Stovebolt.com Forums The Shop Area The Engine Shop New Compression Numbers

The 216 in my 51 Canadian GMC has served me well for over 40K miles. Initially I replaced the rings and pistons and installed a completely rebuilt head. The cylinders have significant wear at the top and one cylinder has been honed out some by a previous owner. That cylinder yields quite a bit of piston slap noise. The motor runs well and does not use a lot of oil. However I am planning to eventually do a rebuild and have the block bored. Several times in previous years I have done compression checks. The numbers ranged from 80 to 90 lbs. This year I converted to a 12 volt system keeping the 6 volt starter. So it turns over much quicker. I did a compression check the other day and all the numbers are between 110 and 120 lbs. The only conclusion I can come to is that the compression numbers can be greatly affected by the speed that the starter turns the engine over.

Correct, also ambient air temperature and oil viscosity.

One of the principles of physics is that certain loads increase in a geometric fashion, not linear, as a function of speed. For instance (being VERY inexact) if the load on a surface is 10 lbs. when it is hit with a hammer at 10 ft/sec, that same surface will see 100 lbs of force when the hammer speed is doubled- - - - -the load squares! Now apply that to the pressure compression exerts on a piston ring- - - - -it pushes the ring out against the cylinder wall, and down against the top of the ring land below it to help the ring achieve a seal. As the speed of the piston increases, the force applied to the ring also increases, and the ring gets a better seal. Today's "low tension" rings rely almost entirely on cylinder pressure to get a seal- - - - -there's very little spring tension against the cylinder wall involved.
Jerry

I have read over and over that the differences between the cylinder readings is really the important part of the test. I guess this sort of points that out!

Jerry, GREAT description Sir!

Okay Jerry, I’ll really show my ignorance and ask a question...... with the key off do you crank the motor over just a couple of seconds and then read the pressure gauge or do you spin it over longer? Maybe you’re supposed to read the gauge with the engine running??? Is this a one or a two person job?

When I check compression, I count the revolutions, you will hear it hit the gauge each time it comes up on compression, I usually go five hits per reading.

Joe

Ditto- - - -I usually count 4 or 5 compression strokes at cranking speed with all the spark plugs removed and the throttle blocked wide open. I look for cylinder pressures within 10% of each other, not any specific pressure reading. A 5% tolerance is better.
Jerry

“When I check compression, I count the revolutions, you will hear it hit the gauge each time it comes up on compression, I usually go five hits per reading.”

When I checked my compression last I didn’t hear anything “hit” the gauge. I was in the cab stomping on the starter button. Guess I was supposed to be under the hood and manipulating the starter linkage?? I’ll try that method this week.

Jerry, I didn’t know I was supposed to take out all the spark plugs. I only took out the one and replaced it with the screw in hose to the pressure gauge. I’ll remove the other five next time.

Once you get all the plugs out, you will hear the one cylinder hit. Be sure to wire the throttle wide open and have the choke wide open. Pulling the air breather also helps. You want the engine to pull air as easily as it can.