) Porker specialist and he tells me that the header length should be as short as 600mm for power to 850mm for torque. Mine are 925mm, and there are other issues. The 3 into 1 collector is very short, only 80mm and my exhaust program calculates it at about 125mm (sorry I don’t have the print out here) which would give a smoother transition from the 3 to the 1. As I understand it, the pressure wave travelling down the pipe should bounce back from the collector, back to the closed valve and down the pipe again causing a low pressure area behind the valve head ready for when it opens. This encourages the burned gas to exit the cylinder and head off down the pipe. If the timing of these pressure waves is way off, I might have a positive pressure behind the valve head at the affected engine speeds. There would in any case be quite a bit of pressure in the cylinder when the valve opens, so even with the positive wave, flow from the cylinder into the pipe is probably going to happen, but perhaps, at the end of the exhaust stroke, when the pipe ought to be sucking and evacuating the cylinder ready for the fresh charge, I am getting burned gas back into the chamber just before the valve closes, and this prevents a complete fill of fresh fuel/air ready for the next pop.
The question really is whether exhaust system design could have this much impact. The aftermarket exhaust specialists would like us to think so, but do they really expect to get 15% more power and increased economy like their advertising says, maybe when stacked up against an old cast iron manifold, but whether there is that much room for improvement with a modern equal length header I don’t know. And if you got it wrong, would it give rise to the kind of problems I am trying to resolve?

I am going to install those Lambda probes when the loom comes from Autronic, but that could take a while. Any modification to the exhaust is both fairly quick and relatively inexpensive. I could get the car in, alter the pipe, and have it on the dyno later in the week to see if there is any change. Even with my inferior ECU, I ought to be able to see if I am on the right track or not. However, there’s not any point in just stabbing away wildly at this project, there has to be some logic applied and some structure to the procedure leading to (hopefully) a successful conclusion. This is why I am bouncing these ideas around to see if anyone has tried any of this before.
So that’s it for this post, see what you think about some of the content, and if you have any questions about project porker, I will do my best to find the answers. I don’t think there is a simple quick fix just around the corner, but can’t help thinking that maybe this exhaust system has something to do with it.

Regards,

Simon.
 
A big flat spot wont be ignition - its far more likely to be a fueling problem or possibly silly cams not matched to the rest of the engine (but thats not likely).

Exhausts in my experience dont cause huge flat spots either. They can be used to tweak the perfirmance but Ive never seen an exhaust cause your issues.

Imho you are overcomplecating things .....

Ignition and fuelling are the two main reasons for running badly If the ignition side is ok then 2 or 4 plugs or the way it works wont be the issue .

My best guess would be the fuel system needs properly setting up by throughout the rev range . This is pretty much confirmed by the fact they cant get the af ratio right,

It sounds like you have an ecu that can be mapped ok so that only leaves a fault in the fuel injection or the actual mapping of it .

And you did say the af is wrong where the flat spot is.

There isnt anything else left that I can see.Unless of course the engine spec is the problem which does seem very unlikely.

Again can youi confirm if you are running the normal bosche k-tronic injection
This has known issues on tuned 911 engines.


BTW the definition of a specialist/expert isnt someone who can tell you that you have a problem .

It is someone who has fixed it and told you what it was.
 
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Sleeper,

I am glad you are prepared to keep on reading my posts, and I hope you are coming to realize that I have tried to carry out this rebuild/modification using best practice, tried and trusted methods. Like I said right at the beginning, there must have been so many 2.7 engines hotted up over the years that all we need to do is find the successful formula, and copy it. Sadly, mine out of all the 2.7s handled this way didn’t work out and my job now is to find out why.

I am a mechanical engineer, trained in the U.K. (I am originally from Stratford-on-Avon) working with heavy, high precision, industrial machine tools and have always done my own engine rebuilds, servicing and maintenance and never had any real issues with any of my cars until this one. Giving up is not an option, I will carry on with this engine until I understand what is going on inside it, and work out what needs to be done to make it right.

