Compression test numbers
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Cross-Country
Joined: Nov 2011
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Compression test numbers
74 F250 with stock 360 engine. What should I expect to see for compression test numbers? Rebuilt the motor
around 4000 miles ago, honed cylinders and installed new iron rings. Right after rebuild I saw 100 pounds
in each cylinder.......I am not sure of the numbers on the low Comp. 360. Thanks guys.
around 4000 miles ago, honed cylinders and installed new iron rings. Right after rebuild I saw 100 pounds
in each cylinder.......I am not sure of the numbers on the low Comp. 360. Thanks guys.
Freshman User
Joined: Apr 2018
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I just did compression on a 74 390 with 111K on it and I got 3@130ish 3@120ish and 2@110ish
this motor hasn't spun for a couple years though.
I have searched for factory specs for any FE and haven't found them .
Can anyone share what the service manual says?
this motor hasn't spun for a couple years though.
I have searched for factory specs for any FE and haven't found them .
Can anyone share what the service manual says?
Laughing Gas
Joined: Nov 2013
Posts: 1,207
Likes: 60
From: S.E. Louisiana;so far USA
Using that specification, yours would be O.K.
Last edited by Ozzie H.; May 10, 2018 at 03:03 AM. Reason: added information
Lead Driver
Joined: Jul 2017
Posts: 9,577
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From: San Jose, CA
Over the years I've seen very few listed values in the books. They all say the same thing yours did, by giving the acceptable range, but not the numbers.
And traveler, did you change the cam too by any chance? If so then any factory numbers will likely not matter. While that 100 does seem a bit low from most experiences, from what I've heard that is not out of line for some engines. Like said, it's the variation between cylinders that's more important in some ways. But yeah, I would think you'd want to know overall pressures too, in order to gauge health.
When you got those numbers though, did you happen to do the test with all 8 plugs removed, and the carburetor throttle held all the way open?
If not, that would easily explain the lower than expected numbers and I would think it would go up at least 20 to 30 points when re-done.
Good luck.
And traveler, did you change the cam too by any chance? If so then any factory numbers will likely not matter. While that 100 does seem a bit low from most experiences, from what I've heard that is not out of line for some engines. Like said, it's the variation between cylinders that's more important in some ways. But yeah, I would think you'd want to know overall pressures too, in order to gauge health.
When you got those numbers though, did you happen to do the test with all 8 plugs removed, and the carburetor throttle held all the way open?
If not, that would easily explain the lower than expected numbers and I would think it would go up at least 20 to 30 points when re-done.
Good luck.
Freshman User
Joined: Apr 2018
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I don't have the shop manual for 1974. The 1976 Ford Truck Shop Manual for the 360 & 390 engines under specifications for compression pressure does not give any PSI values. It says "When checking compression, take the highest compression reading and compare it to the lowest reading. The lowest reading must be within 75% of the highest".
Using that specification, yours would be O.K.
Using that specification, yours would be O.K.
Thank you Ozzie, I guess that explains why I couldn't find anything specific on the gorgler
To the OP,
I guess if it runs, smile and drive. I would personally expect readings higher that 100 after its broken in completely. I've typically seen that within 500-5000 miles on gas engines and my last diesel took well over 20k for the rings to seat .
If its running well, I'd personally make sure timing, valves, and carb tune were good and get through an oil change or 3
If its still 100psi you could do a leak down test to see if there's some issue with the heads or rings .
Then again 100 could be totally normal
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
'64 Shop Manual specs
COMPRESSION PRESSURE* - Sea Level @ Cranking Speed
144 Six ......................................150-190
Other Engines.............................130-170
*Allowable tolerance between cylinders +/- 20 p.s.i.
-----------------
Book sez fresh charged battery, compression test needs Plugs removed, throttle & choke blocked open, engine fully warmed up to normal operating temperature. The latter hardly anyone does. Who wants to pull plugs on a hot engine? I always figured if the test is OK when engine is cold, it would only improve once warmed up right? There will be a film of oil and the pistons will have expanded to fit the cylinder bores.
With all that said 100 pounds sounds pretty low for new rings. Does it burn oil or smoke? Maybe rings never seated and cylinders glazed over. Another (easier) test than compression is using a mechanic's vacuum gauge. It will tell you a whole bunch about internal engine health including compression, at least indirectly and doesn't torture the battery and starter.
COMPRESSION PRESSURE* - Sea Level @ Cranking Speed
144 Six ......................................150-190
Other Engines.............................130-170
*Allowable tolerance between cylinders +/- 20 p.s.i.
-----------------
Book sez fresh charged battery, compression test needs Plugs removed, throttle & choke blocked open, engine fully warmed up to normal operating temperature. The latter hardly anyone does. Who wants to pull plugs on a hot engine? I always figured if the test is OK when engine is cold, it would only improve once warmed up right? There will be a film of oil and the pistons will have expanded to fit the cylinder bores.
With all that said 100 pounds sounds pretty low for new rings. Does it burn oil or smoke? Maybe rings never seated and cylinders glazed over. Another (easier) test than compression is using a mechanic's vacuum gauge. It will tell you a whole bunch about internal engine health including compression, at least indirectly and doesn't torture the battery and starter.
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