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Old 07-25-2010, 05:52 PM
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compression test results

I am testing 2 chevy 350 engines. One is on a stand, no carb, no exhaust. Second is in my 65 Elky, large 4 barrel wide open throttle, headers and 3 in. pipes.
I am assuming I will get a much higher reading on the one on the stand, with no accessories. How can I compare which is the better engine as far as compression goes ? Also should I remove the v belt from the water pump and Alt, will it matter ?
Thanks, Crash

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Old 07-25-2010, 05:57 PM
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Quote:
Originally Posted by crash
I am testing 2 chevy 350 engines. One is on a stand, no carb, no exhaust. Second is in my 65 Elky, large 4 barrel wide open throttle, headers and 3 in. pipes.
I am assuming I will get a much higher reading on the one on the stand, with no accessories. How can I compare which is the better engine as far as compression goes ? Also should I remove the v belt from the water pump and Alt, will it matter ?
Thanks, Crash
You're not measuring power output at the flywheel, so it won't matter what's hooked up and what's not. As long as both engines have the ability to breathe unrestricted, you will get valid results. Wire the throttle blades wide open on the carb. Disable ignition.

Last edited by techinspector1; 07-25-2010 at 06:27 PM.
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Old 07-25-2010, 06:17 PM
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Also make sure you perform the test on the elky with the engine cold as a warm engine will tend to read a little better as tolerances close up with thermal expansion. You wont burn your arm either
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Old 07-26-2010, 06:14 AM
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compression test

Both engines are cold, but I'm getting much higher and consistent readings on the stand engine, which is supposed to be rebuilt. I had my doubts about the stand engine but it appears to be tighter than the one in my elky, so I wanted to be sure I was doing it right. Thanks for the answers.
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Old 07-26-2010, 08:33 AM
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Unless one is giving readings in the basement,consitancy cyl to cyl would be what I would be after
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Old 07-26-2010, 08:41 AM
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Quote:
Originally Posted by crash
I am testing 2 chevy 350 engines. One is on a stand, no carb, no exhaust. Second is in my 65 Elky, large 4 barrel wide open throttle, headers and 3 in. pipes.
I am assuming I will get a much higher reading on the one on the stand, with no accessories. How can I compare which is the better engine as far as compression goes ? Also should I remove the v belt from the water pump and Alt, will it matter ?
Thanks, Crash
For practical purposes, the two engines will be able to be compared as you described the way each is set up. To get a valid reading for comparing them, use the same number of "hits" per cylinder on both engines. You'd like them to spin at the same rate (the flywheel size can affect this as much as anything), but that is less important than having the carb WFO, plugs out so they can spin freely as possible and the same number of pumps per cylinder.

I agree w/looking for uniformity between cylinders, as well. Differences in cam timing, for instance, can change how one engine pumps compared to another- even if they seal the same.
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Old 07-26-2010, 10:00 AM
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compression test

After 3 hits the one on the stand is at 110/120 psi in every cylinder. Haven't got to do the elky yet, its already 92 out there. Thanks
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Old 07-26-2010, 08:11 PM
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Quote:
Originally Posted by crash
its already 92 out there.
You fellows are having a little cold snap, huh? 108 on the back patio today here.
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Old 07-27-2010, 08:43 AM
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Quote:
Originally Posted by techinspector1
You fellows are having a little cold snap, huh? 108 on the back patio today here.
But it's a 'dry' heat! lol

Seriously, though- as a long time former Floridian, I can attest to how miserable the 90's are @ 95% humidity. Not saying 108 ain't hell, just sayin'.
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Old 07-27-2010, 09:51 AM
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Doing a leak down test at TDC is a more important test to do.
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Old 07-28-2010, 09:35 AM
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compression test

Thats my next step. Its only 92 this am, maybe it'll cool off this pm, yeah right.
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Old 07-28-2010, 11:33 AM
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Quote:
Originally Posted by crash
Thats my next step. Its only 92 this am, maybe it'll cool off this pm, yeah right.
The suggestion to do a leakdown test IS a good one. Most guys don't have the equipment on hand to do one (it takes a compressor and the leakdown gages), but if such could be secured, it does take the cam timing out of the equation.

If not, work w/what you have available and use your best judgement.

Good luck.
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Old 07-28-2010, 04:05 PM
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compression test

After a physical exam of the engine and comp test i am pretty sure this will be a good engine, has nice oil pressure also. My only reservation is the flywheel, it seems to be bent or something, it looks crooked when cranking over, although the flywheel bolts are tight. I'll have to pull it and see.
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