I thought we had finally sussed it when I found the 2.7 heads. I mean, how can you expect the engine to run well with heads from completely the wrong engine? Small ports and compression ratio up to maggots? Once I had them babys slapped on there I was honestly expecting to see a big difference, even if it was not perfect, I would have thought it would at least have improved things. The only thing that did make a difference was the Dougherty cam, even that cylinder head job did not improve peak power, or help with my flat spot. Can you imagine how frustrating that was?

I spent hours yesterday studying exhaust design, looking at photos of aftermarket systems for clues and learned a few things. It seems that the reason the guys racing these things use a 6-2 system is that it will produce higher peak power, but usually at the expense of some mid range power. For street use a 6-2-1 system may not make for quite so much power, but with the available power available through a much wider rev range, the car may actually be quicker under normal driving conditions. In the light of this, and the fact that it is mid range torque that is missing, sticking with the 6-2-1 layout looks like being the way to go. I would also be quite surprised that the exhaust could have this much effect on the torque curve, and maybe it is the fuelling, but then how come more fuel, or less fuel does not make any difference.

On the dyno we can sit with the car at any engine speed under any load conditions and the output is displayed on the screen to the last couple of watts. This dynapac system is so accurate that if you switch the lights on it registers the reduction in axle power. From there we can increase or decrease the fuel in fractions of a percent and try to improve the situation. The Autronic ECU has an autotune function so all you have to do is activate the autotune and sit with your foot on the pedal. The dyno holds the engine at the right revs, and the ECU sets the fuelling and ignition map for the correct A/F ratio and best power. Once each block is mapped, the ECU prompts you to move the pedal to the next condition so that mapping can be done there. Once the map is complete, the ECU lets you know that everything is hunky dory and Bob’s your uncle. We are using the Dicktator at the moment, so all this has to be done manually.

If the injectors are capable of providing enough fuel for my peak power, then why can they not deliver the correct fuel for the mid range, or maybe they are delivering too much, as the map tells me that it is rich. I am going to try and paste the Torque and Lambda map here and see if it comes out in the post. If this doesn’t work, I will figure out some other way of making it available. O.K. it didn't work, do you know how I can slip in a link to this map?

Torque is on the left side and the readings from the Lambda probe is on the right. My tuner tells me that what he likes to see is the A/F map starting on the left near the top (lean) moving to the lower right (rich), so we are on the safe side when we are at high revs and WOT (wide open throttle). You can see my map is O.K. by about 2000 revs where it feels quite strong, but plummets at about 3500 revs, where the flat spot starts, then ends up somewhere near the ballpark when the power feels good over 5000 revs up to the redline.

Just look at the torque, once you are past my rough bit at 1500 revs the car feels very promising and like it is going to go really well, then as you pass 3000 revs the torque decreases so acceleration is very disappointing until you hit the second peak at which time we are seeing the 211 Hp and off she goes. It is necessary to either stay below 3000 revs so you are in the lower torque peak, or drop a gear or two to place you in the second torque peak. This is a real pain when driving the car, and on top of that, I ought to be seeing another 40 Kw (54 Hp) at the peak anyway. This would be a theoretical sea level figure at the crank, corrected for altitude and allowing for drive train losses.

The injection is not Bosch jetronic, or any standard Porsche system at all. When I took off the carbs we built a system from scratch using Bosch injectors (I thought, turns out they are made by Siemens) and these are controlled by the ECU, whether this is the bargain basement Dicktator on there now, or the top shelf Autronic SM4 (some people dream about being able to afford one of these). One thing that has crossed my mind is that I could take a weekend to put my carbs back on, and take her out for a spin. If the problem is fuelling, this ought to prove it one way or another. If they were Weber carbs I would either have retained them, or sold them on Ebay for a small fortune, but they are the Zenith carbs, so I wouldn’t want to leave them on permanently, but this test may or may not give me a result. Whether the result is better or worse, would not matter, it would tell me that fuelling is the problem. If it makes absolutely no difference at all, like so many other things, then I am back to the exhaust thing, or something else that so far has completely eluded us.

