comp ratio?

[69sixstang]

this whole extra head volume 62 cc on my 79 head vs 52cc on the orig 66 head . has me a little baffled!! Yes i understand the diff. and the head gasket thickness. But one of my questions is if i put it together without changing anything will it run ok???? my goal is to run Pump gas (cheap stuff) about what will my compression be? my rebuilder says it will not drop as far as they are saying and we will be in the 8.5 range?? or should i just mill the head less, say .040 instead of the .075 to make it more exceptable to todays gas??
Mark

 

[wallaka]

It will run fine with no modification. Will it be down on power? Yes. It won't hurt the engine, but it won't be running as good as it could be. As long as you have it at the machinist, there is no reason not to take some off. Around 9:1 is a good ratio for crap gas, up to 9.3:1 or so would be fine with probably 89 octane. Anything above that will need the good stuff. However, even if you use the car as a daily driver, the cost won't be more than a hundred bucks a year to use premium. With gas prices so high anyhow, 10 or 20 cents is nothing, not nearly the difference that it did with gas at a buck-ten.

If it's a weekender, I would go with the high compression. Static compression ratios don't really mean anything anyhow. A bigger cam will bleed off that pressure nicely. Dynamic compression ratio is what really matters, and the 200 is severly undercammed anyhow. There's plenty of advice already on the board for camshaft selection, you just have to search for it.

 

[CZLN6]

Howdy back Mark:

Thanks for the clarification. I went to our website and ran some numbers for you. Assuming the chambers on the '79 head are 62 cc and you are using a NAPA Victor head gasket at .045" compressed thickness your CR will approximately 7.7:1. A CR this low will not hurt anything, but performance and MPG will suffer- that hurts.

IF you were to mill the head .050", reducing the chamber volume to 52 cc and with the same NAPA headgasket your CR comes out to 8.6:1, which is about ideal for the generic cheap stuff (?) with an octane rating of 85.

FYI, know that the 200 is relatively good for being knock resistant with its small bore, dished pistons, and a wedge chamber. The thicker aftermarket head gasket will hurt quench and combustion efficiency a little, but seals better then the OEM steel shim type did. Carbon build up is the real culpert in causing pre-ignition.

Is that what you needed?

Adios, David

 

[XFlow_Fairlane]

as mentioned....a turbo will help this out a bit