compression questions
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Freshman User
Joined: Mar 2007
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compression questions
Hey,
maybe this is a stupid question but I need to ask anyway. I am getting ready to put my 352 in my truck and I thought since ido not know anything about the motor that I would do a compression test on it before I put it in (thinking that I could). How am I doing that U say? well maybe I am an idiot (probably) but I thought I could just hand crank it. Well I am getting around 20 psi on some and nothing on others. Can I not test the compression this way?
bubba
maybe this is a stupid question but I need to ask anyway. I am getting ready to put my 352 in my truck and I thought since ido not know anything about the motor that I would do a compression test on it before I put it in (thinking that I could). How am I doing that U say? well maybe I am an idiot (probably) but I thought I could just hand crank it. Well I am getting around 20 psi on some and nothing on others. Can I not test the compression this way?
bubba
Cargo Master
Joined: Apr 2004
Posts: 2,574
Likes: 4
From: Golden, CO
Not stupid really, it has come up before. That time someone wanted to use an impact on the crank snout.
The consensus was that you should use the starter to turn the engine. If I remember it right, the starter spins at 300 RPM. I've heard that number for different brands as well. Never thought about it this way, because I never questioned it; but when you see the specs for a compression test, isn't it based on a standard procedure?
Enough rambling! If you can bolt a bell housing to the block, you can spin the engine out of the truck.
Be safe! Mike
The consensus was that you should use the starter to turn the engine. If I remember it right, the starter spins at 300 RPM. I've heard that number for different brands as well. Never thought about it this way, because I never questioned it; but when you see the specs for a compression test, isn't it based on a standard procedure?
Enough rambling! If you can bolt a bell housing to the block, you can spin the engine out of the truck.
Be safe! Mike
Tuned
Joined: Feb 2003
Posts: 355
Likes: 0
From: Oregon
Not stupid really, it has come up before. That time someone wanted to use an impact on the crank snout.
I know somebody that did that on a 454 Chevy. Snapped the snout right off.
I was highly amused. He was........not so amused. Typical crappy cast GM crank. Back to what you were asking, the compression will bleed off unless you can spin the crank at starting speed, like Hypoid said.
Postmaster
Joined: Jul 2003
Posts: 2,711
Likes: 2
From: Pittsburgh
The test you can do with a motor on the ground is a leak down test if you have a leak down tester and an air compressor.
You pressurize one cylinder at a time with the compressor, then the leak down tester tells you how much is bleeding off. If it's major, you listen at the intake, exhaust, oil fill, etc until you hear the hiss of escaping air. Then you know where the issue is an how major it is or isn't.
It's actually a better test than a compression test, but takes more tools.
Justin
You pressurize one cylinder at a time with the compressor, then the leak down tester tells you how much is bleeding off. If it's major, you listen at the intake, exhaust, oil fill, etc until you hear the hiss of escaping air. Then you know where the issue is an how major it is or isn't.
It's actually a better test than a compression test, but takes more tools.
Justin
Senior User
Joined: Jan 2001
Posts: 193
Likes: 0
From: Ohio
I did something like that but I had all the plugs out except the one I was checking. Used a ratchet and spun by hand on the crack on a 351M. Seens to me I was getting around 70 psi. I ended up rebuilding it before I put it in. Upon tear down it wasn't in bad shape it probably would have been ok.