What do you think about giving it a bash with the carbs?

Simon.
 
Assumimg it is fuelling it can only be the map or the fuel system. And as it can apparently supply sufficient fuel at high revs its unlikely to be the latter
The only other thing I can think of is have you got a fuel pressure regulator in the system .
This could cause an issue at lower revs especially with large injectors because that may make it harder to map.

I honestly still beleive you are overcomplicating things .
Its all very well explaining how well the system works and how you map an engine . And regardless of how uch theoretical work may have been done if the fuelling is wrong its wrong and you have said it runs too rich.

So prersonally I would try the carbs but - they have to match the engine spec you have and be the correct size ones and be set up properly for the engine you have now.

On the plus size with carbs you can manually play around with the jets

And with carbs it is much simpler but simply having the wrong jets can cause a large flat spot - a main jet will flood the engine (not literally) with petrol if its too large.

Honestly I doubt it is the exhaust especially if it is running too rich- that can only be fuelling so fix the problem dont look for another one.

BTW the answer to...........

If the injectors are capable of providing enough fuel for my peak power, then why can they not deliver the correct fuel for the mid range

is.......

They can but....
they need to be set up for each part of the rev range to do this .
Engine charachteristics vary with revs .The a/f mix isnt linear in fact its far from it.

So if its too rich you need to change the af mix.

At the risk of repeating myself if you have found a problem ...too rich and it seems to be where the flat spot surely you should fix that before looking at the exhaust.
.
Playing with the exhaust that will not change the mixture. playing with the fuel system will.

good luck
 
Sleeper,

Yes there is a regulator in the fuel system, I think we are running at 2 Bar, but I would just have to check that. The system was adjusted with a very accurate gauge, but I don't have a fuel pressure indicator in the cabin.

I think you are right about trying it with the carbs, just to see what happens. These carbs ran fine with the engine before it was tweaked, so all that can have changed is slightly more airflow caused by the increase in displacement. If they worked O.K. before, they ought to work O.K. now, as they are still exactly as they were when they were stripped off.

I can understand that the rate of flow will be greater at WOT and high revs, but there is no need to test the car there, it is the mid range that's the problem, and under those circumstances the flow rate will be similar to what it was before all this work. If I am right about that, then there should be no need to play around with jets and all that, just fit them and go for a drive. It should be apparent very quickly if the mid range flat spot has dissapeared, or just improved a bit. In any case, unless the car is immediately absolutely fantastic, I will be taking them off again at the end of the test and reinstalling the fuel injection.

If hat turns out to be the case, then I know exactly where to look.

O.K. then, that's the plan. This weekend might be out as I am having a birthday bash, and the next weekend I have to install a solar geyser in my roof. Maybe I can take the car off the road for a day or two and work in the evenings. I will just commute in my X-1/9 until I finish the test.

Regards,

Simon.
 
That is correct the carbs should be ok but maybe only where the flat spot is now ; remember they wont be jetted for the new power ; in fact it may run too lean at high revs so i wouldnt hammer it - try it gradually and keep an eye on the af ratio

With you new more aggressive tune the rate isnt going to be similar at lower revs or even close.
In fact the flow rate low down could well be the cause of the flat spot.

just a guess but I suspect the flat spot is just before the steepest part of the power curve as it comes "on cam" and you can confirm this by looking at the power curve on the dyno printout . that is where your current system may not be able to cope with the sudden change in af mix.
 
Hi guys,

it has been a while since I last posted any progress on project porker, but I have had plenty of other things to do, and only got back to it recently. My latest efforts have really made a difference, and it is quite interesting what I found.

First off, I didn't get around to putting the carbs back on, so no result from that, but I did take it in for a bit of work on the exhaust. Without doing any other mods, we improved the mid range torque dip significantly, and got another 9% peak power. I did all the calculations myself, and got the job done by a guy in Kempton park, East of Johannesburg. Very professional and competent, and I actually only went there because my usual guy was booked up till the new year. This guy is now my usual guy.

