compression numbers ?
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Elder User
Joined: Feb 2004
Posts: 830
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From: Indiana
compression numbers ?
What is the typical psi of a compression test on a rebuilt motor. I picked up a old beat up truck for the motor to replace mine. Got the motor switched out and it runs as good as rebuilt motor. Did a compression test today to check out the motor and got 150 to 160 psi on all cylinders. On the first revolution the needle goes between 100 and 120. I'm thinking this motor is going to last me a while. Have a couple of small vacuum leakes to track down and a little carb work but that won't take long. 
Logistics Pro
Joined: Jan 2002
Posts: 4,907
Likes: 39
From: suburban atlanta
The psi will depend on a few things: 1)compression ratio, 2)piston ring seal, and 3)valve seal. Generally, mechs look for even compression, with not more than about 10% difference between cylinders. If you are in the 150-160 range, you should be fine and are in the ballpark with average compression ratios.
tom
tom
Senior User
Joined: Dec 2005
Posts: 261
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From: Columbus, Indiana
My old engine was at 170PSI, so you are still in real good shape.
I do wonder why they just go by the 10% rule. As the engine wears, if it wears evenly, the compression will drop evenly in all cylinders. An engine that puts out 60PSI is probably not that mechanically sound even if they are all within 10%.
However, I do understand that it isn't a very scientific method of determining wear and has a lot of factors involved, including a portion of human error, so that is probably why it isn't used.
I do wonder why they just go by the 10% rule. As the engine wears, if it wears evenly, the compression will drop evenly in all cylinders. An engine that puts out 60PSI is probably not that mechanically sound even if they are all within 10%.
However, I do understand that it isn't a very scientific method of determining wear and has a lot of factors involved, including a portion of human error, so that is probably why it isn't used.
Fleet Owner
Joined: Sep 2005
Posts: 28,609
Likes: 18
From: Lake Tahoe, Nevada
Originally Posted by 79F150EFI
I do wonder why they just go by the 10% rule. As the engine wears, if it wears evenly, the compression will drop evenly in all cylinders. An engine that puts out 60PSI is probably not that mechanically sound even if they are all within 10%. .
On the other hand, ten percent is generous for a variation. But mechanics will pay more attention to variation than the actual number because the pressure will vary due to differences in compression ratio or cam grind from one year to the next, even in the same engine type. Also, owners may have made changes that will affect the numbers as well. (Not to mention variations in compression testers.)
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Logistics Pro
Joined: Jan 2002
Posts: 4,907
Likes: 39
From: suburban atlanta
Not to mention altitude and starter motor and battery and .... variations, which would make absolute numbers totally untranslatable to the real world.
At 60psi, you would get more power out of the starter motor. Maybe. But if you got it going, it would keep spinning when you shut off the ignition.
tom
At 60psi, you would get more power out of the starter motor. Maybe. But if you got it going, it would keep spinning when you shut off the ignition.
tom
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