Quote:
Originally Posted by ATL BRZ
No reason to lower the compression, IMO. It will just make your turbo less responsive and introduce more lag, while requiring more boost pressure to make the same power you'd make at 12.5:1 with less boost. I dont think you're planning to buy a bigger turbo, so this also means your GT2860RS will struggle to hold those higher boost pressures required to make decent power with low compression.
Unless you are stuck with 91 octane fuel and you have no access to anything higher octane, I recommend a 12.5:1 block.
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I understand he is selling his car now, but check out
this post from
this thread.
I think a tuner could do boost by gear and progressive boost to reduce the risk of knock and preignition. They could also reduce the effective compression ratio below the static compression ratio with cam timing like the WRX does, but it might just be easier and safer to just use a lower compression motor. The detriments might not be that bad like the guy mentioned in the link. Like you said, if all he had was 91 and modest power goals like lower than 300whp then definitely he shouldn't lower compression. With E85, the risk diminishes, and he could hit his 400whp goals, but a lower compression motor would have more potential for more power later on, so I would advise reducing compression to at least 11:1, but closer to 10:1 would be ideal. The lower the compression ratio the more power potential the engine will have, but there obviously can be more lag.
Porsche 718 Cayman uses 9.5:1, the BMW S55 uses 10.2:1, the B58 uses 11:1, the Mercedes engine below uses 9:1, but because it does, the torque comes on higher in the rpms, but as you can see, they are running 30psi on pump gas, so the motor hits over 400hp. An Evo's compression ratio was 9:1 and most would agree that 500hp was closer to the limit on that motor before piston/rods were needed, but I don't know how the stock piston/rods compare to the FA20. Just something to think about.
https://carbuzz.com/news/mercedes-am...-making-416-hp
https://www.motortrend.com/news/amg-...r-engine-specs