Compression ratio and forged pistons
 

There has been alot of talk about forged pistons for the P5. I saw that you can get the Mazdaspeed HC pistons (10.7:1). What is the difference between a forged piston and the HC piston.

Also what Compression ratio does the stock P5 run at, and what compression ratio would you need to run about 8-12psi of boost on a FI P5.

Thanks.

I am also a bit confused on the matter of compression ratio. The aussie model P5 has a compression ratio of 9.7:1. If i wanted to increase my boost to about 12-15psi would i increase or decrease my CR. Also, our petrol is 98 octane.

higher compression pistons increase the likelyhood of detonation ( knocks and pings ) in your engine. Adding forced induction will also increase the likelyhood of detonation. In north america, oour octane ranges between 87 and 91 ( up to 94, depending where yo live ). If you have 98 available, I would guess that the engine would be fine.

Generally speaking, higher compression goes along with NA motors, and lower compression goes with FI ones. The compression ratio is the combustion chamber space (once the piston is at TDC) compared to the entire space (when the piston is at BDC). I'd speak with a mechanic/tuner before upping the boost with 9.7:1 CR.

I'd say depends on your tuning.

if you have a real good tuner, and real good tuning, you can run higher cr pistons with higher boost, but there is more of a chance of detonation, and you should keep with the 98 octane if you have it.

I cant remember, but i think it was the higher the octane means the more the gas can compress before it detonates, or the more temperature it can take. soemthing along those lines. so yeah.

if you are using forced induction, with higher cr pistons, then yeah imagine that turbo, or supercharger cramming all that air in and then being crompressed so much, you definately want to stick with the higher octance so it doesn't pre ignite

generally to be on the safe side, people run lower cr ratios with higher boost.
the higher cr ratio will give you more power, but is more prone to detonation, which can cost you a new motor.

thats my .02 cents, take what i said with a grain of salt, cuz i'm not sure how correct i am.