The Stovebolt Site Forums The Shop Area Engine Shop Compresssion...???...not like what you guys are getting..

Measured the compression on my 292 this morning after I got her runnin' again.

I ended up with 80,75,75,75,75,80.

But I see people getting over a hundred here on the board, so why are mine so low? Neighbor says it's fine and good that they are all close.

Am I ok here?

The shop manual reads 130 pounds per cylinder. That puppy is tired.

Larry

sablesurfer,

There is a pretty specific procedure to follow when you do the compression check, and it can make a difference in the figures. Do you have that recipe, and did you follow it?

Stuart

Only trust a screw in compression check guage. The ones with the rubber push in fitting generally leak and give bad readings.

I would try a different gauge. It,s rare that they would be that close to each other & be that low. I know your engine doesn,t have a lot of blowby. Did you have the throttle open at least partway when you ran it?

OK, I guess I wasn't all that scientific about it. I used a screw in for the first one...but it got stuck, wouldn't unscrew as the tube just spun in the screw end.

After getting that out we used a push in rubber ended one...so the numbers are 'close' since it wouldn't set a pin like the screw in one.

Also, I was turning the engine over without the ignigion coil hooked up. I though this was done just by cranking the engine using the starter...is that not correct?

sablesurfer,

It sounds like you did things properly. Your readings were all low, but consistently low so that means you probably didn't flub a cylinder while measuring.

Although I am not sure this will make a difference, you could try the test again making sure the choke plate was wide open, and the throttle was blocked wide open.

My compression gauge is the screw in type too. It has a quick connect between the hose and the plug adapter...it does spin.

Stuart

I would warm up the engine and then take a compression reading then squirt some oil in the cylinder and see if it comes up. If so, time for an overhaul. If a little or not at all may be a bad gauge though they are quite close. Should be 130 or there abouts. 110 would still be good but much lower and.......its TIRED or time to RE-tire it to the machine shop. Put in 307 pistons either the 8.5 or 9:1 ratio. The 8.5 would run real well especially with this watered down gas. They were originally 8.0:1. Would bring the compresiion to about 160 on a fresh rebuild including a valave job. Would gain about 25hp over stock. Let us know what you found out please. H.

First, check the gauge! Use a known source of air pressure, like your air compressor, and hook it up directly to the gauge. I've seen compression readings vary as much as 25 pounds by using different testers, just due to gauge calibration. Regardless of the pressure reading, compression pressures should be within 10% or so from cylinder to cylinder.
Jerry

Just one thing to add, which you prolly already did, is to remove all of the spark plugs so the engine will spin faster.

IMHO screw in compression checkers are relatively cheap. Buy another and check it again using the new one.

If by chance the cam has been changed, the readings can be lower then a stock engine due to valve overlap. Why are you checking the compression? Is the engine worn out and tired? Are you just wondering what it is? With them all them the same, thats a good sign, at least they are all worn the same.
Recheck with a different gauge, engine warm, no spark plugs, good battery, and spin each cylinder atleast four compression strokes. If they are all still the same, you did it right, if they are higher, even better! Joe

Hey Joe...I ran the test just 'cause I wanted to know. I figure now is the time to look, since I was test driving after reinstalling the exhaust.

The only thing I didn't do, according to the posts here, is pull all the plugs. I just pulled one at a time.

I guess I have no idea how the choke and throttle matter since the engine isn't getting any fire since I pulled the coil wire to the cap....???

The engine doesn't seem to burn any oil. The exhaust is real clean, so I can't see how the rings or pistons would be bad...??? (Or does a worn out piston not show oil burning?) I'll know more about oil burning after I run her a couple more miles.

I'll pick up my own compression tester this week and do it again. It may be a few days before I have an update.

Compression tests are performed with the throttle and choke wide open. This is so the cylinder can pull in a full gulp of air. (scientific term) With the throttle closed, airflow into the cylinder is reduced, thus decreasing the amount of air being compressed and causing low readings. RPM is also critical, like others have said pull all the plugs, and make sure the battery and starter are in good shape. Perform the test warm for the most accurate results. If the numbers are still low, squirt a little oil in each cylinder and repeat the test. If the numbers improve significantly the compression rings are worn. If the numbers remain low your leak is elswhere, most likely an intake or exhaust valve. I doubt you have any problems, just re-run the test with the throttle wide open and I bet your numbers fall right in line.

John

John

John1 beat me to it. :hammering:

In the back of my '54 Motors Manual is a procedure for compression tests. I hadn't paid much attention to it until now. A interesting point...if all the plugs are removed for this test, it isn't necessary to hold the throttle plate open because there are always two or more intake valves open which provides a path to the intake manifold proper...who would have 'thunk it'.

I was always under the impression that the process of adding oil to the cylinder during this test was a way to differentiate between poor rings or valve seating problems. If the compression figures came up with the introduction of the oil, rings would be suspect. If the oil made no difference, then the valves might be the cause.

Stuart

all plugs out,,coil off,,throttle opened only once(use something to hold open,,,check each cyl 4 comp hits,,,batt should be at full charge when this is done...doc

I bet the compression test wasn't properly done or a faulty guage was used. I don't think an engine would run on 75 psi, or at least it would run very poorly. Remove all of the plugs and use a known good screw in tester. Crank the engine until compression stops building. The majority of the reading should be reached within the first two revolutions.
Good luck,
Scott

Its more important that the readings be all within 15% of each other. It sounds like your engine is just fine, if the compression was really that bad it wouldn't start easily.

dry test first:
first pull all plugs out
comp ea cylinder and write them down.
crank about 30 sec for ea cyl.


wet test second:
(this tests your cyl. by sealing the rings a little better w/ oil)
squirt wd or light oil in ea as you do that cylinder
write ea cyl down
(if your comp. improves after the wet test your rings are poor, other wise it may be that your valves &/or seals are poor.) so you only have to do a top end job.

One thing to look out for: if the valve core has been replaced in your gauge, it might not be the right one. If you replace the valve core and use a tire valve core your readings will be way low. I really don't know what the difference is but they are NOT the same. I have run into this several times in the past. Joe