Compression check on a 340

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Compression check on a 340

Post by swinger1969 on Mon Sep 21, 2015 2:29 pm

Did a compression check on my 70 340 and it was pretty much 100 psi all across , and the check was done cold ( not run in a couple of weeks) because I have the intake off. Sound ok? Thanks! Larry

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Re: Compression check on a 340

Post by JGL on Tue Sep 22, 2015 7:05 am

Should be in the 120 ballpark warmed up(I think), so 100 cold sounds ok. Don't rely on my opinion.
you may get the proper info on google.

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Re: Compression check on a 340

Post by philcollins on Tue Sep 22, 2015 7:22 am

should be around 150 psi for a stock 1970 340.

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Re: Compression check on a 340

Post by Gazoo on Wed Sep 23, 2015 3:34 am

100psi does sound low, even with 8.5:1 pistons as opposed to 10.5:1.
Does it have a really high duration cam in it?
The difference between a cold engine or hot engine is negligible, depending on components.

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Re: Compression check on a 340

Post by Keith Mopar on Wed Sep 23, 2015 7:17 am

I's throw some oil in the cylinders and see what happens. Is the gauge accurate? You could always try it on a different car.
I'm with gazoo seems low. I'd think a low comp car would be a least 120 - 130.

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Re: Compression check on a 340

Post by swinger1969 on Fri Sep 25, 2015 5:00 pm

My buddy did a search on it and apparently th "cranking pressure" on a cold engine should be 100 PSI which would make the warm pressure about 130-150 so mine should be fine. Thanks!

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Re: Compression check on a 340

Post by Gazoo on Sat Sep 26, 2015 3:21 pm

I checked 125-150 psi on a 1991 "318" (5.2L), with 300,000 k's, it was cold, the engine hadn't run in 3 years. Now it did have a bit of carbon deposit, but nothing substantial.

I really don't see where you're going to gain 30-50psi with a hot engine. That's a 30-50% increase in compression, the heat growth coefficient factors is nowhere near this amount, on any material.

Theoretically speaking, cold air is denser, and will give you a higher reading of compression for the same volume. But these readings would be fractionally different then hot air.
On the other side, hot gas gets denser with heat...But you're not doing a compression check with the fuel running, right?

Having said this, if you have 8.5:1 pistons and a high duration cam, thicker head gasket etc..., you will get a lower compression reading in psi.
If you are getting an even number of compression on all cylinders (give or take a few psi's)...maybe your gauge is not 100% accurate on it's calibration of the actual psi readings.

I unfortunately, haven't looked through my shop manual, for the recommended specs.
I avoid paging through it as much as possible.


Like you said, it should be fine, considering you have an even reading straight across, so you can feel confident about, no bad head gasket or burnt valves (assuming you did a bleed test also).

I personally wouldn't lose any sleep over it.




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Re: Compression check on a 340

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