it's not volumetric efficiency that will remain unchanged (amount of air entering the cylinders relative to actual displacement) you just lose overall efficiency because it won't burn as explosively with the reduced pressure(less horsepower per liter) higher compression yeilds higher fuel economy and about a 5% increase in power for every point of compression added without adjusting timing. So if you ran 12:1 compression without detonating you would see about a 5% increase in power. Similarly if you dropped from 11 to 9:1 you would lose 10% of your power but you would be able to run enough boost to more than make up for it. Say the highest psi you could run on the stock motor was 10 making 600hp, you go to 9:1 you would only make 540hp. but if you added another 10 psi thanks to your reduced compression allowing it to not detonate you could make 795hp. if you run that same 20psi on a motor with higher compression you could make 834 on 10:1 or 874.5hp on 11:1 but it would probably detonate and blow up.
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