AFR fight btwn me and friend

[XiPPiLLi]

Ok, I'll try to make this as simple as possbile (new here too). Buddy of mine, brand new dodge srt4, with TBE, Intake, boost controller (set@12lbs), 750cc injectors, FIC (that already had a tune on it), wideband AEM AFR guage and some other guage that says he's getting a fuel overload (what he's telling me at least).

Were having an argument about proper AFR readings or what should be seen. I told him the sweet spot would be between 12.4 to 13.5. When he goes wot the AFR guage goes right away to red on the guage (I forget what reading he told me he was getting). And gets bad hesitation in the upper RPM. I told him he is running WAY lean, and he says he is running WAY rich. This is why I am throwing this question out there. I've tuned cars before, and I've built narrowband AFR guages that were nothing more than a light show to give me an idea of where I was at when tuning so I'm pretty sure I'm right, but I'd rather make sure.

 

[pgarner]

as far as im aware you should be looking for around 14.1 in most applictions

 

[old-git]

as far as im aware you should be looking for around 14.1 in most applictions

Agree. The theoretical stoichiometric air/fuel ratio for 'pure' fuel is 14.7:1, but with all of the additives in modern motion lotions it is around 14:1. Lower is considered to be rich and higher lean.

 

[HDi fun]

If there is not catalyser to worry about (or a simple, unregulated 2 way cat) then you can lean the mixture off during light load. Unfortunately this also increases temperaure in the combustion chambers.

It was not uncommon once upon a time to use over rich fuel AFRs to provide additional cooling.

 

[XiPPiLLi]

Ok thanks. And even under full throttle (when the map switches from closed to open loop) the AFR should still be reading around the 14.1 region?

 

[HDi fun]

Most cars run richer than 14:1 when under full throttle. Basically because the whole combustion process happens so quickly at high revs that a correct 14:1 ratio would leave it running lean.

Don't think I explained that very well.

 

[Country Bumpkin]

At WOT you should be looking at around 11.7. as a guide and tune from there by reading IDC's, knock etc.
750cc injectors ? What do you have controlling/metering the fuel for them ? What type of fuel pressure regulator are you using and at what pressure is it set ? You'll obviously have a form of timing control there too with injectors that size.
You really do need to run a wideband o2 sensor when tuning at this level, a narrowband sensor can't cope with demand of info quickly enough. We use a wideband gauge/reader for update readings. Please don't confuse my wording between gauge and sensor.
Maybe more of a point, what turbo is it running ?
I would suggest that if it is getting too much fuel it may be due to a lack of injector control or that it possibly needs to come down in injector sizing. What are the IDC's ?
I would suggest that your friend is correct and the poor motor is being strangled with fuel. I would also suggest correcting this matter before you suffer bore wash and start to lose rings then pistons.
The above is a brief put together of words as without the whole and accurate story, it really is hard to decipher what is going on.

 

[HDi fun]

I can see what you're at. Excess fuel is fine provided that

a). It's delivered intentionally rather than as a result of poor design or regulation

b). If it can't all be burnt during the expansion phase then the excess is fully vented during exhaust valve open time

c). If this is not the case then the fuel will cuase oil dilution within the combustion chamber and act as a solvent, very effectively washing away the boundary lubrication between rings and sidewalls

 

[Country Bumpkin]

There are so many aids to use when tuning an engine. Checking for IDC's [injector duty cycle], EGT's [exhaust gas temperatures] too high and it's running lean, too low and it's running rich, AFR's [air fuel ratio] too low is rich, too high is lean, [Knock] detonation in relation to weak mixture, poor timing or a combination of both. Incorrect settings on a boost and or fuel controller will just destroy an engine.
Another point to factor in is running too lean will provide excess heat = detonation = melted pistons !
FMIC's are great but they do need to match the required air flow, you can't just chuck one on the car and expect it to work if it's not suited for CFM.
Ultimately, you can not tune any car, let alone a turbocharged one without good tools and in essence that does require some quality time on a dyno as you just can't achieve the speed and control out on the road to do this.

 

[Sid447]

The Original Poster is correct,

Best AFR for most engines under load is between 12.5 and 13.2:1

Don't bother arguing with fools (your buddy with the SRT4) they just drag you down to their level and beat you with experience.
Running higher than this AFR-wise is considered too weak and will raise cylinder head temps and probably cause detonation which can very likely lead to engine damage.

1. Maximum Torque occurs at a 13.2:1 Air Fuel Ratio.
2. Transitional Fueling and Maximum Boost Air Fuel Ratios are about 12.5:1.
3. At around 13.2:1 or air fuel ratios of 0.75, EGT’s will peak.

 

[HDi fun]

detonation is extremely damaging. A knock sensor should pick up this and alter the timing and fuelling for that particular cylinder, or all of them if it's a global problem.

Over time it can damage piston crowns, small end bearings, exhaust valves and even the bores because the detonation is trying to force the piston downwards before it gets to TDC. That's where the metallic noise comes from. Although it sounds relatively benign it isn't and the cause shoule be investigated and rectified.

 

[Sid447]

HDI,

KR can only pull timing. That's just the problem.
If AFR's are set too weak when the car goes into Power Enrichment Mode, there's nothing that can be done about it.

