Compression Check Interpretation

[astark]

A friend of mine did a compression check on my 302 in the Bronco and it had 120 lbs in one cylinder, 110 in another and the rest were 115.

What does that mean and why are they different %)

This motor was rebuilt ten or more years ago but I don't know what was done back then, and it does run pretty darn good. Has good oil pressure and no banging, clacking or ticking.

 

[bax]

Lots of reasons for the variation in #'s The good thing is they are all close. 110 lbs is not a lot of compression but it will work just fine. If the compression test was done on a cold motor all it would take to have the variation in compression is a few drops of oil from a valve stem or something like that. If the motor was hot the extra oil would of been burned off and
you would have to suspect the condition of the cylinder wall and the valve seats. Still nothing bad about your #'s Carbon build up (10 year old rebuild) could cause the difference. I think your good, Let her eat.

 

[Master Chief]

A 5 lb difference between cylinders is nothing to be worried about, your cylinders are all wearing the same.

Was your engine warmed up prior to your friend doing the compression check? That will make a difference to the lower reading since metals in the engine expand and contract differently with temperature. Ideally you want to see numbers around 150 - 180 psi. Since yours are all evenly lower your rings are wearing the way they should.

A few squirts of oil in the cylinder and a recheck of the compression will verify the ring wear, if the # goes up it's rings, if the # remains the same it's generally valves. Big number differences between side-by-side cylinders can be attributed to head gaskets.

As long as you have good oil pressure, strong power, no noise and are not leaving a smoke screen behind you I'd continue driving your rig. You may want to consider freshening up the engine in the future with new rings/bearings and a quality valve job.

Senior Chief

 

[Master Chief]

Bax beat me to it, we must have typing at the same time!

 

[Broncobowsher]

I will confirm it as well, all is good.
It is nothing more then standard variation in production parts. These are not ultra precise machines. If they were you couldn't afford to buy one. Mass produced engines with slight variations in head chamber volume, ring sealing, rocker arm ratio, cam lobe profile, etc. It all adds up to give a little variaton.

When one of those numbers starts dropping to 80~90 then you have a problem. Until then, keep driving it.

 

[73stallion]

as a general rule the numbers should be within 10% of each other.