I have now got my own laptop and loaded the software so I can tune on the fly, rather than paying a fortune for a session on the dyno every time I want to see if there has been a change. So I can see what is going on, I have an NGK wide band heated oxygen sensor installed in the exhaust, so I have a display of my A/F ratio in the cabin. Even so, I decided on another dyno session with my tuner, and we just couldn't get control of the A/F ratio. For some time I had noticed that the TPS reading was jumping about a bit, sometimes it was enough to activate the throttle pump routine, and if this was flicking on and off rapidly, it was basically throwing fuel into the engine and choking it on unburned fuel. To counteract this, I had the throttle pump settings as low as I could go, any further and throttle response was rubbish, but if I put in more it would choke the engine with fuel. We did our best on the dyno, but I still wasn't happy with the power run, so I was looking around to see if I could get another TPS but it looks like it will have to come out from the U.K.

The following weekend, I had a bit of time on my hands, so I decided to take off the TPS, look for any loose bits, give everything a good clean etc. When I removed the retaining plates, I could see the D shaped spindle of the throttles engaged with the D shaped hole in the rotating element in the TPS, but it didn't seem to go in very far, a couple of millimeters at most. I didn't think this looked right, and had a think about it while I cleaned things up, ran a tap down the threaded holes and so on. Once I had finished, I examined everything and did a trial fit with the TPS the other way round, with the outside on the inside. Straight away it went in much further, the D spindle came almost all the way through the TPS and I thought I had found the problem. It wasn't as easy as that, there is only just over 90 degrees of movement in the TPS and there was no way the screws would end up anywhere near the mounting holes, on top of that, because I had flipped it over, the direction of rotation was wrong. I thought I wouldn't be able to leave it like that, but then I had an idea, and moved it over to the right bank of throttles. It fitted straight on, the other throttle rotates the opposite way, and the screws lined up, bingo!

This must have been like this since the Jenvey throttles were installed, and I have had trouble with it ever since. I think that there must have been a small amount of movement between the throttle spindle and the TPS and this has been giving an unstable reading, enough to activate the throttle pump routine and enrich the mix when it shouldn't have. It was a very easy mistake to make. The flat base of the TPS looks like the mounting surface, and the top looks like it goes to the outside. I am a bit disappointed that the guy who did the installation did not notice that it wasn't fully engaged, but if you have been following my story, you will know how I have been getting on with our local experts. It might be an idea for Jenvey to drop a slip of paper in the packet showing which way it goes. It has caused me all these problems for 2 years, and I only found it now.

Once I had the TPS mounted, I did a calibration and went for a drive. I could tell straight away that the enrichment problem was gone, so now I could tune and actually get a consistent result, and just as important, I could set up the throttle pump values to improve throttle response, and not worry about it activating when it shouldn't. Since then I have driven around with the laptop on the passenger seat, and keeping one eye on the A/F ratio reading, things are far better than they have been since this project started. I am getting 126 Kw at the rear axle and we are at 5800 feet in Johannesburg, so 15% for drive train losses, and 15% for altitude and theoretically I have 167 Kw at the crank (at sea level) and the old 2.7 was rated at 150 Kw, so at last I am seeing some progress. Bear in mind that this reading was before I sorted out the TPS issue, and we had an A/F ratio in the low 9s during the power run, so with things now sorted, I am going to drop it back on the dyno in the new year and see what we have got. This is still only 80 Hp per Litre, and I set out to get 95, so I am looking for another 20% improvement over the last run. Any comments on that? I was advised that this was quite an ambitious target for a 2 valve per cylinder engine that was built 35 years ago.

One question for you guys, can anyone recommend some sort of knock gauge, knock sensor equipment that is suitable for use by a clueless herbert like me who wants to optimise my ignition timing and minimise the risk of damaging anything through detonation. I have seen a few advertised on the internet, but could do with anyone who has had good results, letting me know, so I don't waste money on a piece of rubbish, or heaven forbid, trash my engine and have to start project porker all over again. I don't mind spending a bit on good stuff.

