Compression test reading

fastsc92

Registered User
What an average number for compression in this motor? I got some results, but not sure whats too high and whats too low. All the numbers were consistant at 5psi difference in the worse situation.
 
Not sure what they are on the lower compression engines either. The should all be consistant though between 15-20%. If I remember correctly on the norm they should be 175 or greater depending if the plugs are in or out. I would imagine ours are closer to 140-150 with the plugs in.

Chris
 
Normal for a high mileage motor is anywhere from 135-150psi. New is about 165. 5psi is a good variance. Ford specifies 75% variation but I'd call it bad long before that.

Mine makes about 140 +- 5psi.
 
Well, I did the test while the car was cold, and all the remaining spark plugs stayed in. I just did one cylinder at a time. The highest reading was about 122, lowest was 116ish. I figured they were a little low, but that may be because the car was cold. It'll have to last until the fall when the motor gets yanked for something good ;)
 
Uhhh...I think 75% is the range not the variation. If you had a 75% variation then you could have one that is 150 or so and then one that is 112. Does that look correct?

Chris
 
Range/variation, call it what you want. An example of Ford's acceptable range would be 113-150psi on the same motor. Obviously that's not very helpful as I'd be pulling a motor apart long before it was that bad.

112-120 is poor. Make sure your gauge is accurate and the motor is at least somewhat warm. Also make sure that you have the throttle open, the injectors disabled and a strong battery. With only 5psi variation it doesn't seem right that they would all be that low unless the rings are just that badly worn out. Try squirting some oil into a cylinder and see if the compression jumps up a whole bunch. Since you know that all the cylinders are even, just test one out and see what happens.
 
car was stone cold....and all the other plugs were in.
i'll do it again tonight and see what i get. I doubt they are all that low...could be, but they were all pretty even. The IC tubes were off, so throttle wasn't a concern.

Tonight it'll be up to operating temp and all the plugs will be off of it, as well as the battery on a starter charger.
 
OK did the test again, took all the plugs out this time around, and the engine was also quite warm. Results are as follows.

1-140
2-137
3-140
4-137
5-137
6-140

Looks much better...enough for me to last the season at least.......
 
Last edited:
XR7 Dave said:
Range/variation, call it what you want. An example of Ford's acceptable range would be 113-150psi on the same motor. Obviously that's not very helpful as I'd be pulling a motor apart long before it was that bad.

112-120 is poor. Make sure your gauge is accurate and the motor is at least somewhat warm. Also make sure that you have the throttle open, the injectors disabled and a strong battery. With only 5psi variation it doesn't seem right that they would all be that low unless the rings are just that badly worn out. Try squirting some oil into a cylinder and see if the compression jumps up a whole bunch. Since you know that all the cylinders are even, just test one out and see what happens.
I think maybe that the instructor said it should be closer but for some reason that looks like something Ford would do. I do agree though with the pressures being that low.

Chris
 
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