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Ok, here goes... try to follow allong.
If you get 8.5:1 compression pistons then yes, you are going to have more room in the combustion chamber for more air. Keep in mind though that since you lowered your compression, your going to need to boost more to get the same power out of a higher compression lower boost setup. If you get 8.5:1 compression pistons then you are going to have approximately 0.5725 in³ more room in the combustion chamber assuming that these are going into a DOHC KA that was previously equiped with the stock 9.5:1 compression pistons. This is figured out using the displacement volume along with the clearance volume. In N/A form, compression is referred to as static compression which means the amount that the air inside the cylinder is compressed. When a form of forced induction such as a turbocharger or supercharger is added, compression is referred to as effective compression.
How to figure out your efective compression:
(Boost / 14.7) + 1) x Stock Compression = Effective Compression
A general rule that you should follow is that you should not exceed an 18:1 or 19:1 Effective Compression while running on available "premium" piss gas. With very good tuning, a 20:1 effective compression is achievable on pump gas. On some C16 race gas, effective compression ratios of 25:1-26:1 can be run on street cars with good tuning and proper preperation.
So say that you are running 8.5:1 compression pistons. To get an effective compression of 18:1, you are going to need to run approx. 16.5 psi of boost.
If you were running 9.5:1 compression pistons, then you would only need to run about 13 psi to achieve the same amount of power.
I'm not saying to go out and buy super high compression pistons so that you can run low amounts of boost and still get the same power, because at a certain point, the high compression:low boost ratio would no longer be worth it. With a higher compression, it will have more low end torque and power due to the higher compression before the boost starts building.
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