Once again, thanks for the advice I have been offered on this site. Everything has nudged me toward a final solution, and this latest issue with the TPS confirms everything that was offered as a possible root cause of my tuning problems. However, until I forked out for a laptop I was unable to do much about it, and although that certainly helped, it wasn't until I got the A/F meter that I could get a clear picture of exactly what was going on.

The finish line is in sight!

regards,

Simon
 
great news

Unfortunately I have no experience about knock levels/guages on them . This isnt because there isnt any available its just because Ive never needed one as mine was/is still standard.

However I cant see it being a problem.
Even early (1991) skylines have knock sensors fitted as standard so I am sure many others do as well . You can also get aftermarket alarms/guages which are adjustable to give an alarm at a pre set level.
It may take some working out but it shouldnt be too difficult bearing in mind what you have done already
I realise that as you are "playing" with the mixture etc at the moment it would be a woerthwhile saftey guage but once you get the fuel sorted it should fine .

Look forward to the next stage.
 
Hi everyone who has chipped in over the last year or so, and still keeping an eye on project Porker.

Towards the end of summer I had the car going well, and it seemed that it was consistent enough to book a day on the dyno for the final touches. I have to say that without my own laptop and the NGK wide band heated exhaust gas thing I would never have found the fault (TPS sensor on the wrong way) or been able to tune the map properly. This was the turning point.

As we are Southern hemisphere we are now getting into winter and so I am getting to set compensation values for cold starting. Here in Johannesburg at the beginning of June it is an icy 15 degrees C when I am starting in the morning and it takes a bit of extra juice to coax the old girl into life. This is easy to set in the compensation table, there is a value for engine temperature, as well as one for air temperature, so keeping an eye on the A/F meter lets me add a bit for cold weather, as well as a bit more for a cold engine. All sound good so far, but I have come across another small issue that might help somebody else going down the same road as I have with this project.

My temperature gauge reads the oil temp. as this engine is one of the old air cooled units. Actually it should be called oil cooled as there are 15 Litres of the stuff in a tank just behind my seat. The thing is, that the guy who did my first engine management set up drilled one of the casings and installed a temp. probe to pick up engine temperature, but this was changed by the guy who installed the second ECU. He simply tapped off the oil temperature probe and used that as the input for the ECU. This sounds fine, and for the whole summer it was. I was "cold" starting between 25 and 30 degrees, so very little compensation was required. What happens now is that I have to add about 25 (I am not sure 25 what this is, but less isn't enough and more is too much). The engine starts straight away, and in a few seconds it settles down and we are good to go, however, when I drive off, unless I have sat for a while to heat the oil up, the engine sometimes dies from too much fuel when accelerating.

Having given this a lot of thought, I am pretty sure that what is happening is that the engine is warming up quickly, but it takes a while for all 15 Litres of oil to get there. Consequently the engine is overfuelled until the oil heats up, then everything evens out and we are good again.

I am planning to take the car off the road for a couple of days to install my new idle control system recently arrived from the U.K. When I do this I can take off the air ducting covers and have a look at a better spot for this probe. I am thinking that somewhere up by the cylinder heads would be best so the sensor responds very quickly as the engine heats up and the ECU reduces the fuelling as soon as this happens.

The other reason I need to carry out this strip is to look at fitting the knock sensor I managed to find. It can be bolted on pretty much anywhere in the cylinder head area, and it would have been easy to fit it underneath, but I am worried about the heat from the exhausts, and it would be a bit exposed to stuff thrown up by the tyres, so I plan to find a place for it on top.

