compression test

[kamstra]

Any idea of what my compression ratio would be. I did a compression test and it averaged between 150 and 160 psi

[Billydelrio]

Kamstra
There are too many unknowns to determine a compression ratio by looking at the compression readings alone. I have a 8.7:1 engine with a mild cam advanced 4 degrees before split overlap and it reads 155 to 160 psi. I have seen 13:1 engines with BIG & WIDE solid lift cams only get low 120's on tests.
Sorry for not being able to answer your question exactly, but I would guess around 8:1 if it is a factory build.
Billy

[kamstra]

Thanks,
Here is some additional info.
Then engine is a 1968 440 Mopar replacement pistons; closed chamber 915 heads and a MP509 cam.

[cuda66]

Simple deal:
Pull one head and measure the distance of the piston to the face of the block, CC the combustion chamber add in the compressed thickness of the head gasket, do the math and you'll know for sure....I know you didn't want to hear this did you?

There is no exact way to calculate compression ratio from compression know to man or beast and unless you know it's real hard to figure out plug gaps and best fuel octane and how do you select a cam without knowing this.....maybe a crystal ball, or of course there's always the Smummitt catalog hahahahhaaaa

Don

[kamstra]

OK, the eays answer would be nice, but I figured as much.
can I rotate the engine to TDC and just measure with a feeler guage?

[cuda66]

I doubt it....you need to be within .001 to be right on the number

[PRO]

My 8.5:1 440 w/509 cam had 110 lbs of cranking comp so Id guess your around 9.8:1 or so...............PRO.......

[kamstra]

Thanks Pro, The heads are off again for some screw ups on the rebuild. I am having the heads CCed at the same time. I will try to measure the piston depth too. Most guesses were close to 10:1

The machine shop is throwing in Free STainless Steel valves for me re and re the engine.

[Walkercolt]

kamstra, I think that's close, because my stock 5.2L Magnum in my Dakota shows about 135PSI and it's spec'ed to be 9:1CR. If you have a "stock" engine without a radical cam, you can get an idea of where you are with a guage. I think it's better than just trying stuff and hoping to find the "right" combination. I know a tear-down is the only way to be certain, but that can be an exspensive thing nowdays( empty and re-fill A/C, gaskets, hoses, etc.) Actually, there are pressure transducers which can give you the actual combustion pressure and temperature with-in the cylinders...I've seen them used on 5000HP stationary Waukesha natural gas fueled compressor engines, but those things are bigger than my house! Bore of 30" and stroke of 36" is average, twelve or sixteen of those cylinders and 550 RPM idle or red-line!