Compression Ratio Choice

[60DegreeTurbo]

Hi,

I'm new to the board, and figured you guys are the closest cousins to my engine and have had lots of experience with turbocharged engines.
I've had my 94 V6(3.4L - 60 degree block) camaro turbocharged for a few years now. I'm wanting to put a nice engine together for it with forged pistons and I'm trying to decide what compression ratio to choose. The current engine is compleatly stock internally with a compression ratio of 9:1. It also has 216,000 miles and is still running great(I beat on it every day)!
I'm running a TO4E-60 turbo with a very large t-3(~stage 3) turbine wheel and a decent intercooler boosting to 10psi on this motor with no problems. I have a custom ECM setup for fuel control.

I'm wanting to aim for these pistons taking 400-450 hp with the right components on the engine at up to 20 psi boost.

So what compression ratio should I shoot for with this engine running on 93 octane? If the only way to get that much power would be to run a race fuel I could deal with that. I can just keep the boost low (~15?) for the street.

Thanks!

[kenmosher]

In general, lower compression and making it up with boost is more efficient in higher boost configurations. What you lose in efficiency in compression, you make up for with VE from boost. The TRs run about 8:1 compression stock.

Practically, it depends on where you want things to live and how you want to make power ... there are tradeoffs either way. Too low a compression ratio and you get poor low RPM efficiency and need a quick spool turbo (which can limit overall flow). Too much compression and you are detonation prone with pump gas and normal driving, which will require lower boost levels which can underutilize the turbo compressor ("fall off the map"). A lot depends on combustion chamber design, swirl, etc.

Personally, it might be good idea to drop the compression a bit (maybe to 8:25-8.5:1) so you can run a bit more boost and possibly be a bit more pump gas friendly at higher boost levels. Bottom line ... cylinder pressure is cylinder pressure and at a certain point, you have to have the octane to support it whether it's low boost/compression or high boost/low compression.

Aux. injection (alcohol, propane, etc) can help with lowering charge temps (which helps with detonation), as can intercoolers (again, lower CAT), etc.