engine upgrade

goofdog

Newbie
Points
36
Location
TY-Lesotho
Car
BMW325is
I have a beemer 325is 1990 model with a 2.7L engine. Mods include a racing chip, alpina cams, oversized valves, gas flowed head, 535AFM.

She has recently developed a bearing knock and i am considering some power mods whilst the engine is opened. I am looking at either the possibility of a turbo, or a stroker conversion.

I already have 320i crank( and rods) which can drop the compression if I do the turbo route..

Alternatively I can get a 328i crank reasonably from a friend, which i can also use with the 320 rods for the stroker conversion. I also have 728i pistons (86mm) that i am not if they can be used to increase the bore without too much modification. (is this possible?)

I am looking for a fast street car that can be for everyday use and not an outright racer (reliability issues).

I would also like to do this conversion without breaking the bank. Please may I have some sound advice/ informed opinions on which would be the best way to go.
 
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We get asked this question a lot so have a look at these threads which cover the basics. I have to admire your ambition though!:D And I hope you pull it off.

A turbo conversion would yield the most power but there is quite a bit of work involved.

http://www.torquecars.com/forums/f101/adding-turbo-nasp-engine-15003/
http://www.torquecars.com/forums/f118/fitting-turbo-some-dos-dont-s-24176

Cheap turbos can be purchased from breakers yards which are ideal for a try it and see type project.:bigsmile:

Adding a turbo is a major engineering task and without a kit with full instructions and all the parts you need there is a big risk of blowing up the engine. Expect to spend 50 hours upwards on a project of this type. This is not the sort of modification you should undertake unless you are mechanically competent and know what you are doing.

It is worth looking at superchargers as these are generally much easier to install than a turbo is.
 
Hi and welsome.

Once again, I think Waynne is being very optomistic with his estimate of 50 hours to convert a non-turbo engine into a turbo engine.

This is a complicated job, especially if there is not a specific kit for your engine. The time taken to complete this task doing all of the work on your own, including research, sourcing parts, fabricating parts you cannot buy, ECU and wiring mods, setting up etc could easily reach 10 times Waynne estimate. This sort of project ALWAYS takes WAY longer than estimated.

For example, simply making brackets to fit the intercooler can take a couple of hours on their own, once you have the material and the tools to hand.

I am not trying to put you off, but what you are thinking of doing isn't a weekend task. If you enjoy a challenge and have the skill sets and tools required, or are prepared to acquire them, then this could be a fun and satisfying project. However, many of these projects never get finished due to the work involved.
 
Fitting a turbo will be the more expensive option and will take far far longer.

A decent ECU and map can cost into four figures unless you are lucky and find the right ecu second hand.

That by the way definately isnt the end . its just the beginning . Second hand turbos are risky unless you know the history and then there is the rest of the work.

It is therefore unlikely that it will be anything other than an expensive project.
 
Good advice from all. Been there done that and you will find it a wallet lightening process.LOL
Turbo power is my drug of choice and beware it is easy to get hooked on the surge of power when it hits full boost and you can end up wanting more when you get used to it and it seems slow.
 
That's the old problem. 400lbft feels rapid at first. After a week it becomes the norm and we start craving more.

If you fit the crank from a 320 will that not have a shorter throw and thus reduce the swept volume as well as compression?
 
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Hi there,
don,t all decreases in compression also result in a decrease in windswept volume? I would assume that either using shorter rods or a crank with a shorter throw will result in the piston not going all he way up to TDC and hence decreasing both compression and WSV. Please help me understand the concept if my understanding is wrong.
 
Hi there,
don,t all decreases in compression also result in a decrease in windswept volume? I would assume that either using shorter rods or a crank with a shorter throw will result in the piston not going all he way up to TDC and hence decreasing both compression and WSV. Please help me understand the concept if my understanding is wrong.

To answer the first question NO

In theory lowering the CR by any of the methods below would allow a turbo/Supercharger slightly more space to force in more air
Remember the static CR of the pistons will not be the actual CR the motor sees when forced induction is added .
My motor has 8.7-1 static CR pistons BUT when I add 17 psi boost the actual CR increases to app 16-1 so that is why it is so important to have it tuned properly and use the best fuel

To lower the CR in a NA motor can be done in 4 ways
1 fitting a decompression plate
2 thicker head gasket
3 porting combustion chamber to increase volume.
4 fitting lower compression pistons

Re crank shorter rods are coupled with a lower pin position to ensure the piston still comes to the top of the block.
Conversely in my stroker with a 12mm increase in stroke the piston gudgeon pin is 6mm higher so the piston still comes to the top of the block at the top of the stroke
 
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Now I'm getting really confused.
In the case of a stroker conversion, it is advocated to use shorter rods together with a crank that has a longer throw to increase compression ratio. In light of what you have told me, will this then not simply result in the piston going above the top of the block at the top of the stroke?
 
Goofdog it seem that you have little know how when it comes to motors so let me explain
In my case I have a 4G63 that was a 2000cc motor with an 88mm stroke. I have installed the crank out of a 4G64 which has a 100mm stroke and a block with a 6mm higher deck height due to only 1/2 of the 12mm longer stroke extending the piston upwards. Both motors have exactly the same length rods so if I were able to use the 64's pistons ( which I can't as would have to overbore the block past the makers maximum recommended limit) rods and crank in the 63 the pistons would protrude 6mm above the top of the block .To overcome the problem of the pistons protruding 6mm too high I could have used 6mm shorter rods but that would make the rod ratio worse ( But explaining that is for another time )so I used forged "stroker pistons" that have the gudgeon pin 6mm higher/closer to the piston crown/top and are available off the shelf.
LET ME KNOW IF YOU UNDERSTAND THIS BEFORE I/WE GO FURTHER (I am not shouting))
 
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Swept volume is (cylinder bore x Pi) x stroke x no of cylinders.

Crank throw dictates stroke. Rod length is irrelevant with regard to stroke and therefore swept volume. Longer rods simply displace the pistons higher into the combustion chambers.
 
Goofdog the compression ratio (CR) is governed by the swept volume of a cylinder (CV) that is calculated when the piston is at bottom dead centre(BDC) and when that piston has compressed all of the air it had at BDC into the combustion chamber(CC) in the cylinder head when the piston is at TDC gives you the CR

TO simplify it for you lets say that the CV is 100 cc and the CC is 10cc then the CR would be 10-1 BUT you need to understand that there are a number of things that need to be taken into account in calculating the static CR such as
1 the piston top at TDC is it exactly level with the deck of the block?
2 the shape of the piston crown. Is it a flat top, does it have any humps,dishes,fly cuts for valves ?
3 the thickness of the compressed head gasket.
Hope this helps you understand how compression ratios are calculated
 
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Swept volume is (cylinder bore x Pi) x stroke x no of cylinders.

Crank throw dictates stroke. Rod length is irrelevant with regard to stroke and therefore swept volume. Longer rods simply displace the pistons higher into the combustion chambers.

Not quite true, when you build a long rod motor you order custom pistons with the pin height to suit so the piston still comes level with the top to the block @ TDC
Can explain more if needed in relation to the various combinations available for 4G63 & 4G64 motors .
 
Swept volume is (cylinder bore x Pi) x stroke x no of cylinders.

Crank throw dictates stroke. Rod length is irrelevant with regard to stroke and therefore swept volume. Longer rods simply displace the pistons higher into the combustion chambers.

Not quite right. Cylinder bore (diameter) x Pi will give you the circumference, not the area. For that you have to use pi x radius squared. Sorry, Paul :)
 

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