I know that if you are going all motor you should get high compression, and FI you should get low compression, but why. What would the difference be of having a 9.0:1 compression motor compared to the same thing with 11.0:1 compression. Would you have more power through out the powerband or what?

 

its all about cylinder pressure and detonation. with low comp and FI the blower or turbo provides the cylinder pressure. low comp resists detonation as the intake charge temp runs over 200 degrees sometimes (because air heats when compressed), and hotter air with fuel will detonate.
all motor needs high compression, less likely to detonate as its NA, not forced. Intake charge temps are usually ambient air temp or slightly above.

 

In high compression race engines you can have really high cylinder pressures. So high that you can get lower pressures and more power from turbo cars, they dont need such high compression because you can move more cfm in and still not get rediculous cylinder pressure. Thats also read as reliablity.

 

the more cylinder pressure the more power. NA engines use mechanical compression and FI cars use artificial compresssion. Think about it this way. compress normal atmospheric pressure (NA) 11-1. then compress 14PSI (two atmospheres already) 9-1. factor in the 200 degree or higher intake charge from a turbo.

trying to compress 14PSI of air that is 100 degrees over ambient 11-1 and you-re gonna have problems with detonation. an intercooler will lower the intake charge, reducing detonation and increasing density. I useed to run a non-intercooled 2.2 daytona turbo. After a long run the car would lose a substantial amount of power. The intake charge would rise dramatically, over 250 degrees (due to friction from being compressed) fahrenheit. Even though it was at 7 PSI all the pressure barely compensated for the reduced air density because it was so hot. net result was probably zero.