Re: Correct way to do Compression test
[Re: n20mstr]
#2715151
11/12/19 02:46 PM
11/12/19 02:46 PM
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Joined: Jan 2003
Posts: 10,011 Frostbitefalls MN (Rocky&Bullw...
gregsdart
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I Live Here
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Frostbitefalls MN (Rocky&Bullw...
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It helps if you can warm the motor. Round bores and running piston to wall clearances should help too. Then i pull all the plugs and spin the motor till the gauge stops gaining pressure. It may take as many as ten compression cycles since the gauge and line have to be pressurized . Open the throttle if you don't pull the plugs on all the cylinders.
Last edited by gregsdart; 11/12/19 02:49 PM.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: Correct way to do Compression test
[Re: BradH]
#2715158
11/12/19 03:16 PM
11/12/19 03:16 PM
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Joined: Jan 2012
Posts: 2,654 Stuttgart, Arkansas
rickseeman
master
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master
Joined: Jan 2012
Posts: 2,654
Stuttgart, Arkansas
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I have a very UNscientific approach: - Carb wide open - Crank the engine till the gauge doesn't go any higher
I agree with this
2011 Drag Pak Challenger
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Re: Correct way to do Compression test
[Re: rickseeman]
#2715161
11/12/19 03:32 PM
11/12/19 03:32 PM
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Joined: Jun 2006
Posts: 5,163 CT
GTX MATT
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master
Joined: Jun 2006
Posts: 5,163
CT
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I have a very UNscientific approach: - Carb wide open - Crank the engine till the gauge doesn't go any higher
I agree with this This is also what I do
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
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Re: Correct way to do Compression test
[Re: n20mstr]
#2715573
11/13/19 11:17 PM
11/13/19 11:17 PM
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Joined: Nov 2015
Posts: 879 Missouri
jwb123
super stock
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super stock
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Missouri
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By the book, all plugs out, charger on battery, 4 pulses, throttle open. The main thing on the pulses is just do the same on each cylinder. Plugs all out makes it spin faster, and keeps you from missing head gasket blown between two cylinders. Try a compression test with the throttle open and then with it closed, you should see a few more lbs with it open. Also a running compression test can be valuable as well. One cylinder at a time, with the engine idling you will see fairly low compression, blip the throttle and the compression should come up to 80% of the cranking compression, tests for loose valve guides, broken valve springs, worn cam lobes, carbon build up on the intake valves. Also dry and wet compression test, if compression is low, squirt a little oil and retest, if compression goes up it's the rings, if it does not it's the valves or head gasket. I still prefer a leak test, main reasons is battery charge does not affect results, and I can leak test an engine not in the car. When buying a used engine first thing I do is leak it, saved me money several times. After it's built on the engine stand leak it, just to make sure you did it right, again saved me several times.
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Re: Correct way to do Compression test
[Re: KDY]
#2715578
11/13/19 11:23 PM
11/13/19 11:23 PM
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Joined: Nov 2015
Posts: 879 Missouri
jwb123
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Missouri
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At the risk of show my ignorance / inexperience, what are we looking for with:
A) Compression testing? What should I theoretically expect to see and how do we correlate real world results to the expected value?
B) Leak down – measure of ring / valve seal efficiency only?
Compression test measures the ability of the engine to take in air and compress it. Leak testing just checks the ability of the combustion chamber to hold air. Leak testing is also good to check for blown head gaskets and cracked heads, pressure the cylinder, take off the radiator cap and look for bubbles. I also like to use a vacuum gauge when testing. Performance engines have low idle vacuum, but will still show 18 to 20 inches if held above the lope of the cam. Low vacuum means something is wrong, if the needle is bouncing while low its in the valve train, just one example.
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Re: Correct way to do Compression test
[Re: AndyF]
#2715691
11/14/19 12:56 PM
11/14/19 12:56 PM
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Joined: Jan 2003
Posts: 19,376 Las Vegas
Al_Alguire
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I Live Here
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Las Vegas
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As stated carbs open, plugs out(easier on starter), battery on charger to ensures it does not get weak. Engine warm is essential, particularly in an aluminum block. Pay attention to the first pump number and then total number. Some engine builders like to use the first pump number as a bench mark as it compares to total. As for a total c cranking compression number lots of variable there for sure. Aluminum blocks will generally tend to have less cranking compression that their equally done iron brethren.
As for leak downs a lot of variables there. Good to fund think like slightly bent valves etc. I think if you ask engine builders they would rather see a cranking compression number over a leak down number as to engine health. If you ran a vacuum pump that is another way to gauge the engines overall "health". If that number begins to fall you will likely see that in a compression test as well.
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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