Compression Test - Need Help
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Think I'll redo the test on just one of the 90 psi cylinders and cyl#1 this weekend with the throttle plate open. The battery is new and was turning good and hard with all the plugs in, so that should not nullify the test. And since I already have the differences between the cylinders, there's no point to doing them all. Sound logic?
Also, had a bit of an idea though as far as interpreting the numbers. So my compression is 8.2 right? Atmospheric pressure is ~14.7psi here at sea level in Florida. Therefore, 8.2 x 14.7 = 120.5. 120.5 is what I should be reading if everything is good and tight, not 150+. Now, reading my Haynes manual, the only figure it has for a compression test is "Standard : 150psi" which is supposed to cover all 15 years of the C3 line, and I know for a fact that earlier models had higher compression making this null and void. Anyone hear of connecting these dots before or am I speaking nonsense? Because as is, on paper, with this engine stock, it should be physically impossible for me to get readings greater than 120.5 psi w/out pouring water in the damn cylinder.
Oh, and one other thing, I used two gauges initially. Both read 70psi on cylinder #1 so I trust the readings, especially given the consistency on the rest of the cylinders.
Also, had a bit of an idea though as far as interpreting the numbers. So my compression is 8.2 right? Atmospheric pressure is ~14.7psi here at sea level in Florida. Therefore, 8.2 x 14.7 = 120.5. 120.5 is what I should be reading if everything is good and tight, not 150+. Now, reading my Haynes manual, the only figure it has for a compression test is "Standard : 150psi" which is supposed to cover all 15 years of the C3 line, and I know for a fact that earlier models had higher compression making this null and void. Anyone hear of connecting these dots before or am I speaking nonsense? Because as is, on paper, with this engine stock, it should be physically impossible for me to get readings greater than 120.5 psi w/out pouring water in the damn cylinder.
Oh, and one other thing, I used two gauges initially. Both read 70psi on cylinder #1 so I trust the readings, especially given the consistency on the rest of the cylinders.
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Thanks and noted. I knew there would be more to it than just swapin rings.
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Also, you got 165 psi on your '71, which at best had a 9.0 compression ratio. Assuming it's a stock engine, that kind of shoots down my idea of compression ratio x atmospheric = compression test nominal figures
Please tell me your engine isn't stock 
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Taking the plugs out just minimizes the load on the battery when cranking. All you are trying to do is to determine the integrity of ONE cylinder at a time. And, if you don't open the throttle plates, there will be NO AIR TO COMPRESS.
Sorry, folks. But, you need to refer to the Chevrolet Chassis Service Manual for procedures on how to check systems out in our C3's...or talk to someone who knows how to do these tests.
The very low numbers were the "tell" for your problem. I have no doubt that a proper recheck will show those numbers well above 100 ( probably in the mid-teens).
If the numbers still look low, the next test (for a cylinder measuring lower than expected) would be to shoot a bit of oil into the cylinder and do the test again. If you have a piston ring problem, the numbers will go up...as the oil will tend to help seal the leakage. If the numbers don't go up, you have a valve and/or cam problem. Once you have retested all cylinders properly, then tested again with a squirt of oil, the resulting data will tell you what direction to head for correction.
Sorry, folks. But, you need to refer to the Chevrolet Chassis Service Manual for procedures on how to check systems out in our C3's...or talk to someone who knows how to do these tests.
The very low numbers were the "tell" for your problem. I have no doubt that a proper recheck will show those numbers well above 100 ( probably in the mid-teens).
If the numbers still look low, the next test (for a cylinder measuring lower than expected) would be to shoot a bit of oil into the cylinder and do the test again. If you have a piston ring problem, the numbers will go up...as the oil will tend to help seal the leakage. If the numbers don't go up, you have a valve and/or cam problem. Once you have retested all cylinders properly, then tested again with a squirt of oil, the resulting data will tell you what direction to head for correction.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Atmospheric pressure is ~14.7psi here at sea level in Florida. Therefore, 8.2 x 14.7 = 120.5. 120.5 is what I should be reading if everything is good and tight, not 150+
Looks something like this.
https://www.google.com/search?q=pres...vXNU_GsaDcY%3D
You don't need both primary and secondary open to get enough air at cranking speed. Likely the difference would be negligible even if you don't open the throttle manually, not a lot of air is being demanded by the single cylinder that you are measuring cranking at a speed the starter can develop. However, normally I remove the throttle spring and prop the primaries open.
Removing the spark plugs is a must to recuce load on the starter and achieve a good cranking speed. Also hooking up a charger to the battery will net you more consistent starter cranking speed and thereby a more consistent pressure reading cylinder to cylinder.
Last edited by REELAV8R; Sep 15, 2015 at 11:36 AM.
GM Cert Corvette Tech
Joined: Apr 2008
Posts: 2,559
Likes: 585
From: Anaheim Hills California
Pressure and volume are not a linear relationship.
Looks something like this.
https://www.google.com/search?q=pres...vXNU_GsaDcY%3D
You don't need both primary and secondary open to get enough air at cranking speed. Likely the difference would be negligible even if you don't open the throttle manually, not a lot of air is being demanded by the single cylinder that you are measuring cranking at a speed the starter can develop. However, normally I remove the throttle spring and prop the primaries open.
Removing the spark plugs is a must to recuce load on the starter and achieve a good cranking speed. Also hooking up a charger to the battery will net you more consistent starter cranking speed and thereby a more consistent pressure reading cylinder to cylinder.
Looks something like this.
https://www.google.com/search?q=pres...vXNU_GsaDcY%3D
You don't need both primary and secondary open to get enough air at cranking speed. Likely the difference would be negligible even if you don't open the throttle manually, not a lot of air is being demanded by the single cylinder that you are measuring cranking at a speed the starter can develop. However, normally I remove the throttle spring and prop the primaries open.
Removing the spark plugs is a must to recuce load on the starter and achieve a good cranking speed. Also hooking up a charger to the battery will net you more consistent starter cranking speed and thereby a more consistent pressure reading cylinder to cylinder.
Instructor
Joined: May 2005
Posts: 200
Likes: 4
From: Richmond Virginia
It's a stock 1974 L48 motor as best I know, so it's even worse at 8.5:1 ratio. My uncle had it since the 80s and didn't change out the cam (that I know), and I haven't changed it out since I bought it in '02. It's pretty much just stock. Granted, it was a NEW battery and I think I was performing the test wet.
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Actually, if you have a Q-Jet, it is much better to open the secondary throttle plates, as they are so much larger than primaries. Any reduction in flow will affect the results. But, I suspect the secondary openings on a Q-jet would be more than adequate to do the job.
Le Mans Master
Joined: May 2006
Posts: 5,814
Likes: 414
From: Pittsburgh suburbs Pa.
That's not always the case . Many years ago when my money was limited to having to do things on a budget I re-ringed many engines that had very limited ridge at the top of the cylinders or no ridge at all but were smoking and I cross hatched the cylinders with a ball hone and re-ringed with stock rings and got many more years out of the engines with no smoking.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
That's not always the case . Many years ago when my money was limited to having to do things on a budget I re-ringed many engines that had very limited ridge at the top of the cylinders or no ridge at all but were smoking and I cross hatched the cylinders with a ball hone and re-ringed with stock rings and got many more years out of the engines with no smoking.
If it's just smoking, that's something completely different.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
rings rarely cause compression issues this major, except in boats where the cyls got wet and they rusted into the pistons. kiki really needs to redo his compression test or we gotta stop tossing this issue around...
