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

[Concord]

Compression Test

Our engine is winterized and a surveyor would like to do a compression test of the system and is proposing to run the engine for approximately 60 seconds to heat the motor up for the test with no cooling. He proposed to remove the belt so the pump would not spin but that is not an option.

My partner and I would be more comfortable pushing more antifreeze through the block if they need to run but I am not sure we even need to do that. I would think the compression cold would be lower then after it warmed up so if the compression is good cold warming then engine would only improve it.

I would appreciate any input about running a compression test on a cold engine vs warming it up by running it for ~ 1minute.

Thanks

[ndutton]

Your surveyor is entirely unfamiliar with Atomic 4 engines. How can I know? The belt drives only the alternator. Removing it WILL NOT defeat pump function.

Assuming the survey is necessary (otherwise why would you be doing it?) I'd go ahead and appease him and re-winterize after he leaves. He'll likely damage the impeller in the process. This may qualify as destructive testing so if he insists on proceeding maybe you can get him to assume liability in writing for any resultant damage. That ought to back him off.

First sentence, I was being polite.

[Concord]

Thanks for the input, it is kind of ironic because in 20 years this is the first surveyor I found that actually wants to do a compression test.

We are selling the boat and I want the new owner to be comfortable that the engine is good although he sailed with us when we moved the boat in October and he had a great sea trail. (25 knot winds, current against wind direction, chop.....)

My partner did a compression test a few years ago and compression was well within spec so I am confident the engine is good I just do not want damage anything running in an ill advised configuration. Our tentative plan is to run 2 gallons of antifreeze through the block then we would fog the engine to stop, do the compression test then fog the cylinders.

I am still curious on your thoughts of doing a compression test cold vs warm.

http://philadelphia.craigslist.org/boa/4210194610.html

[ndutton]

Checking the compression warm should return better results than cold and I can certainly understand a surveyor unwilling to accept someone else's numbers*. Maybe you could have a discussion with the potential buyers explaining that since they've already seen satisfactory engine operation such a test will be of little or no value, will certainly compromise the previous winterization and very likely damage the water pump impeller (replaceable at a cost) if performed the way the surveyor wants to.

Under no circumstances should you accept liability, big or small, for damaged components (impeller particularly but possible freeze damage as well) due to their (the buyer's) survey. They sound like reasonable folks who are genuinely interested in buying the boat. I doubt they'll want to risk damage either. Approach it with the best interests of the boat in mind. It's yours now but will be theirs soon.

Please keep us updated, this is really interesting.

*edit:
Perhaps the buyers will accept your previous compression numbers eliminating the need to repeat the test for the reasons noted.

[hanleyclifford]

A surveyor who wants to run an engine without a cooling system and especially without a coolant pump is a fool and should not be trusted by anyone.

[romantic comedy]

I had always heard that a compression test should be done with a warm engine, full throttle, no choke, fully charged battery... for best accurate results.