Valid Compression Test

[jkenan]

Could any compression pressure leak out of any of the holes resulting from the removal of peripheral A4 parts? I have removed several items from the engine: Delco Starter, Carburator, Water Jacket Side Plate, Cover to Reversing Gear, Exhaust Pipe (had to in order to remove the engine), Flywheel Cover. I have not removed nor adjusted any of the head or manifold nuts, which are still intact as they have been for the last 20 years.

I am concerned, because I bought a compression guage and after checking compression in all four cylinders, the readings came back very low, between 30 and 60 psi. I am concerned about the validity of the readings, because before I hauled my engine, it had power to run at 2000 RPM, pushing the boat at 5.5 knots (but a few other concerns promped me to pull the engine - compression was NOT one of them - see my other thread "Engine Rattling and Low RPM").

All four plugs are removed, and rotating the crankshaft around with the compression guage firmly screwed into any spark plug hole produces very little compression on the guage. I can also put my thumb over the open hole, and can keep it there on #1, #2. On the other two, a little air leaks past my thumb, and those are ones reading about 60psi.

Thoughts?

John

[Don Moyer]

John,

Not to worry! Compression is always much lower when hand cranking versus turning the engine over on the starter. Starter speed is approximately 350 to 400 RPM, which makes your engine a much stronger "air compressor" than you'll be able to duplicate by hand cranking.

In fact, a good Atomic 4 trivial pursuit question could be: "If you were performing compression checks on an early and late model engine in similar condition, which one would you expect to have slightly higher compression, and why?" A: The early model would have slightly higher compression, and the reason is because the gears on Prestolite starters are a bit larger than the starter gears on Delco starters, and the engine is therefore turning a bit faster during the compression check.

Don

[jkenan]

Interesting.... Thanks for the explanation Don.

My plan now is to reattach the starter and hook it up to a cranking battery, then turn the engine over using the starter. My question is what should RPMs be under just the starter alone versus when the engine is actually running (at 800 RPM or so).

Thanks.

John

[Don Moyer]

John,

Starter cranking RPM is usually between 350 and 400.

Don

[jkenan]

Sorry, I meant to ask, what should the Compression PSI be while starting versus running, not RPM.

Thanks.

John

[Don Moyer]

John,

We consider 85 to 115 psi to be the normal compression range. Compression much below 85 psi will usually result power issues and/or excessive blow-by. Compression much over 115 psi is needlessly high and usually results from an artificial elevating of the combustion pressure by a lot of carbon in the combustion chambers or smaller than normal combustion chambers within the head (repeated milling, etc).

Don