Incidentally, another learning experience I found out the hard way. Setting the compensation for "throttle pump" which does the same as an accelerator pump on a car with carbs. There are four parameters to set: Increase, the amount you want to enrich by : Decay, how quickly you want the enriched value to return to the base value : Dead band, how much the pedal must move to activate the routine, and finally: Deactivate over, above which the routine will not run. The problem I had was with the decay value. I was told by the guy who supplied the unit that this was how long it takes for the value to return, in other words, the lower the value, the quicker it happens. This was giving me problems as I was finding that it would overfuel the engine and the setting didn't make sense. This turned out to be because the value is actually the opposite, it is the speed at which it returns to the base value, the lower the number, the longer it takes. I would not have spotted this without the laptop on the passenger seat at the data page. The engine died after a blip, and when I looked at the fuelling field I saw the value take several seconds to return to the base value. Using the information I had been given, I reduced the value in the compensation table, tried again and it took even longer. I went back in and increased the value and sure enough, it dropped back in a few tenths of a second, which seemed like a good starting point. It was pretty close, and after a few sessions on the way to work I now have this sorted out too. I just though I would mention that, it doesn't matter what some expert tells you, check everything to make sure and wherever possible make notes. I also keep a version of any successful map saved, and if my latest tweaks are not giving me the results I am looking for, it is easy to simply load the earlier map and go back to where I started. With the cold weather here, to be on the safe side I saved my previous map and labelled it "Summer" and am now making small changes to the map and saving it as "Spring / Autumn just to be on the safe side. I now need to save a version and label it "Winter". I did this because I have had to add fuel at certain rev ranges with the colder air. I was running lean at 4000 revs as the car "came on the cam" and this would come right again as the revs increased. With extra fuel at this engine speed the A/F ratio seems O.K. and the car accelerates well again, it will be interesting to see if this situation reverses when the weather warms up again in September.

And another thing. When my idle control stuff arrived, I was excited at the prospect of installing it to help with idle speed now the cold weather has arrived. The air valve has a four pin connector, so I went looking for a suitable plug. The four pins mean that it is kitted out with a stepper motor type of air control, and then I found that it will not work with the ECU I have in the car. To get this right, I need to temporarily install the air control from a VW Jetta or suchlike which works by "pulsing" the air control valve (a three wire system). I am going to install the plumbing and just blank it off for now, then if I find a valve it will be quick to put it in, and when I reinstall the Autronic SM4 ECU there will be no problem controlling the Jenvey air control valve.

O.K. there you go, that's just about where we are right now. Steady progress is being made and most of the major issues sorted out. This latest stuff is just a bit of fine tuning and we are getting better all the time.

Thanks for all your input guys, it really made me think, and I don't think I would have reached this point without your input.

Regards,

Simon.
 
Alrighty then, just another report on things that have happened ICW project Porker and a question I hope somebody can answer.

I was having trouble with the fuelling when the engine hit 4000 revs, and put this down to a change in the weather, (see last post) this might have been a mistake. I now think it is because I have been trying changes to the ignition timing map and this has had far more dramatic effects on other things than I had anticipated.

I have now entered a new map and the problem seems to have disappeared, if I plot the fuelling values in an Exel spreadsheet, I now have a nice curve on the graph with no odd looking spikes or bumps in it. This makes sense actually, with the timing different, I was suddenly having to push a lot more fuel at 4000 revs, and then it came back again by about 5000 revs. It didn’t feel as though the engine was developing any more power, so I hadn’t hit any magic numbers or anything, in fact it now has much better pull all the way through the rev range which I will confirm once I get it on the dyno.

To come up with this trial map I read up on as much theory as I could find, then opened a spreadsheet in Exel and created a chart with all the values for rpm on the vertical axis, and load on the horizontal axis (identical to the map in the ECU). I then started working with a few values out of the original manual when the car had distributor ignition. Static advance 5 degrees, maximum advance 35 degrees (Giving a range of 30 degrees) and another 8 degrees advance for load (which equates to vacuum advance if I still had the distributor). I then entered a formula into a series of cells so that the “mechanical” advance started at 1000 revs (engine idles at 900) and moves to full advance (35 degrees) at 4000 revs, then stays there to max. RPM. I divided up the “Vacuum advance” (Load) into 4 so the full 8 degrees would be applied at 0% load (idle) giving a total of 13 degrees advance, then as there are only five load sites, 0% - 25% - 50% - 75% - 100% I divided it so I added 6 degrees at 25%, 4 degrees at 50%, 2 degrees at 75% and 0% at full load. I programmed all the cells so that I just enter the static timing figure, Max. advance and the amount for load, the spreadsheet works everything else out and I just enter the timing map into the ECU.

