compreshion ratio calculations

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cudaman51

cudaman51

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I got to thinking about calculating compreshion ratio from compreshion tests and this is what i came up with. I think this is corect based on no leaking parts. atmosphereic preshure at see level is 14.7 so a 10.5 cr moter should have 10.5x14.7=154.35 so the gauge should read 154.35 -14.7=139.65.the minus 14.7 is 0 on the gauge is really 14.7 at sea level.or you could figure it this way. your test shows 160psi. if the gauge reads 0 to start then 0 is 14.7 axualy so add 160 + 14.7=174.7divided by14.7=11.8 for your compreshion ratio. well isnt that fun am i right i think so .i know this is based on perfect cercumstanses. second opinions anyone
 
I dont no what you meen. but i think with some cams you would have less compreshion and this formula wouldnt be acurate.I am not a moter builder i just put this out to se if i am right.some one out their can either confirm or deni but i think with a mild cam it would be close'
 
For some reason I don't think that's reliable. I can't tell you why... but I think it has to do with the dynamic compression ratio rather than static. That's a really cool thing to ponder for a rainy day tho...
 
I agree with moper, high overlap cams give some bleed off at low rpms correct?
Given that, you calcs would be off some.
 
Different cams would change the cranking/dynamic you would think.

I mean I have 155psi cranking with 10.5 piston but then consider the cams o lap of 60* so....change that to 40* and 170psi hmmmmm....

btw good morning.
 
A pressure gauge will always give you a reading based on dynamic, but should more accuratly be called cranking pressure.they are all part of the relationship which gets you to dynamic compression ratio.

you should look up Pat Ryans DCR calculator.
Andrew
 
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