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.

 

Thanks and noted. I knew there would be more to it than just swapin rings.

 

Good stuff. Can definitely rule out the battery, and I haven't checked my timing, but I don't see how that would go out of wack w/out messing with it first. I'm hoping that opening up the throttle plate will let in more air and give my numbers a boost.

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

 

I am glad I read this thread. I didnt know you had to take out all the spark plugs for this test.

Thanks Corvette Forum.

kdf

 

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.

 

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.

 

 

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.

 

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.

 

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.

 

Ok, kiki. Did you redo the compression test?

 

An engine with low compression is going to have very worn cylinders and worn or broken compression rings. You won't be able to simply re-ring it with any success.

If it's just smoking, that's something completely different.

 

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...