The car will try and "tell" you by suffering hesitation and maybe backfiring when shutting off quickly but if this is ignored and the condition continues KR can only be pulled a set amount.
Most production pistons nowadays are cast hyper eutectic. They will crack, just like username "zcuvalo's" Jag engine in this thread:-

http://www.torquecars.com/forums/f45/forged-pistons-2002-jaguar-s-type-r-4-2-v8-s-c-8190/index2.html

Picture here:-

http://picasaweb.google.hr/CoberHr/JaguarEngineBreakdown?feat=directlink#

Been there, done that, even got the tee-shirt. :amuse:

 

[HDi fun]

HDI,

KR can only pull timing. That's just the problem.
If AFR's are set too weak when the car goes into Power Enrichment Mode, there's nothing that can be done about it.

The car will try and "tell" you by suffering hesitation and maybe backfiring when shutting off quickly but if this is ignored and the condition continues KR can only be pulled a set amount.
Most production pistons nowadays are cast hyper eutectic. They will crack, just like username "zcuvalo's" Jag engine in this thread:-

http://www.torquecars.com/forums/f45/forged-pistons-2002-jaguar-s-type-r-4-2-v8-s-c-8190/index2.html

Picture here:-

http://picasaweb.google.hr/CoberHr/JaguarEngineBreakdown?feat=directlink#

Been there, done that, even got the tee-shirt. :amuse:

OK , that makes sense. I thought something similar after posting. So basically, once the O2 sensor has its way with AFR then it's down to retarding sparks only?

 

[Sid447]

O2 sensors,

are only capable being accurate around 450Mv (equates to 14.7:1) and a little either side.
As such they are known as narrow-band sensors.

They can be used as a guide for fuel mixture outside the narrow bands, but that takes a bit of explaining and you usually need some kind of scan and logging software to be able to store and read it.

When an engine for example reaches it's Closed Loop Temp Enable setting (referenced from ECT or Engine Coolant Temp) ..the O2's start working and the engine goes into Closed Loop.
As long as you stay generally gentle with the TP and avoid sudden acceleration, or higher rpm (usually above 4,000) and/or high MAP, the O2's will continue to operate and keep the engine in Closed Loop mode. All the time continually sending info to the PCM to keep the fuel trimmed** at 14.7:1.
It's possible to complete a full journey in CL mode by being gentle with your right foot and avoid going into PE (Power Enrichment) mode at all.

**These are known as Short Term Fuel Trims (STFT's) and are capable of adjusting instantly by either increasing or decreasing injector pulse-width to either lessen or add the amount of injected fuel to keep the engine at 14.7:1
In addition....every 20 seconds or so the Long Term Fuel trims adjust to the STFT's average and adjust to try and even them out.

i.e. STFT's have been taking out fuel because of a slightly rich condition, the LTFT's adjust PCM parameters to compensate.

Only the initial start-up and the time taken to get to CLTE is the car not in CL mode. It's in Speed Density mode where it uses a Commanded Fuel While In Open Loop table that has ECT and MAP as reference and at each different ECT and MAP point there will be an AFR setting which the PCM will read from and use until the O2's come online when the correct engine coolant temp is reached.
If the engine subsequently does a re-start and the ECT is above Closed Loop Enable Temp, it still initially uses SD mode for a set amount of time regardless, and then goes into CL mode once again.
(For example if you've driven the car for 30 mins or so then stop for 10 mins and then re-start).

All the while, from engine start the Maf-sensor is sending initial air-mass information to the PCM which uses another seperate fuel mixture table as a base fuel mixture reference, the VE (Volumetric Efficiency) Table.
The VE table is the main table for SD mode and also when in CL mode.
If the VE table for example, is set too much on the rich side everywhere (this table usually uses RPM and MAP as a reference) then the engine LTFT's*** will stay permanently rich (i.e. be taking fuel away because too much is being commanded from the VE Table.
Conversely if the whole VE table is set too weak, then the LTFT's will be adding fuel permanently. (This is how a lot of factory tunes are set, for economy/emissions reasons but it makes the car more prone to KR or detonation).

Therefore when a car is running in what is ideally considered to be 0 to -5% LTFT range and the car goes into PE mode. It commands what is set in the PE versus RPM table quite faithfully (i.e. if 12.7:1 is set there, then that is what you get).

Whereas if the car has been running with the LTFT's on the plus side (weak) and you go into PE mode it adds the fuel based on the Fuel Trim average (which was weak in this instance) and you do not get a true 12.7:1 AFR it would be 12.7 plus the amount it had been running weak as a percentage so the car then accelerates with a weaker AFR then commanded.

***LTFT's are normally up-dated every 20 seconds and this LTFT info is stored in the PCM/ECM. Whereas STFT info isn't stored in memory.

LTFT's and STFT's have something like a plus or minus 25% range to work with.

The maf-sensor will also have an acceptable range to work with also.

Go outside of these without doing a maf-recalibration or VE table adjustment and you are heading for a problem.

>>> This post not poached or pasted from any source. It is in my own words relying on memory.
If anyone can see anything they'd like to raise questions or doubts about, please feel free to either post or PM me and I will happily check, correct if necessary, or back this up with my tuning info/files which I don't have with me at the moment.
Furthermore, anyone interested in seeing actual examples of Fuelling Maps, VE Tables, Timing Tables and anything else they might be interested in, both for petrol and where applicable Turbo-Diesels, again let me know & I'll be happy to do so, after Jun 14. <<<

-- Sid447 --