log1call wrote:When you raise the compression the thermal efficiency goes down. When you raise the compression so much you have to back off the advance then the volumetric efficiency goes down. When you lessen the combustion chamber size you get less air/fuel mix in there. It's been proven to be best to have as larger combustion chamber as possible with as higher compression(courtecy of the turbo) as can be run with as much advance as possible gives the best power and life to the engine. Raisnig he compression ratio negates the advantage of having a turbo.
Those are factors that effect power.
Reliabilityis a different matter but it's influence by most of those factors as well.
Subaru(and all the other manufacturers) know quite a bit about these things.
Sorry...but there's a lot of false in the above post.
-Timing and volumetric efficiency are totally unrelated. VE is a factor of how much of the mixture fills the cylinder...timing is NOT a factor in this regard.
-The size of the combustion chamber also has no effect on cylinder filling...that has to do with port sizing/shape, combustion chamber
shape, valve timing and intake/exhaust manifold design.
I also find it funny that some of you are arguing about what the best comp. ratio is for turbos...as if there's only one number. There's soooo many factors that should be accounted for to determine this...like vehicle weight, gearing, desired fuel, engine displacement, desired power band/usage, etc. There's plenty of very well sorted-out, street driven turbo cars with compression ratios ranging anywhere from 7.5:1 all the way to 11:1. As the compression ratio goes up, tuning becomes more critical for sure, but the upshot of a higher C/R is that the car becomes more drivable, thanks to more torque when off-boost.
