Whats the best compression for turbo setup?

[eurotekm3]

Im just wondering what compression u turbo bimmers are runing? and what u guy recomend? 8.5:1 what the lowest u can run?

[Concentric190]

I belive someone has a 7.9:1 compression with a gt35r.

The lower you go is not necisarily better.

[spooltime]

"Best compression" depends on your goals. Generally speaking, you will be able to make more total power with lower static compression and higher boost, but this sacrifices off-boost response and efficiency. Low static compression engines lose fuel economy due to their lower thermal efficiency off boost.

You should be able to make enough power to overcome the tires any time you want on stock compression with ~8psi if tuned properly. Beyond that, it's a game of compromises.

[coolcarlski]

Im just wondering what compression u turbo bimmers are runing? and what u guy recomend? 8.5:1 what the lowest u can run?

To run more boost and stay somewhat reliable I'd say go anywhere from 8.5:1 to 9.4:1

[trthrrt489]

[QUOTE=spooltime]

You should be able to make enough power to overcome the tires any time you want on stock compression with ~8psi if tuned properly. QUOTE]

I don't know if I'd go that far. 8psi is great, but its not "tire shredding" in 3rd kind of power. ..............at least mine isn't.

[Darwin006]

About how much boost will you be able to run at 8.5:1 compression?

[James Bond]

About how much boost will you be able to run at 8.5:1 compression?

On a S50/S52 I’m sure with a good tune 20-25 PSI would be all possible, depends of course on what size your turbo is and a ton of other variables. There is no real set boost per CR that you can just pull out of a hat

Just about any boost in any compression (of course keeping it realistic) is possible.

The tuning is key here. Idealy when we build our cars (usually 2.0 4 bangers) we recomend keeping it in the low 9's. This keeps for pretty decent throttle response and a safe range for PUMP GAS. Pump gas is really the key word there. On race gas or higher just about anything is possible with the right ignition setup.

We have seen 13-15 PSI on pump gas in Honda motors on more then one occasion (we are talking 10: CR and up) but who knows how long that would last. The 4G63 (DSM, EVO's) will do an easy 18-20 pounds on pump gas (again with the right tune) with a decent size turbo. Im talking 50 trim, T3/T4 all the way up to GT35R's.

Choosing your compression ratio has many variables, what are you goals for the car? What gas and turbo are you going to run? What EMS are you going to run?

If your going to build a motor for boost and keep the car very street-able then stick to the low 9's or high 8's. The lower the go the worse the car feels when your OFF boost. Honestly in a E36, I Would take higher compression with lower boost any day of the week. It would just make for a better all around power band. Lower CR will allow you to use a bigger turbo and run higher boost levels but with more lag and loss of throttle response.

We have 4 bangers making 400+ WHP on pump gas with a low 9 CR setup with no knocking whatsoever. This is on CA 91 octane crap gas. (Most of these are tuned with an AEM EMS)

Lower CR = Safer, more boost somewhat more reliable. Less power per pound of boost. Will allow you to run more ignition timing on pump gas.

Higher CR = More power per pound of boost, Limited on pump gas, less boost, less timing but a better all around horsepower shape.

This subject is allot more complicated then what I have said, but that’s some basics to get your started.

[Concentric190]

About how much boost will you be able to run at 8.5:1 compression?

Pretty much what mr james bond dude said.You can run ungodly amounts of PSI though there but it all depends on the tune and what kind of exotic fuels you're running.

[gsn794]

Jim Conforti wrote a short primer on what he calls "effective compression ratio" at http://forums.bimmerforums.com/forum...d.php?t=183985 Anyone figured out what sort of formula he was using for his calculations?