Crank It
Mark Baldridge, Paola, KS: I've been reading about cranking compression as being the best way to determine allowable compression ratio because it takes into account the effects of cam timing. What's the upper limit for an aluminum-headed street engine on 91-octane gas? What's a really safe number that will never knock even on a hot summer day after some carbon builds up? How much can I increase the cranking compression if I install a water/methanol-injection system? I'm willing to do some things like polish the combustion chamber and piston tops and cut back the side electrode on the spark plugs, but I don't think I'm going to spring for thermal coatings.
Jeff Smith: Here goes, Mark. A good, streetable cranking compression number for a pump-gas 91- to 93-octane engine is around 185 to 190 psi. An engine we purposely built with 8.2:1 compression and a healthy cam struggled to generate 170 psi. It ran OK but was a dog and didn't respond well to tuning efforts. We also built a 383 small-block Chevy that measured between 195 and 205 on the cranking-compression scale and was crisp and responsive and rarely detonated on pump gas unless the engine and the inlet air were really hot.
With water/methanol injection, you could get away with upping the cranking compression to perhaps 210 psi. The hassle is that a small, half-gallon tank won't last as long as you think, and you will have a rattle-fest if it runs dry.
Polishing the chambers and piston tops is probably not a good idea. The Dart cylinder-head people say it tends to promote carbon buildup. The best move, however, is to create as tight a quench area between the head and piston as possible. For steel connecting rods, most engine builders like to keep the piston no less than 0.039 inch away from the head. That usually ends up being zero deck height on the piston with a 0.041-inch-thick head gasket to create 0.041 inch of clearance. This move, more than almost anything else, will help reduce the engine's sensitivity to detonation while also improving combustion efficiency. Sounds like a fun project, Mark, and thanks for a great question.
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