Going to do a compression test on my 71. Cold or warm engine? Plug wires disconnected. All plugs out. Carb plate open. Should coil be disconnected? Does fuel still get pumped when test cranking ?

 

I start out just a little warm and since it takes me about 90 minutes to do this task it's a bit cooler at the end.

Open the throttle 100%.
Open the choke 100%
Disconnect the positive wire on the coil.
If the needle and seat are good you will have no problem with excess fuel.

Oh yeah, you might want to arrange a chiropractor visit first.

 

yup.
mainly you look for balance between all the cyl.
so do it the same way on all.
good battery

all plugs out

 

Thanks......was a bit concerned about where the spark will go with the plug wires disconnected. Actually going to jack up car (jack stands of course) and get some of the readings from under car. May be easier to reach.

 

Disconnect the coil wire at the distributer and ground it or disconnect the coil power wire. Never allow the coil to fire with nothing attached to the wres. That can cause HEI failure or points failure.

 

compression testing is very old school and really just a waste of time because it doesn't tell you where the problem is or about the health of the motor in question. Bubba and his brother still own a compression tester.

Smarter people do leak down testing. you can actually see the health of all the cylinders by the % of leakage and whether it is valve or ring related.

 

True! Compressure testing is old school but it's a good starting point.
Don't you agree that if all the cylinders test within the correct limits that you need go no further?

Also as a hobbyist, I don't have compressed air.

 

If you got a battery charger put it on the battery on 10 amps while doing the test. Keeps the battery up so you don’t end up with ever diminishing pressure readings as you go along due to lower cranking speeds from a battery with lower voltage as you use it.

 

curious about why you are testing.

 

Yes, it takes a lot of CFM to maintain 100 psi on the input side of a leak down tester.

If you had a motor that was brand new and you ran a compression test and every cylinder read out 180 and you kept a note book. then five years later you tested again and now all of them were at 155. you would know that you are really down. but somebody without the original 180 test results might think that they have a great motor because it is even across the board. It all says nothing about if it is valves or rings.

but instead of doing a compression test, I'd just pay for a leak down

 

So with no known control number, the lower numbers on the 5 year follow up on a worn engine might seem good.
That's interesting.

 

Vettesic,
Whenever the compression topic pops up, there are always negative inputs regarding the value of a compression test. The compression gauge is just another tool for diagnostic evaluation. Just because some forum members don’t believe in its value, that doesn’t mean it doesn’t have value in your quest to diagnose a problem. Understanding the gauge and what to look for is key to getting results that are valid. Do a google search for using a compression gauge and any good source will specifically tell you to watch for the strength of the first pump or movement of the needle. That is an excellent indication of the strength of the rings. Make a diagram and record the results as a comparison to the other cylinders. Comparing all of the results will natural tell you if you have a weak cylinder or for example, two low cylinders right next to each other would be an indication of a gasket problem. Another indication of cylinder wear is doing a wet compression test where a small amount of oil is squirted into the cylinder to help seal the rings to the cylinder walls. With the oil in the cylinder, the compression will go up a small amount with even good rings. Excessively worn rings will have a significant increase with the oil.
As already pointed out by others, have the carb open, battery charged, engine hot, stay consistent with the amount of times the cylinder comes up to TDC. Watch the needle on the first pump and write the numbers down. Do the test and form your own opinion on its value for evaluation overall condition. The gauge is a great tester if you take the time to understand it, it doesn’t really cost anything beyond your time. It really is only as good as the skill level of the user, so do some research. Check the condition of the plugs while there out, replace if required, and always confirm the “pre-gapped” plugs are actually correct.
Regarding a leak down tester, this is also an excellent diagnostic tool and I use it often. If you look at the harbor fright tester, it only takes about 10psi to bring the needle up to the test gauge line so even a small compressor will keep up with it. I prefer my OTC tester at about a third of the price of my previous Snap on. If I suspect a valve sealing issue, the compression test will only confirm a compression loss, the leak down will point me to which valve is at issue. Regardless, leaking valves usually tell me to pull the head.

 

Black c3.....Thanks for the informational post. As with others that have offered great info. Initially, I wasn’t sure about what to do with the coil spark during the compression test. So that was the basic reason for this post. But I got some valuable info along the way. As with many posts, members may disagree but end result for me is good. My Vette runs great but am just interested in it’s basic health within its original motor. If compression test results are good I can stop there. If not I may have a few leak down test questions.

 

Exactly..... I always start with a compression test, I make sure to do it the same way every time, and always log the results and the mileage....Every couple years I check it when I change plugs...Just to keep tabs.

A compression check is a good "first test" if you suspect a problem. Then I move on to leak down testing from there with a GOOD leak down tester IF I think there is a problem.

No harbor freight junk tester....You can get skewed results very easy doing the leak down test and make a cylinder look a LOT worse then it is, if you don't perform the test right, and use good equipment.

Good on you for wanting to keep tabs on the health of your engine.



And yes, always pull the coil wire off of the distributor or pull the 12V feed to the distributor (HEI). You can test with the throttle blades wide open (usually recommended) or closed, it will still pull enough air in there for the test...(how would it idle if it didn't?).

 

Here are the results. I did a dry test then injected small amount of oil into each cylinder and re tested. All plugs had a light tan color at the tip of them. I think the numbers are not bad. Let me know.

 

Thats good. Only about 10 lb difference between cyls. About 5 lb increase with oil. Means the rings are A-OK.
# 3 & 7 did not respond to the oil introduction but the plugs look ok.
And sometimes a few lbs lower is just the battery getting tired depending on which order you tested cyls in and if a battery charger could keep up with the cranking.

 

If 3 and 7 didn't respond, maybe the oil didn't coat the walls well enough. I usually put the oil in and turn it over by hand (socket wrench) a couple of times to spread the oil. If that is not the issue, I think I would suspect valves and do a leak down test on them. They should of at least come up a few lbs.

 

I actually did a repeat test last night on both cylinders and they did come up another 5lbs a piece. Maybe I was kinda rushing through end of test or I just did not add enough oil. I know the heads were done few years ago and as stated before Vette runs great. Should I do leak down test anyway?

 

Not if you have to pay mucho $. You are good to go with those numbers.

 

If they came up the 5 lbs, that good enough. I wouldn't do a leak down with those numbers unless your very bored.