so what is the highest compession ratio that i can run (say in a long rod engine) on gas from the pump.....whats the best compression you can run on good ol cheap 87?

I had 0.100" taken off of my slant's head and I can't run less than 92 octane with my inital advance set to 2*

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where did you get your machine work done? I've been looking for a shop I can trust here in PA.

It is better to think about this question in terms of cylinder pressure instead of calculated compression ratio.

Calculated compression ratio is simply the volume ratio between a full cylinder and a fully compressed cylinder.

Cylinder pressure is the cranking pressure (in psi) that the cylinder sees during a compression check. The cam profile and cylinder "leak-down" has a big influance on this number. This is why big cam profiles are combined with higher compression ratios, the cam "bleed-off" is off-set by the higher C/R to yield a desired cylinder pressure.

Here is my opinion on cranking cylinder pressure readings and the fuel you need to use:

Anything under 100 psi = rebuild the engine.
100 to 140 psi use 87 octane
140 to 170 psi use Premium (92 octane)
170 to 200 psi use Race gas (100+ octane)
200 to 240 psi use Super Race Gas (114)
240+ alcohol

Finding the true cranking cylinder pressure for a new combination is difficult to do, you really have to build the engine and check it.
That is why it is nice to find a "proven" combination of parts which is already known to produce the proper cylinder pressure for you application.
DD

I had my head done at ABC automotive in souderton PA. they got me for almost $500 for cleaning, testing for cracks, shaving 0.100" off the mating surface, new guides, an intake valve, valve and seat resurfacing and a gasket set. This place has had a good reputation since I was a kid working at a GMC truck shop(I'm 35 now). I think I must be in the 160 PSI range now. My brother-in-law si going to let me borrow his compression tester this weekend when we get back from the MOPAR get together near king of prussia.

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