Thread: Compr ratio from a compr test?

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Dumb question: Can you calculate your compression ratio by doing a compression test? My buddy tells me you can't do it...

Originally Posted by DennyW

Yes you can, your buddy is wrong. Simple, if it's over 150 lbs, use the premium fuel. Around 110-120, it's only about 8-8.5 to 1. About 175-180 is 10.7-11.3 to one, with a picked cam. With a straight up 3/4 race cam, it's over 11.5.

Denny W, would you take a look at this cranking pressure calculator. I am missing something. I plug in numbers for my soon to be assembled motor, as follows: 9.1 CR, cam 260, stroke 3.3, rod length 6.32, alt. 300 m, and it comes back with a cranking pressure of 189 psi.
http://not2fast.wryday.com/turbo/com...pressure.shtml

Originally Posted by Gusaroo

Dumb question: Can you calculate your compression ratio by doing a compression test? My buddy tells me you can't do it...

I'll have to disagree with Denny. I all depends on the cam in the motor and the intake closing point ground into the intake lobe. Compression does not start until the intake valve closes, so you could have a 10:1 motor with a very short cam in it that would test out at, for instance, 200 lbs. Change the cam to a very long duration unit with a delayed intake closing point and you could read 120 lbs. I'm not being scientific on these figures, just trying to make a point.

This would be taking into fact that the rings and valves are in good condition and working properly. You could have 11.5 to 1 compression, but with a couple broken rings and some carbon under a valve it might only show 60 pounds of cranking compression..... With an engine it good condition and all cylinders having equal cranking compression, it should get you ball park, at least.....

good stuff!!

Originally Posted by techinspector1

I'll have to disagree with Denny. I all depends on the cam in the motor and the intake closing point ground into the intake lobe. Compression does not start until the intake valve closes, so you could have a 10:1 motor with a very short cam in it that would test out at, for instance, 200 lbs. Change the cam to a very long duration unit with a delayed intake closing point and you could read 120 lbs. I'm not being scientific on these figures, just trying to make a point.

i second that you cannot tell from a compression test .

Originally Posted by DennyW

The amount of compression you have, is, and will be your the compression ratio at that point in time. In other words, if the compression is lower, but all even, and the engine runs good, you can estimate the compression ratio for the lbs of compression you show.

sorry , you cannot tell c.r. from a pressure test . example i have a smallblock 350 compression test said 125lbs , new motor 9.5 to 1 . i also had a 540 with 130lbs , new motor 15.0 to 1 . because the 540 had a camshaft that had a lot of overlap it released cyl. pressure .

Originally Posted by DennyW

You can disagree, it's ok. You didn't read my reply right, or I didn't explain it enough. This is in general rules. I started with 11.5 to 1. milled the heads .030 from clean up. 224º duration, 514 lift. .060 over in the bore, not decked. My compression is 175lbs. All within 5lbs. With that being done, I am actually over 11.5. I run premium 91 octane, no ping.

That's what I was getting at on the cams. You need all the specs to know what you will end up with. I agree on that totally.

i could take that same motor change the camshaft and you would have only 125lbs , and then your static compression would still be the same just lower cranking pressure .

Originally Posted by DennyW

That's called relative conpression actually. Relative to either the cranking rpm's, or the running rpm's.

If you crank the engine over, and it reads 125lbs, then, that is the compression ratio.

Same as if you run a 4 barrel carb. At idle, and light aceleration, it is a 2 barrel, but you have the 4 barrel option.

i think you have confused with cylinder pressure or cranking pressure with compression ratio or static ratio .
and the way he posed the question he was asking for compression ratio or static ratio , example 9 to 1 or 8.5 to 1 ratio .

if you crank the engine over ,and it reads 125lbs . then thats cylinder pressure , not the C.R.

My take on this is........static compression is kind of like theoretical max pressure.The dynamic is the actuall pressure the engine will see.There is a guide to cranking pressure on the internet that was put out by some recognised names in performance.David Vizzard says he likes to see 190 + cranking pressure on a hot street motor with the proper associated components.The cranking pressure can be used to determine the approx dynamic pressure and aslo octane requirements of the engine.With the proper cam specs one can also estimate the actuall static copression ratio.

Considering unless you cc every piece on the engine most calculations are approx estimates anyway...........
I have found there is a converter /estimater for just about anything you could want to convert or estimate from just about any measurement you could possibly take on a engine.Whether these are anywhere close to actuall numbers is a different story..............but the conversion tables are out there.

IM DONE TRYING TO EXPLAIN , good luck .

my head hurts...

if it has a stock or small cam you can tell close to what the C.R. with a comp. check. its not a 100% but you'll should know if you have a hi or low comp. motor.