This sounded far too simple but it seems to be giving me surprisingly good results.

Now for my question. I know from watching vacuum gauges in other vehicles that manifold vacuum is proportional to throttle position, so vacuum makes a good indicator of load. In fact some ECUs (usually on turbocharged engines) will use manifold absolute pressure (MAP) as the primary load sensor rather that the throttle position sensor (TPS). My spreadsheet works out the ignition timing as if I still had a distributor in there with mechanical advance and vacuum advance. The mechanical part is easy, and I worked it out exactly as if I was setting up a distributor, but I do not know whether I have got the vacuum (load) part right. Can anyone tell me whether the vacuum advance is “all in” (in this case 8 degrees) at high vacuum (closed throttle) and decreases steadily as the throttle opens until the last bit of vacuum advance falls away between 75% and 100% load, or is it “all in” at idle, falling away quicker than that, giving no additional advance over 25% (for example)?

The only way I can see to check it would be with a car fitted with a distributor on the dyno, a timing light, and then to run the car at 25% load, check timing and pull off the vacuum pipe and see what happens, then repeat at 50% load and check again etc. That is unless somebody knows the theory and can enlighten me.

Whatever the response, I can reprogram my spreadsheet to accommodate the theory and that will be great. What I could do is paste the spreadsheet on this site so anyone could have a go with it. I don’t think it is any substitute for a session on the dyno to work out the optimum ignition timing map for peak power and checking that there is no detonation, but seems to work as a good place to start, which is what you need when you finish a rebuild like this and have to start putting numbers into the map. I should have thought of doing it a couple of years ago.

Looking forward to your input,

Simon.
 
Hi, me again,

all a bit quiet after my last post, so I am going to keep this brief.

Can anyone tell me how much vacuum advance you would get out of a Porsche distributor on, say, a 2.7 Rs?

I can find how much static advance, and how much maximum advance should be, but nowhere can I find anyone to tell me a number for the vacuum advance. I am trying things by increasing the value in my spreadsheet (and then in the ECU program) and had my first attempt this morning, with a noticeable improvement in light throttle response. I will have another go and see what happens when I go another 2 degrees. However, I am a little worried about overdoing it, so a baseline number would be a great place to start from.

Anyone ever looked at this, and if so, what can you tell me?

Thanks,

Simon.
 
A big flat spot wont be ignition - its far more likely to be a fueling problem or possibly silly cams not matched to the rest of the engine (but thats not likely).

Exhausts in my experience dont cause huge flat spots either. They can be used to tweak the perfirmance but Ive never seen an exhaust cause your issues.

Imho you are overcomplecating things .....

Ignition and fuelling are the two main reasons for running badly If the ignition side is ok then 2 or 4 plugs or the way it works wont be the issue .

Twin sparks I have been told/led to believe are only used to overcome a poor CC design that does not make for good flame spread.

My best guess would be the fuel system needs properly setting up by throughout the rev range . This is pretty much confirmed by the fact they cant get the af ratio right,

It sounds like you have an ecu that can be mapped ok so that only leaves a fault in the fuel injection or the actual mapping of it .

And you did say the af is wrong where the flat spot is.

There isnt anything else left that I can see.Unless of course the engine spec is the problem which does seem very unlikely.

Again can youi confirm if you are running the normal bosche k-tronic injection
This has known issues on tuned 911 engines.


BTW the definition of a specialist/expert isnt someone who can tell you that you have a problem .

It is someone who has fixed it and told you what it was.

Simon I tend to go along with Sleeper on this as I have found that there are Tooners and Tuners and IF the spark and fuel maps are set properly there will be no flat spots.
 
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