Cold vs. Warmed Engine Compression Check

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Bodyperson

Bodyperson

Pedal to the metal
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I checked it cold because it isn’t in running condition. I just wanted an idea before I pulled it. 145 Across the board. forged piston 30 over 340. I am wondering if anyone has an idea how much of a gain in pressure a warm engine might make over a cold (70 degree) engine. All of the plugs pulled with WOT. Popped up pretty good after a couple revolutions. It’s been sitting for years In a controlled environment.
 
Is it stock cam and if it isn’t do you know cam specs? And did you squirt some oil in the cylinders beforehand? I’m with Kimmer, I’ve seen a .040 over 340 built with 10.5:1 and a stock replacement sealed power 340 cam come up to 168 on all eight after the rings seated in.
 
Garrett Ellison said:
Is it stock cam and if it isn’t do you know cam specs? And did you squirt some oil in the cylinders beforehand? I’m with Kimmer, I’ve seen a .040 over 340 built with 10.5:1 and a stock replacement sealed power 340 cam come up to 168 on all eight after the rings seated in.
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Basically stock 340 lift. 107 lobe separation. 216@.05 270 duration as far as I can tell. It's an old isky cam. Been getting some conflicting specs. I have no cam card or any inclination of the cam timing. No oil squirt.
 
I never have paid much attention to cold or hot, throttle open or closed. I just get it done. Besides, all you're looking for is the difference between each cylinder. 10% is the general accepted difference. If I'm really doing hard diagnosis looking for something, I'll get it warmed up and do it with the throttle open, but as just a general well check, it really doesn't matter.
 
RustyRatRod said:
I never have paid much attention to cold or hot, throttle open or closed. I just get it done. Besides, all you;re looking for is the difference between each cylinder. 10% is the general accepted difference. If I'm really doing hard diagnosis looking for something, I'll get it warmed up and do it with the throttle open, but as just a general well check, it really doesn't matter.
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Just curious RRR.
 
Bodyperson said:
Just curious RRR.
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If it's been sittin a while, there could be a substantial difference once it has run some. That 145 could become 165 or so once it's been running again and the cylinders are sealed up good again with a coating of oil. But if it has 145 pretty evenly all around, that's good.
 
RustyRatRod said:
If it's been sittin a while, there could be a substantial difference once it has run some. That 145 could become 165 or so once it's been running again and the cylinders are sealed up good again with a coating of oil. But if it has 145 pretty evenly all around, that's good.
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It ran pretty good up high. Pretty sure it has a wrong converter. It was a dog out of the hole. Really bogged down when engaged. I plan on different trans and converter plus some 65 cc promaxx heads and good exhaust. I have not visited this car in 20 years. Hopefully this summer but that's another thread.
Thanks.
 
Make sure you have a good quality comp guage as when I tested comp with my elcheapo guage (was brand new) my 318 only registered 110 psi and my 410 said 140. seeing that my 318 was supposed the have 10.1 KB pistons with 61cc alloy heads and my 410 registered 140 psi and it was supposed to be 11.0 to 1 I was a little bit worried until I borrowed a mated Snap On tester. On the snap on tester my 318 showed 160 psi and my 410 showed 205 psi. So it just goes to show you their is crap tools and good quality tools out there!
 
James Clews said:
Make sure you have a good quality comp guage as when I tested comp with my elcheapo guage (was brand new) my 318 only registered 110 psi and my 410 said 140. seeing that my 318 was supposed the have 10.1 KB pistons with 61cc alloy heads and my 410 registered 140 psi and it was supposed to be 11.0 to 1 I was a little bit worried until I borrowed a mated Snap On tester. On the snap on tester my 318 showed 160 psi and my 410 showed 205 psi. So it just goes to show you their is crap tools and good quality tools out there!
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It would be interesting to compare with other testers. Mac is my brand.
 
If it's been sitting for a while ( more than 2 years ), I would expect the rings may need to re-seat, which may boost the cranking compression significantly, 20 psi or more.
 
missing linc said:
If it's been sitting for a while ( more than 2 years ), I would expect the rings may need to re-seat, which may boost the cranking compression significantly, 20 psi or more.
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That would be nice. I was thinking the same, especially with forged pistons. My Subaru rattles for a bit before the pistons swell.
 
Compression test should be done at the engine operating temp. That is where the compression is needed. No oil either, that can seal bad rings & give a false reading.
Once an initial test has been done without oil, then a squirt of oil can be added. if the numbers improve with oil, it suggests worn bore/rings.
 
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