Compression Spec?
06-19-2006, 10:42 AM
#1
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Compression Spec?
Checked the compression on the '72 this weekend and found about 120 PSI all the way around. The spec in Haynes says normal is 128-152 with a compression ratio of 8.2. Doing the math (8.2 X 14.7), I come up with a theoretical max of 120.54 PSI.
Why the large discrepancy between Haynes' spec and the theoretical max? More importantly, do I truly have good compression??
(For the test. all plugs were removed, air cleaners off and WOT.)
TIA
Jim
Why the large discrepancy between Haynes' spec and the theoretical max? More importantly, do I truly have good compression??
(For the test. all plugs were removed, air cleaners off and WOT.)
TIA
Jim
06-19-2006, 12:09 PM
#2
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Hi Jim,
I never really thought about how they calulate compression ratios, but I've had engines over 140, so the numbers in the Haynes must be fairly correct.
At 120 pounds, you are a little low, but if it's running pretty good, I woulnd't worry too much. Overall, how does the motor run?
Matt
I never really thought about how they calulate compression ratios, but I've had engines over 140, so the numbers in the Haynes must be fairly correct.
At 120 pounds, you are a little low, but if it's running pretty good, I woulnd't worry too much. Overall, how does the motor run?
Matt
06-19-2006, 12:29 PM
#3
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Thanks, Matt.
I'm not totally convinced that the compression test was as good as it could have been. The engine was completely warmed up (drove it for 10 miles and let it sit for a half hour so as not to be real hot) and the compression gauge didn't screw into the spark plug hole as well as it might have. (Or maybe this is my lame excuse for low compression!
Just getting back into it after 3 years of not much driving/maintenance. Seems to run fine after warming up, though the idle is not particularily smooth. Had some difficulty starting, then didn't want to run on a cylinder or two for a minute, but this was partially (but not fully) rectified with a new blue coil and a "point-less" insert. Might need new wires, too?? Planning on a valve adjustment/oil change/carb sync next.
Tends to run a little toward the warm side (230-240 cruising around on a 90 degree Texas day), but good overall. Got the 1.9L "Big Bore" kit, mild cam and dual Webers, but no external cooler (yet). It just seems like I should be able to find a little more power somewhere with the existing set up....
Jim
I'm not totally convinced that the compression test was as good as it could have been. The engine was completely warmed up (drove it for 10 miles and let it sit for a half hour so as not to be real hot) and the compression gauge didn't screw into the spark plug hole as well as it might have. (Or maybe this is my lame excuse for low compression!
Just getting back into it after 3 years of not much driving/maintenance. Seems to run fine after warming up, though the idle is not particularily smooth. Had some difficulty starting, then didn't want to run on a cylinder or two for a minute, but this was partially (but not fully) rectified with a new blue coil and a "point-less" insert. Might need new wires, too?? Planning on a valve adjustment/oil change/carb sync next.
Tends to run a little toward the warm side (230-240 cruising around on a 90 degree Texas day), but good overall. Got the 1.9L "Big Bore" kit, mild cam and dual Webers, but no external cooler (yet). It just seems like I should be able to find a little more power somewhere with the existing set up....
Jim
06-20-2006, 09:30 PM
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Stock- 8.2 to 1
The book says the ratio should be in the 120-155 psi range for the "W & EA" motors with the Calif low compression models less.
I had a set of P&C's off a low mile 1.7l that I used to rebuild this motor, which was a low mile 1.7l with bus pistons (7 to 1). Main bearings, rods etc were in spec but I did replace the cam and lifters to a Raby FI cam. Hope I pick up 5 hp doing that.
The book says the ratio should be in the 120-155 psi range for the "W & EA" motors with the Calif low compression models less.
I had a set of P&C's off a low mile 1.7l that I used to rebuild this motor, which was a low mile 1.7l with bus pistons (7 to 1). Main bearings, rods etc were in spec but I did replace the cam and lifters to a Raby FI cam. Hope I pick up 5 hp doing that.