Better or worse?tighter quench.

Hippie, there was nothing on the site you linked even approaching the 11.4-1 compression that the OP is trying to get away with.

Point I was trying to make is virtually all solid dome pistons have the potential to be cut down, this is what we did before you could buy todays variety of pistons.


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The Dynamic Compression Ratio impacts octane requirement. Higher static compression ratios are used to offset the cylinder pressure lost from long duration cams. A high compression engine with a big cam can end up with the same DCR as a lower compression engine with a milder cam therefore requiring the same octane. If he had a big enough cam he could run that thing on 87 Octane with 12 to 1 static compression, he probably wouldn't want to run it on the street but it could be done. You want to compare the DCR numbers they came up with against the Octane they were using. The switch to aluminum heads would let him get by with about a full point higher static compression.




I was just stating that he needed to look closely at what he had because the new stuff is different.

Hippie, I've been meaning to tell you how much I appreciate your Bear Alignment sign. That sure takes me back to my youth.

By the way, can you tell me how you got it on there. I have tried to get a personal picture up as an avatar and apparently don't know how to do it. Thanks, Richard.

I worked at the factory back in the 70's building balancers. I just asked Jon if I could have it as an avatar and he hooked me up.

My pistons are speed pro part #H618CP they are hypereutectic and my cam is the edelbrock rpm duration 308/318 lift is .488/.510 lobe separation is 112 and intake centerline is 107.Thanks for the info Guy.

OK, with that cam and a standard Fel-Pro head gasket at .041" thick with a gasket bore of 4.166" I came up with 10.72 to 1 but your DCR was only 7.36. While the quench distance isn't optimal that does put you in pump gas range, you could run 87 Octane. With the .020" head gasket figured with a 4.100" gasket bore and a better quench of .045" static compression came in at 11.38 to 1 and DCR was 7.8 to 1, tough call given the improved quench I'd say 89 would work, maybe even 87 with good cooling, tune and the correct converter and gears. If it's borderline you can also retard the cam to bleed off a little cylinder pressure at low RPM. I'd go for the tighter quench. If you go to a milder cam though you're gonna be in trouble.

Thanks for your reply Hippie,When I tried the DCR calculator i came up with a 9.79 i must have done it wrong.Just out of curiosity have you ever built a high compression street engine like this one?I gonna be running a T-5 tranny and 3.42 gears.

Only one of my own, a 400 Pontiac, it was 10.75 to 1 with a little milder cam and Sunoco 260 was $0.49 a gallon. Remember 50 cents a gallon 104 Octane? I have been closely involved in several builds of similar engines to your's over the past 35 years. Pontiac, Olds, 440 Mopar and Big and Small Block Chevy's. The rest of mine have been considerably tamer, the lumpy cams and constant tuning grew old real fast for me. I'm an amateur but I do have a fair amount of hands on with real engines not just magazine articles and computer programs.

What DCR calculator did you use? I ran your numbers through at least a dozen times and with different gasket thicknesses and advancing and retarding the cam. I use the calculator at the KB piston website but with the Intake closing point calculated using advertised duration instead of at .050" plus 15 degrees, this gives the same results as Pat Kelly's calculator and is a little easier to use, one of the guys over at Chevy Talk clued me in on the KB calculator using advertised duration. Most people I have talked to feel Pat's results are more accurate. I'll check my inputs again I could have been off but 9.79 sounds WAY high.

Thanks Hippie I think im gonna be alright for pump gas.You were right with your dcr I used the same calculator but used the intake closing angle at.050 lift ,thats why my numbers were so high .Thanks again i feel alot more comfortable with this build.Lots of good info.