To add to what was said above, compression helps to carry a motor past it's peak. I went from 15:1 compression to 11:1. Heads were unchanged. The cam was slightly smaller. What I noticed is the low compression motor made 50 hp less than high compression (800 -vs- 850) but also fell off much quicker after peak. With the high compression the motor basically flat lined from 6500-7500 rpm. With lower compression it was more like 6400-7000.