Max Compression
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Joined: Dec 2002
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From: great state of the finger
Max Compression
Im in the process of building a 460 and am looking into a set of hypereutectic pistons. These are rated to produce 9.14 : 1 compression with 92cc heads. Now, I am really close to acquiring a set of old DOVE heads that will no doubt raise my compression with these pistons to 10.5 : 1 or more. My question is: Will these pistons be able to withstand that amount of compression, or should I purchase a set of forged ( $$ I don't want to if it is at all avoidable $$)? Also, with compression looming close to 11 : 1, would I even be able to get away with premium pump gas?
Thanks for your input.
Thanks for your input.
Posting Guru
Joined: Dec 2002
Posts: 2,003
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From: chicago burbs
Max Compression
camshaft selection has a lot to do with actual operating cylinder pressure. a higher amount of valve overlap will reduce cylinder pressure, allowing you to run a higher static compression ratio and not have excessive cylinder pressures. the overlap occurs while the exhaust valve is near closing and the intake valve starts to open. some cam profiles don't have any overlap while others have more or less. it can be figured out from the cam specs.
Elder User
Joined: Jan 2000
Posts: 966
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From: Crooked River Ranch, OR
Max Compression
Don't underestimate the hypereutectic pistons...they are actually stronger than a forged piston according to a number of sources, but they are more brittle. Their worst enemy is detonation and they will fracture if subjected to prolonged or severe detonation. The other thing is you MUST follow the MFG.'s ring gap procedures or you WILL break a ring land if the engine gets hot enoughh and the ring-ends butt. I'm currently using hypereutectic pistons in my blown Vette with the addition of a 250 HP nitrous shot...but I mix Av gas and have a MSD timing retard to keep the engine from detonating.
Also, I'd say 11:1 compression will be too high for 92 octane. Most consider around 10:1 to be max with iron heads and 10.5:1 with aluminum heads but as the previous poster pointed out your cam profile has a lot to do with it also. Other things that effect detonation are too lean of a fuel mixture, too much timing, too hot of heat-range for spark-plugs, engine water temp, outside air temp, engine load, carbon build-up, etc.
Deen
Also, I'd say 11:1 compression will be too high for 92 octane. Most consider around 10:1 to be max with iron heads and 10.5:1 with aluminum heads but as the previous poster pointed out your cam profile has a lot to do with it also. Other things that effect detonation are too lean of a fuel mixture, too much timing, too hot of heat-range for spark-plugs, engine water temp, outside air temp, engine load, carbon build-up, etc.
Deen
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