Hi,
Looks like the engine on my UTCC entry is toast. I have a spare engine sitting in the shop. How can I do a compression test on a "loose" engine without a starter or anything? Can I just crank it by hand?
I know this is a dumb question ... but I'm learning.
Not really. You can do a leakdown test however, but that probes some slightly different things.
You kinda need at least a starter and something to keep the engine stationary but that's about it. I've been involved in a compression test on an engine only very loosely installed in a car. The most fun (sketchy) part was rigging it up to temporarily run to burn out the coating of storage oil in the cylinders. I'm not an expert but I'm pretty sure it needs some amount of speed to produce realistic results so cranking by hand wouldn't work beyond telling you whether it has compression at all.
A leak down test is probably your best tool at the moment unless you can jury rig a starter motor.
On an only slightly related topic, I once bought an 460 Ford motor from an interesting character who fired it up sitting on the floor using a gas line stuck in a bread pan full of gas. He cranked the starter by touch the jumper cables to the lug on the starter. Between the sparks from jumping the starter, the flames from the open headers and the cigarettes he chain smoked I'm still surprised we all didn't go up in a ball of fire.
Yeah I had a similar experience, except the engine was very loosely installed in the car. I know I said "cylinders" in my last post, but it was a 12A rotary. No intake or exhaust, coils rigged to power one way or another, jumper cables to the starter, a squirt bottle with gas in it spraying directly into the intake ports. Extra exciting since the intake and exhaust ports are about an inch away from each other on the same side of the engine. We did have somebody standing around with a fire extinguisher, at least.
In reply to LanEvo :
You can do it on a stationRy engine but you need a leak down tester and a way to move the crank. (Socket and breaker bar or a pry bar to lever the teeth on the flywheel)
A leak down tester not only measures "compression" but tells you where you are leaking from. Valves? Intake or exhaust? Rings? a leak down test is done to a stationary ( non cranking ) engine. You put each cylinder on top dead center with both intake and exhaust valves fully closed. Get your % of leakage. Then listen to where the air is escaping from. If you hear it in the crankcase the cylinders are leaking. If you hear it at the exhaust pipe the exhaust valve is leaking( burned) in you hear it at the intake the intake valve is leaking. Then squirt some oil in the spark plug hole and crank it over again until you are back to top dead center. You will get better reading wet than dry but not as good as if the engine had recently been run. That doesn't matter, what you are looking for is consistency if all the cylinders read roughly the same leakage you have a decent engine.
Realize that both a compression test and a leak down test to be absolutely accurate need the engine warm and recently run.
But you can get a decent idea if all the cylinders are near the same and you do both a dry test and a "wet" test ( squirt some oil in the cylinders for a wet test)
Compression, or even a leak down test on an engine that hasn't run for a few months is going to give you pretty inaccurate results. It needs to run for a minute or so to get rid of little chunks of carbon that have flaked off things, make the valves that were held open smack shut a few times to seal, work the rings a bit..
I'd be more likely to just look at the spark plugs and make sure none are covered in an inch of carbon, and none of them are smashed flat.
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