What compression ratio is best for my set-up

[B33m3|2b0Y]

I have finally decided to go inside the engine but I am having trouble picking what compression ratio forged pistons I should install on my 2002 e46 m3. I recently purchased better connecting rods and Shrick camshafts that will be installed alongside the pistons.

On the car I have currently installed the following:
Rogue Engineering power pulleys,
Dinan high flow cold air intake w/ air mass assembly,
Dinan throttle body,
Dinan stage 4 engine software,
Eisenmann section 1 & 2 backpipes, Eisenmann exhaust

And I plan to install the following (at same time):
Shrick 280/272 camshafts,
Arrow Precision connecting rods,
CP S54 Forged Pistons. (either 11.5:1 or 13:1)

Like I said I am having trouble with the compression ratio, since I have no clue what the diffrence is between the two. My main goal with the car is to achieve +400hp without a supercharger or as close as I can get. If someone could tell me the diffrence between the two compression ratios I would really appreciate it. Also I plan next year mid 2005 to purchase the Euro headers/catalyst from Turner motorsports. So keep this in mind when recommending the proper ratio or any other parts I could add on.

[m28]

wow how much did u spend on all those mods so far?

[B33m3|2b0Y]

Not as much as I should have. So far on all the mods I have spent around $6000-$8000 without labour. The Dinan throttle body and intake I purchased new off a guy I found on ebay who stocks Dinan product. I got both the throttle body and Intake for $2500 so I saved alot there. Gonna be scared when I find out labour charges on the cams, rods, and pistons when someone helps me pick the right compression ratio.

[Jd886]

Is this for racing?

If your going to the extent of getting new pistons/internals, I would suggest looking into building the motor to a higher displacement. Apparently the S54's can get to about 3.4 liters. This way you'll be able to squezze a little more juice out of the motor, especialy torque. With a 3.4 displacement plus 13:1 CR, you'll have 390-400hp fairly easily (maybe more, from what I'm told). Everything must be race spec if you're going to have the high of compression, this will you'll be able to achieve high RPM's and still be safe. I would also race-prep everything in the engine bay to keep down heat. Heat is detrimental to engines that are that high of compression.

Just remember, with that high of a compression, the engine will not last as long. That high of compression wears out parts much quicker than the usual streetable compressions that coincide with warranties etc..

[B33m3|2b0Y]

I am building this car for street that sees occasional track.

Originally I never planned to toy with the C/R, since I dont know alot about how it affects preformance. I recently went to my bmw dealer for a check-up and they found that I had 2 cracked connecting rods (a bmw blunder?). Luckily that was discovered before they broke. Went out and purchased new Arrow Precision connecting rods. I got a deal for $220 a rod with the purchase of new forged pistons. The dealer is still waiting for me to decide what C/R to get.

Not sure but I think the standard C/R of the S54 M3 engine is 11.5:1. Since I dont want to harm the engine, I have been leaning more towards the 11.5:1 C/R, but I am still willing to install the 13:1 if someone can convince me its the better purchase. Right now I think the car has good enough HP, just needs more torque, hence plans for the euro headers/catalyst in the future.

[Jd886]

Then that should be plenty. The higher compression will make it a little more rev happy and pull in higher RPM's, which will be usefull for the track. It will be strange for street use though. But with the 13:1 CR you will loose the longevity of the engine. If anything I'd make a compromise and get a slightly higher CR like 12.1 or so. That way you'll get a little boost of power, and you'll see some higher rev power, but still keep it streetable.

[m3ltw98]

Do yourself a favor and go with either the 11.5 pistons or have a custom set built to 11.75-12.1 13.1 compression is way to high for a street car. Your gonna have to run 116 leaded gas along w/ some 104 so you dont blow the headgasket right off. The leaded gas will eventually destroy your catalytic converters. My brother ///M3MATT runs 13.1 comp on his race car and the longevity of the motor is prob a full season of racing before it needs to be checked over.

[B33m3|2b0Y]

Thanx for the help. I am calling the dealer tomorrow to confirm 11.5:1 C/R pistons. I dont want to be hurting the engine in the long run. When I have all the parts installed I should be running around 370-380hp, hopefully a little more. Thanx for your help guys.

[minus500]

I agree with Jd886. If your looking to gain more horsepower then stick with the stock C/R(maybe a tick higher) and bore + stroke the motor to increase displacement. Keeping the stock C/R should maintain reliability while the increased displacement will yeild some more HP and LBft. You may also want to upgrade the valvetrain to support the additional lift and duration on those camshafts and any additional revs you might through at it. Good luck with the install and make sure to post if you make your 400(+)hp mark.