Compression specs

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May 30, 2017 | 11:52 AM

mrpotatohead

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From: NAMPA, ID.

Compression specs

Decided to check compression on my 390. Shows 125 on all cyls. This seems a little low. The Ford factory manual doesn't give specifics, just comparisons from highest to lowest.

What are others finding?

May 30, 2017 | 01:24 PM

JEFFFAFA

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From: Phoenix, Az.

As I recall you've mentioned that you've rebuilt this motor. Did you use dished low compression pistons?

May 30, 2017 | 04:15 PM

mrpotatohead

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From: NAMPA, ID.

I used direct replacements, Speed pro I think from Summit. I noticed when I was putting it together that they looked like they were down in the hole a little too far at TDC. It runs excellent though.

May 30, 2017 | 05:48 PM

CougarJohn

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From: Cupertino

The numbers are lowish but even. Of course you blocked the throttle open when you made the test.
Nothing alarming. Just drive it for twenty more years and repeat the test then.

May 30, 2017 | 06:17 PM

mrpotatohead

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From: NAMPA, ID.

Quote:

Originally Posted by CougarJohn

The numbers are lowish but even. Of course you blocked the throttle open when you made the test.
Nothing alarming. Just drive it for twenty more years and repeat the test then.

Of course, and it was fully warmed up. I added some motor oil to one cylinder and re-ran the test. No increase which is a good sign.

May 31, 2017 | 09:36 AM

Red69

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From: Rock Hill, SC

150 psi is average for a stock engine. If your rebuild included a performance cam the static pressure readings will be lower. The fact that they are all the same is great. I wouldn't worry.