Dumb question: Can you calculate your compression ratio by doing a compression test? My buddy tells me you can't do it...
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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...
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.Quote:
Originally Posted by DennyW
http://not2fast.wryday.com/turbo/com...pressure.shtml
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.Quote:
Originally Posted by Gusaroo
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!!
Quote:
Originally Posted by techinspector1
i second that you cannot tell from a compression test .
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 .Quote:
Originally Posted by DennyW
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 .Quote:
Originally Posted by DennyW
Quote:
Originally Posted by DennyW
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. :HMMM: