Compression Test With Motor Out?

[kimokalihi]

Can I do a compression test on a motor that's on an engine stand? Do I just turn it over by hand with a breaker bar or could I use an impact gun?

[Buffman]

I'd be more concerned about snapping that bolt off. If the flywheel is on the motor, why not let the starter do the work for you. Unless starter bolts to trans then that wouldn't work. I'd more or less use a breaker bar.

[kimokalihi]

I really doubt that bolt would snap. It's quite large. I would use the starter but I don't have a remote start and it bolts up through the tranny so that wouldn't work.

[PhyrraM]

If you have or can borrow a compressor, just do a leakdown test instead.

I wouldn't trust turning it by hand or an impact wrench. Consistent RPM is a bigger part of a compression test than most realize.

[Buffman]

I really doubt that bolt would snap. It's quite large. I would use the starter but I don't have a remote start and it bolts up through the tranny so that wouldn't work.

I've seen quite a few people snap the hub retaining bolt on SBCs. That's a 7/16"-20 bolt, so it's no small fry either..

[kimokalihi]

If you have or can borrow a compressor, just do a leakdown test instead.

I wouldn't trust turning it by hand or an impact wrench. Consistent RPM is a bigger part of a compression test than most realize.

Don't you need a leak down test gauge and stuff for that? I have a compressor.

[PhyrraM]

Not too expensive.

http://www.harborfreight.com/cpi/ctaf/d ... mber=94190

[glennda5id]

I have that leak down tester. I question how usable it is. Also note that there is an error in the instructions. I think steps 4 and 5 need to be switched. You need to calibrate it before you hook it up to the hose going into the engine.

[jp233]

i think it could easily be done on a stand, with an air impact gun. Just start out low on the supply pressure to the gun, as in normally I run 90 psi on mine, maybe start at 50 and work up to maybe 80, taking compression readings along the way. I'd do this on the lowest setting on the gun, if it has different torque settings. build a small matrix of compression numbers for each cylinder, and you'd get a good number if you average them (and watch for trends etc).

that being said, I haven't done this on a Subaru engine, even though I have one on the stand right now. the heads are gone and I'm re-ringing/honing it anyways, so I'm not that interested in what the compression was before.