Compression Readings
05-12-2005, 02:26 PM
#17
Rennlist Member
As you know, the 10:1 compression comes from the ratio of the cylinder displacement vs what it is compressed into when the piston is at TDC. (ie the combustion chamber volume) but, i see your point. 10:1 should be 10x 14.7 + 14.7, so close to 164psi. obviosly, it can be lower due to cam profiles, but certainly, the max pressure should represent the ratio in pressure.
GREAT question!
anyone?
MK
my 5 liter that i actually measured the ratio was 9.8:1 and i only measured pressure of 155psi. (so right in range) . my 84 with a 9.3:1 rating, was in the 130psi range, the Holbert car with a theoretical 10:1 compression ratio, puts out 185psi and who knows what the REAL compression is. maybe another underating from porsche. Ive also measured compression of a stockish, 6.0L stroker at 220psi. hmmmm 14:1 compression??? maybe !
Hmmm.
GREAT question!
anyone?
MK
my 5 liter that i actually measured the ratio was 9.8:1 and i only measured pressure of 155psi. (so right in range) . my 84 with a 9.3:1 rating, was in the 130psi range, the Holbert car with a theoretical 10:1 compression ratio, puts out 185psi and who knows what the REAL compression is. maybe another underating from porsche. Ive also measured compression of a stockish, 6.0L stroker at 220psi. hmmmm 14:1 compression??? maybe !
Hmmm.
Originally Posted by sublimate
Can anyone explain to me why the pressures are so high? Since atmospheric pressure is 14.7psi I thought that the max you would see from a 10:1 compression engine would be about 147psi (or 125psi with 8.5:1), and that would be assuming a 100% volumetric efficency. Is there really significant overpressurization (due to pressure pulses or intake momentum) at cranking speeds? Or is there something else I'm missing?
05-12-2005, 02:52 PM
#18
Gluteus Maximus
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Originally Posted by mark kibort
As you know, the 10:1 compression comes from the ratio of the cylinder displacement vs what it is compressed into when the piston is at TDC. (ie the combustion chamber volume) but, i see your point. 10:1 should be 10x 14.7 + 14.7, so close to 164psi. obviosly, it can be lower due to cam profiles, but certainly, the max pressure should represent the ratio in pressure.
GREAT question!
GREAT question!
A compression ration of 1:1 would be no compression as I understand it. Therefore the air stays at 14.7 psi but the tester reads as zero since it's they same as atmospheric.
A compression ratio of 2:1 would half the volume and double the (absolute) pressure to 29.4psi. But the tester would read 14.7psi (29.4 - 14.7).
By that reasoning, an 8.5:1 ratio would yeild 110psi and a 10:1 ration would yeild 132psi. But obviously I'm way off. What's up?
05-12-2005, 09:29 PM
#19
Had a car a few years back that had not been run for some time. After I had run it for a while I did a compression test. All high one low at 120. Did a wet test with marvel mystery and the compression shot up to normal and never went back down. Now I never start an engine that has been sitting long without oiling the cylinders, cranking without plugs. putting plugs back in and cranking again. The funny thing was the ring had stayed stuck even though I was running the engine. Not too much oil though. You don't want to lock the engine up.
