Hot compression test - Team Camaro Tech
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post #1 of 9 (permalink) Old Jul 12th, 09, 03:26 PM Thread Starter
MM
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Mark
 
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Question Hot compression test

OK, so I've been reading that when I do a compression test, the engine should be warmed up first. It's been decades since I did a comp. test and cant remember if I did it warm then or not. My question is, how do I go about removing all the plugs and performing the compression test on a very hot engine? (carefully, right?) Gloves? Special tools? Any tips would be much appreciated!

Thx

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post #2 of 9 (permalink) Old Jul 12th, 09, 05:43 PM
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Re: Hot compression test

Warm or not doesn't matter. You'll get slightly higher numbers warm but the numbers themselves aren't important. What counts is how close all the numbers are to each other.
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post #3 of 9 (permalink) Old Jul 12th, 09, 07:09 PM
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Re: Hot compression test

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Originally Posted by markw View Post
Warm or not doesn't matter. You'll get slightly higher numbers warm but the numbers themselves aren't important. What counts is how close all the numbers are to each other.
Beg to differ.The numbers are important. They should fall into known combos.O% differance across the board is the ultimate goal..but all eight at 60 psi when they should be 180 psi is unacceptable.This gross example would have you looking at valve timing events perhaps.

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post #4 of 9 (permalink) Old Jul 12th, 09, 07:50 PM
 
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Re: Hot compression test

when the motor is cool enough to work on with out gloves is when I do my comp test
I agree that consistancy is very important but the number is important
for street i like the 190 to 220 range on premium
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post #5 of 9 (permalink) Old Jul 12th, 09, 08:01 PM
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Re: Hot compression test

I have always done mine cold, remove the dist HEI 12 v wire. Start on 1 pedal to floor and turn over a few times. Never did it warm. Do I need to start?
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post #6 of 9 (permalink) Old Jul 12th, 09, 08:23 PM
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Re: Hot compression test

It's true the numbers should be in a certain range but I was thinking more of a diagnostic check for a bad cylinder. Most people build the motor to a known compression and degree the cam to a known value and let the compression test numbers fall where they may. Does anybody move the cam timing around to maximize cranking compression? I've heard of this being done and then heard others say it only maximizes the motor at cranking speed and is not correct for max power. Some engines rattle with 180psi and some can go 220psi on the same gas. To find a bad cylinder the test can be run cold.
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post #7 of 9 (permalink) Old Jul 12th, 09, 10:07 PM
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Re: Hot compression test

I do mine cold too. The wide open throttle has never gotten me any gains either. I just crank it up on 7 cylinders and let it run till the guage stops moving. Only takes mabe 3 seconds per cylinder.

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post #8 of 9 (permalink) Old Jul 12th, 09, 10:30 PM
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Re: Hot compression test

If you're looking for a major mechanical problem then cold is fine. If your looking for the servicable life of the engine then I'd go hot. A high mileage engine could very well tighten up with some heat. And running it on 7 cylinders is not a valid test. LOL
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post #9 of 9 (permalink) Old Jul 13th, 09, 07:03 PM
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Re: Hot compression test

Quote:
Originally Posted by markw View Post
It's true the numbers should be in a certain range but I was thinking more of a diagnostic check for a bad cylinder. Most people build the motor to a known compression and degree the cam to a known value and let the compression test numbers fall where they may. Does anybody move the cam timing around to maximize cranking compression? I've heard of this being done and then heard others say it only maximizes the motor at cranking speed and is not correct for max power. Some engines rattle with 180psi and some can go 220psi on the same gas. To find a bad cylinder the test can be run cold.
I could see how an engine builder that has a customers home built engine on his dyno..wiggle the cam timing and valve lash if cranking compression is higher than expected and peak torque and hp is less than anticipated.I would expect he would then recommend a cam swap..most likely one with with more duration and a later intake valve closing point.
I agree 100% with you..cranking compression can isolate a weak cyl.To high or to low with a healthy cyl seal and a known combo has you thinking valve timing events..wether it be a wiggle here or there,or a complete cam swap to achieve the goal.
For the most part..average Joe with no dyno (myself) can only come close based on recommendations.
In a perfect world,we would have total access to the dyno,unlimited time and 10 camshaft profiles to test.

George
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