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compression ratio
https://www.slantsix.org/forum/viewtopic.php?t=8736
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Author:  leaningtowerofpower [ Tue Mar 16, 2004 3:41 pm ]
Post subject:  compression ratio

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?

Author:  70valiant [ Tue Mar 16, 2004 4:18 pm ]
Post subject: 

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*

Author:  leaningtowerofpower [ Tue Mar 16, 2004 4:25 pm ]
Post subject: 

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

Author:  Doc [ Tue Mar 16, 2004 4:34 pm ]
Post subject: 

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. :cry:
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

Author:  70valiant [ Tue Mar 16, 2004 8:01 pm ]
Post subject: 

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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