Compression test

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john in kzoo
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Compression test

Post by john in kzoo » Fri Nov 13, 2020 11:30 am

Guys,

Interested in doing a simple compression test. A couple of questions 1. Warm or cold engine? and 2. All plugs out as each cylinder is being tested?

Thanks

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Mark Nunn
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Re: Compression test

Post by Mark Nunn » Fri Nov 13, 2020 11:42 am

Don't forget to fully open the throttle during the tests. You can't compress air that can't get in.

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Re: Compression test

Post by RajoRacer » Fri Nov 13, 2020 12:45 pm

1: I prefer warm - 2: yes & 3: what Mark said !

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Re: Compression test

Post by Mark Gregush » Fri Nov 13, 2020 1:28 pm

It is ok to have the throttle closed or open, I really have never seen a difference between the two. I do my test cold, I am more interested in starting compression then running. Again, you can do them both to see how much difference between the two readings.
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john in kzoo
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Re: Compression test

Post by john in kzoo » Mon Nov 16, 2020 7:53 am

Compression test completed Saturday on a very cold engine (30 degrees) got 45#, 40#, 40#, and 40#. Plugs looked good #1 was sooty though
Thoughts??

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Tim Rogers
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Re: Compression test

Post by Tim Rogers » Mon Nov 16, 2020 1:31 pm

Run the wheels off of it or do a complete rebuild.
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Re: Compression test

Post by Quickm007 » Mon Nov 16, 2020 1:52 pm

How to do a compression test on engine without starter ?
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Re: Compression test

Post by john in kzoo » Mon Nov 16, 2020 1:54 pm

I opt for running the wheels off........

Quickm007 - Crank and crank and crank


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Re: Compression test

Post by Fire_chief » Mon Nov 16, 2020 2:53 pm

Mario, it is still the same process.
The only thing different is that you get to turn the engine over with the crank


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Re: Compression test

Post by kmatt2 » Mon Nov 16, 2020 2:58 pm

Your cold compression test numbers, 45,40,40,40, would sujest why #1 plug was sooty. You are getting a little oil past the rings in #1 and #1 plug has a intermitid misfire. Try doing a wet compression. test with a little oil in the cylendars and all numbers will come up, with #1 coming up the least. Your cold dry numbers of 40 lbs are not that bad. I would keep driving the car, keep up with your maintenance and check the #1 coil & timer connections, replace #1 plug if needed.

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john in kzoo
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Re: Compression test

Post by john in kzoo » Mon Nov 16, 2020 4:21 pm

Thanks Guys.....

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Re: Compression test

Post by Charlie B in N.J. » Mon Nov 16, 2020 5:59 pm

I much prefer comp testing a low comp engine like the T when it’s cold. It gives a truer situation concerning starting the T when cold. Testing hot will give a false reading regarding cold starting. A modern with a comp of say 150 lbs. or better is relatively in affected by a 5 or 10 lb. drop. On a T that could be the difference between easy starting or not on a cold day. Do a dry then a wet test with the choke & throttle wide open. Write the results down. Don’t trust to memory. A VERY informative test concerning your engines condition. Especially if you’re pulling the head for no reason.
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Re: Compression test

Post by Oldav8tor » Mon Nov 16, 2020 6:23 pm

Sooo.... what is the "good" compression range?
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Re: Compression test

Post by Tim Rogers » Mon Nov 16, 2020 6:30 pm

55 PSI would be nice...
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Re: Compression test

Post by Quickm007 » Tue Nov 17, 2020 9:34 am

Thank you guys, I will crank, crank and crank but my shoulder is painfull right now. Hope I will recover soon... :lol:
Super Mario Bross ;)

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john in kzoo
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Re: Compression test

Post by john in kzoo » Tue Nov 17, 2020 11:00 am

It's really easy with all but one plug out and the throttle positioned all the way open as well


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Re: Compression test

Post by KeithG » Tue Nov 17, 2020 1:33 pm

Super Mario, Befriend a football player with wide shoulders and strong arms.... :)

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