Compression testing??? [Archive] - Chevelle Tech

: Compression testing???

Apr 24th, 00, 9:25 AM
The other day I got a compression gauge from Sears.I really don't know how to use it.I kow how to set it up(I think).Put the threaded snd in the spark plug socket right?Did that but how exactly do I get the reading.Do I actually start it and run it.Thats what I did.I ran it for like 5-10 secs.I can't just set there and crank it because it starts on the first try always.The readings I got are all under 100 psi.I know this isn't good but what is a good rating?Thanks for the help.

Apr 24th, 00, 9:43 AM
I'd pull the coil out of the ignition. That way it can't start. Unfortunately, compression testing leaves a lot to be desired. Factors like cranking speed and cam overlap make a BIG difference. As long as all the cylinders read relatively the same amount, you're in good shape.

Apr 24th, 00, 10:56 AM
Ditto above about the coil wire and crank the engine until you get the highest reading. Write it down, reset the button to zero and do the next and so on till your done.

Apr 24th, 00, 11:01 AM
Remove all the plugs, prop the carb wide open, disconnect the ign (if pulling the coil wire out of the cap ground it) and make sure the battery has a full charge. Let it turn till you hear the compression hit the cyl with the tester in it about 4 times and do it the same for all 8 cylinders.

Philip Valentine
Gold Member #42
Sometimes I think I understand everything, then I regain consciousness
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[This message has been edited by 64elcamino (edited 04-24-2000).]

Apr 24th, 00, 11:48 AM
64elcamino is right, only way to do it is as he has written, PERIOD.

Apr 24th, 00, 3:39 PM
I did it like 64 said.These are the readings I got.
cylinder 1:86
Is this good or bad,I think its bad.It's a 307 with about 79,000 miles.It was rebuilt once before I got it for some odd reason.

Apr 24th, 00, 8:27 PM
It is kind of weak even for a 307. My 400 sb tested about the same and I found it to be in need of an overhaul when i disassembled it. Here is one more thing to check. And it will be easier if the plugs are still out. Pull off the distributer cap.Place a socket on the crankshaft bolt and turn the crank until the rotor turns. Mark the damper at the end of the timing tab in the direction you were turning. Now turn the crank the opposite direction until the rotor just starts to move again. If your mark moved more than a 1/4" you have enough play in the timing chain to lose power and cranking compression.

As far as being rebuilt, it is a phrase that everyone interprets differently. For me it is a fresh engine, all castings machined, all moving parts replaced with new. To some it means rings, rod bearings and a lap of the valves. Unless you see some proof you have no idea what has been done.

Apr 24th, 00, 9:02 PM
It could have the cam retarded a tooth as well. This would give the oow readings it now has. If this isn't the case, the engine is in need of a serious fixin'. I take it the engine runs kinda poopy. soggy on the low and mid range, kinda decent on the top end, thjis would indicate late valve timing, like cam in one tooth back. Does it backfire at idle?

Apr 24th, 00, 9:12 PM
Gotta go with Phil on this one. I once had a relative who considered a "rebuild" slapping in a set of bearings, and then adding a set of rings without even checking the cylinder bore.

Once you have made up your mind, facts are but a mere annoyance.

Apr 25th, 00, 2:07 PM
Like Ignition man said it runs pretty soggy but I might just get a new engine anyways.
Theres a place here that is selling 350 short blocks for $350.SOunds like a good deal to me.Then I could put in a mild cam,camel hump heads,my performer manifold,holley and headers.Then I'd be set for the rest of high school.