Compression ratio; Every 1 point you increase in compression gives you a 4% increase in horsepower. 9.0 to 10.00 on a 100 horse engine will yeild you 104 horse engine if everything else remains the same. And the same holds true for every point you increase. That is why a 12.5 engine runs better than a 9.5 engine. Now that holds true no matter what atmospheric pressure is. So a 12.5 to 1 engine will make more power than a 9.5 to 1 engine at 7 pounds of boost, at 14 pounds of boost and at 30 pounds of boost. BUT... the increase cylinder pressures require you to control detonation. As long as you can control detonation, you can run these boost levels.

Lets make this simple. We have a 100 hp engine with 8.5 to 1 compression. We put 15 pounds of boost on it, now we have a 200 hp engine. Now lets build a second engine with 12.5 compression. The first jump we goto 9.5 and get 4% to 104 hp, now we got to 10.5 anf get 108.16 horse, now to 11.5 and 112.48, and finally 12.5 we get 117 hp, now we add our 15 psi of boost and we got 234 horse power engine. So at 15 psi a 12.5 to 1 engine will make ruffly 17% more horsepower. That wins races.

Now because of detonation you may need to pull more timing out, and may loose part of that horsepower at high boost levels, but you will get there quicker, and on the bottom end of the race track you should make enough of a difference that the low compression car can't run you down on the big end of the track. With the electronics in timing we have a 12.5 to 1 15 psi street motor is a reality. Now if you go all out on a race engine. the 12.5 motor may be limited to 28 psi, where a 8.5 to 1 may be able to go to 30 psi without detonation, but I would put my money on the 12.5 engine.

an excerpt taken from http://forums.2gnt.com/dcboard.php?az=show_topic&forum=5&topic_id=10494&mode=full read that.