How is compression ratio figured?
11-10-2014, 09:38 AM
#1
How is compression ratio figured?
I was playing with some numbers the other day and something intrigued me.
Per the manual, a 502mag should have a cold cranking pressure of 150psig (164.7psia, imagine we're testing in the Keys) and a compression ratio of 8.75. However, 14.7*8.75 does not equal 164.7, it's 128.6.
Something else must be taken into account here, so what is it?
Per the manual, a 502mag should have a cold cranking pressure of 150psig (164.7psia, imagine we're testing in the Keys) and a compression ratio of 8.75. However, 14.7*8.75 does not equal 164.7, it's 128.6.
Something else must be taken into account here, so what is it?
11-10-2014, 10:18 AM
#2
Registered
iTrader: (5)
Cranking compression and compression ratio have very little to do with each other. Valve timing events have much more of an effect. Same engine with different cams can have 140-190 cranking compression.
Compression ratio is the volume of a cylinder at bdc, versus total combustion chamber volume at tdc.
Compression ratio is the volume of a cylinder at bdc, versus total combustion chamber volume at tdc.
11-10-2014, 10:29 AM
#4
Registered
iTrader: (3)
The later the intake valve closing point, with all other things being equal (Static compression, altitude, rod length), the lower the cranking pressure. Hence why, lets say you have a stock 502 mag with stock cam. you may get 150psi. Now, throw in a more radical cam, like with an intake valve that closes at 56*, it may fall to 135psi. To get cranking psi back up, you'd raise the static compression up to say 9.5:1.
This is why, cam choice and compression ratio, go together. Too radical of a cam, with not enough static compression, leads to a soggy turd until the engine speed can catch up. At low RPM, there is simply more time, for the cylinder pressure to bleed off. As rpm increases, that bleed time is reduced, and the engine starts to come alive.
I've seen guys put together retard combo's, where the engine barely made 100psi on the starter. Then they scratched their heads about why the engine was an absolute mutt until 4000RPM, wouldnt idle worth a dam, etc.
This is why, cam choice and compression ratio, go together. Too radical of a cam, with not enough static compression, leads to a soggy turd until the engine speed can catch up. At low RPM, there is simply more time, for the cylinder pressure to bleed off. As rpm increases, that bleed time is reduced, and the engine starts to come alive.
I've seen guys put together retard combo's, where the engine barely made 100psi on the starter. Then they scratched their heads about why the engine was an absolute mutt until 4000RPM, wouldnt idle worth a dam, etc.
Last edited by MILD THUNDER; 11-10-2014 at 10:32 AM.
