Compression check................
Thread Starter
Drifting
Joined: Jul 2008
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From: Sonora CA
Compression check................
I have the exhaust manifolds off my 73, 350 automatic, so now would be a good time to check compression. Engine has a little over 100K on it original, never apart and will be cold. I have done compression checks before, but I was just wondering if there were any tips, tricks and or advice?
Team Owner
Joined: Aug 2006
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From: Columbia Missouri
Unplug the power wire to the distributor.
Leave all plugs in while doing them one at a time.
Make sure you let the engine crank at least five times per reading.
Write down the findings.
If you get any lower than average readings, squirt oil in the hole and re-do the test on that cylinder and compare readings.
Leave all plugs in while doing them one at a time.
Make sure you let the engine crank at least five times per reading.
Write down the findings.
If you get any lower than average readings, squirt oil in the hole and re-do the test on that cylinder and compare readings.
Team Owner
Joined: Aug 2006
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From: Columbia Missouri
Why exactly? That's how I was taught, and I was told that you wanted the engine conditions to be almost the same as when it's cranking over to start. That would have the plugs installed and the carb as it would be at idle with a cold choke.
Why would you want the plugs removed and the throttle wired open?
Thread Starter
Drifting
Joined: Jul 2008
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From: Sonora CA
Hummmmm......... Interesting, I would have guessed that all the plugs would have to be out. Would compression on the other seven cylinders influence the reading on the eighth due to slower cranking speeds?
Drifting
Joined: Dec 2002
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From: Long Beach CA
That's how the specs are determined -- at a certain cranking speed range that you get with all of the plugs out and the carb wide open. If you do it with the plugs in and carb closed, the engine won't crank fast enough or pull in as much air. You'll get lower compression readings than spec and think the engine is going south.
Drifting
Joined: Aug 2005
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From: League City TX
That's how the specs are determined -- at a certain cranking speed range that you get with all of the plugs out and the carb wide open. If you do it with the plugs in and carb closed, the engine won't crank fast enough or pull in as much air. You'll get lower compression readings than spec and think the engine is going south.
Yeah, what he saidI know it wont make or break the test, but it is the "correct" way.
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Drifting
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From: League City TX
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Team Owner
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From: Columbia Missouri
That's how the specs are determined -- at a certain cranking speed range that you get with all of the plugs out and the carb wide open. If you do it with the plugs in and carb closed, the engine won't crank fast enough or pull in as much air. You'll get lower compression readings than spec and think the engine is going south.
I didn't think the goal was to see how 'high' the compression numbers were, since there is no baseline for comparison...just to make sure the cylinders all have 'similar' compression numbers. I mean, if I someone were to ask you what their compression 'should' be with a 100K engine that was virgin...how can anyone say, without knowing it's maintenance history or complete driving style history...what the compression should actually be. I don't think an average of 130 would mean much compared to a higher number of 150...would it?
Melting Slicks
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From: Chester Virginia
Originally Posted by LB66383
That's how the specs are determined -- at a certain cranking speed range that you get with all of the plugs out and the carb wide open. If you do it with the plugs in and carb closed, the engine won't crank fast enough or pull in as much air. You'll get lower compression readings than spec and think the engine is going south.
I think LB66383 meant to say 'going North'.
Roger
That's how the specs are determined -- at a certain cranking speed range that you get with all of the plugs out and the carb wide open. If you do it with the plugs in and carb closed, the engine won't crank fast enough or pull in as much air. You'll get lower compression readings than spec and think the engine is going south.
I think LB66383 meant to say 'going North'.
Roger
Team Owner
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From: Columbia Missouri
If I'm wrong I'll certainly concede to the proper way...I just would like to know the specific reasons why. Getting more air in and getting higher readings is one thing, but why is that important if you still get an average the other way and you can see if any are specifically lower than the others?
Drifting
Joined: Aug 2008
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As stated above - you remove all of the plugs so that the engine will crank easier, faster, and at a consistant speed. The carb gets blocked open because you need to supply the cylinder with air. All GM service manuals (Doge and Plymouth too) specify this method.
Melting Slicks
Joined: Sep 2000
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From: Chester Virginia
A leakdown test is a better indication of cylinder 'health' than a compression check. Takes a little longer, but you get what you 'pay for'.
I have done both when checking my engine.
Roger
I have done both when checking my engine.
Roger
Thread Starter
Drifting
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From: Sonora CA
All very good answers and information, thanks. I did do the compression check over the weekend and as stated above, the engine was cold and hasn't been started for a couple of weeks. Both manifolds were off so it was quite easy to get this done.
On the very first compression stroke on each and every cylinder the compression shot up to 90psi. The lowest reading I obtained was 164 psi and the highest reading was 178psi. I did the test twice, with the plugs in and with the plugs out and give or take a pound or two, ........ the readings were the same, even the initial 90psi Now if I'm not mistaken the service manual says that the compression is suppose to be 160psi. Again, the engine has well over 100K on it and has never been apart. Anyone have any comments or thoughts on this?
On the very first compression stroke on each and every cylinder the compression shot up to 90psi. The lowest reading I obtained was 164 psi and the highest reading was 178psi. I did the test twice, with the plugs in and with the plugs out and give or take a pound or two, ........ the readings were the same, even the initial 90psi Now if I'm not mistaken the service manual says that the compression is suppose to be 160psi. Again, the engine has well over 100K on it and has never been apart. Anyone have any comments or thoughts on this?
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
On the very first compression stroke on each and every cylinder the compression shot up to 90psi. The lowest reading I obtained was 164 psi and the highest reading was 178psi. I did the test twice, with the plugs in and with the plugs out and give or take a pound or two, ........ the readings were the same, even the initial 90psi Now if I'm not mistaken the service manual says that the compression is suppose to be 160psi. Again, the engine has well over 100K on it and has never been apart. Anyone have any comments or thoughts on this?
Those are good readings.
Did you try squirting any oil in the lower reading cylinders and trying it again? Sometimes a compression check can shed light on a valve seat problem. Go ahead and post the readings per cylinder in a list.
Next try a leak down test per cylinder, and compare the notes from the two tests.