The Stovebolt Site Forums The Shop Area Engine Shop Compression Test Question?

I want to do a compression check of a 292 that I’ll have at the Stovebolt Reunion this weekend.
My brother in law who has been a foreign car mechanic his whole life says, the the old timers made sure the throttle was wide open during the test.
My question is, does it make a difference if the throttle is open or not?
I’ll have plenty of help this weekend and we will see if it does make a difference.

Don

Yes

So, should the test be done open or closed throttle?
I will be doing this as a how to seminar with some other Stovebolter’s and would like to explain why it may make a difference.

Don

Open throttle

I believe with a closed throttle there won't be enough air drawn in for the compression stroke to compress, thus, recording a very low number or possibly no number for the amount of psi in the cylinder being tested.

Air restriction such as closed throttle or restricted exhaust (i.e spud pounded into tailpipe) will lower compression readings.

Not only open throttle, but number of rotations of each cylinder. Count as the engine rotates, I usually go six to seven compression strokes per cylinder. Be sure battery is up to the task, slow cranking can also lead to low compression test results. Be sure the air cleaner is off.

Compression pressure readings are not particularly useful unless the same exact procedure is followed for each cylinder. Throttle open or closed, all spark plugs removed or only one, number of compression strokes measured, and a lot of other details need to be considered. Bottom line- - - -pick a routine and stick with it, no matter what it happens to be. You're looking for a percentage variation between cylinders, as whatever the PSI reading happens to be is totally irrelevant. Use 5 different compression gauges on the same cylinder, and you'll probably get 5 different pressure readings. Also, use a known air pressure such as the tank pressure of an air compressor to test the gauge before you start the compression test.

A cylinder leakdown test is a far more useful test than a compression test. Then there's the most effective test of compression-related performance, a "Power Balance Test" where the engine is run at a constant throttle setting on only two cylinders, and RPM and intake manifold vacuum is recorded and compared. Compression testing is better than no testing at all, but not much better.
Jerry

Thanks Jerry, that is the info I was looking for. Consistent compression on all cylinders is what I will be looking for.
If Mongo has the equipment and someone knows how to set up and use it, we will do a leak by test also.
On air operated valves (AOVs) in a power plant, it is called a drop test.

Don

All a leak down test needs is a source of air pressure and an adapter to put the pressure into the spark plug hole. I use an air hose quick coupler threaded or brazed into a spark plug shell. If the air source has a pressure regulator, set the "deadhead" pressure with no air flowing to 100 PSI. When air is introduced into the cylinder, any pressure drop represents a percentage of leakage- - - -90 PSI = a 10% leak rate. Either lock the crankshaft with the piston at the top of the compression stroke with both valves closed, or disconnect the pushrods and let the air pressure push the piston to the bottom. A "Top of the cylinder" test is better- - - -that's where most of the tapered cylinder wear is located.

Listen at the carburetor, tailpipe, and crankcase filler for air leaks, which will pinpoint the leak to intake or exhaust valves or worn piston rings. That's something a compression test can't do. Bubbles in the radiator indicate a blown head gasket and/or a cracked head or block.
Jerry