Checking compression by hand?

[slantin_77]

I have an engine from a 1969 Valiant with 80k miles on it and am wanting to turbocharge it and drop it in my '77 Dodge D100 pickup. I bought the engine and put it on an engine stand and don't know anything about it other than he said it ran like a top before he took it out to replace it with a 318 and that it only had 80k miles on it. I want to make sure it's stout enough to make the power the turbo will add, but I don't know if I want to do a full rebuild or just do the bottom end and maybe re-ring the pistons. I would like to do a compression check, but I do not have a starter so I'd have to do a compression check by hand, and that kinda scares me because the engine is so heavy on the engine stand that I would not want to have it tumbling down on top of me while turning the crank with a ratchet. Has anyone done this?

 

[nm9stheham]

Mmmmm, I am having a hard time seeing how this will work well at all. Cylinders have a leak down rate, where the pressure built-up leaks down over a few seconds due to the gases escaping past rings etc. If you don't turn the crank constantly like the starter would, then you can't get consistent or valid compression readings. So, I don't think you are going to be even close to getting good readings this way.

If you can rig it up with compressed air and hold the crank rock steady, you could do a leak down test, which is a very good indicator of cylinder health, and is done with the crank static, not turning. Can't tell you what to expect for numbers though, maybe someone here can. If you do this, hold on to the bar holding the crank still for dear life; there can be a lot of rotating force on the crank if the piston moves a bit off dead center with the compressed air on, and it can rotate the holding bar with speed and force and hurt you pretty badly.

Personally, I would not bother with a turbo on any used engine without a solid rebuild. Any oil leakage will quickly show up as damaging detonation with turbo pressures. 'Running like a top' normally aspirated don't mean it is good for turbo'ing....unless you just want to try.

 

[67Dart273]

The above pretty much nailed it. You have two choices........either rig it so you can run a compression check, or do a leak down. You can buy a leak down tester at Horrid for only 40 bux. Either should really be run on a warm engine.

And, really, you should rebuild the engine if you are going to stress it with a turbo, if you don't know the condition.