Compression Test vs. Leakdown Test

[BryanL]

First I haven't searched anywhere for this information and know it it out on the net somewhere. I just thought I would post to see if anyone knew of a good article to read or knew from experience.
I have done each only one time but that was 13 years ago. I have borrowed a compression gauge but didn't know if I should also do a leakdown test. Not sure how different the tests are or if I need to do both? Basically I want to verify the LS1 in Bitchin Black. I had some oil leak issues over the weekend and am concerned about blowby or anything else for that matter.
Thanks for the help.
Bryan

[Wirtz]

They can both provide some good data to help you figure out what is going on.

Compression test at least has some dynamic element to it as the piston is moving. I mainly use it to track the wear in the cylinders over time and watch them to make sure they stay somewhat balanced out.

Leak down is more useful to me. It is not dynamic but it can indicate where a problem might be. When you run the test, listen at different places to try to figure out where the leaks are going. If you hear noise in the intake, intake valves or seats going. Exhaust valves or seats if you hear it at the tail pipe. Hear something in either the oil fill or the rad fill and it could indicate a crack or bad head gasket. Another possibility for noise at the oil fill is bad rings or cylinder wall.

To me the important thing is to baseline an engine and track them every so often to look for changes. Also, I try to do these tests with the engine warm. I'm not positive that really makes a big difference with a modern engine but it was how I was taught to do it.

Jeff

[BryanL]

With the kids soccer game and birthday party I only had a few minutes to hook up a compression gauge on 3 cylinders so far.
#3-90
#5-170
#7-120

Will finish the rest in the morning and check the pushrods. I am asuming that the rings are bad but my question is how big of a deal is this? I know it isn't good but is it going to hurt anything if I continued to run the car? Will it make things worse or do more damage? I don't need anymore power so that isn't a concern right now.
Bryan

[BryanL]

Thanks for the info Jeff, that is what I was wanting to know. The rest of the leakdown showed 193-196 except for #8 at 175. I didn't have a leakdown tester but decided to rig the compression line to pump some air into the cylinder. Though the engine wasn't warm, plugs were out, not at TDC and rockers were off the two worst cylinders. On those two cylinders air seemed to come out around the head itself like the spring/valvestem area. I put the rockers on and air leaked out through the intake on the three worst cylinders which is all I checked at the time.
I pulled the cylinder head that had the worst readings and can see that two pistons show signs of hitting something like the valves. But on my worst cylinder it doesn't show any signs of hitting a valve. Dropping the head off at a machine shop this morning to be checked. I hope they give me information that corresponds to the bad compression readings.
Bryan

[BryanL]

Thanks for the info Jeff, that is what I was wanting to know. The rest of the leakdown showed 193-196 except for #8 at 175. I didn't have a leakdown tester but decided to rig the compression line to pump some air into the cylinder. Though the engine wasn't warm, plugs were out, not at TDC and rockers were off the two worst cylinders. On those two cylinders air seemed to come out around the head itself like the spring/valvestem area. I put the rockers on and air leaked out through the intake on the three worst cylinders which is all I checked at the time.
I pulled the cylinder head that had the worst readings and can see that two pistons show signs of hitting something like the valves. But on my worst cylinder it doesn't show any signs of hitting a valve. Dropping the head off at a machine shop this morning to be checked. I hope they give me information that corresponds to the bad compression readings.
Bryan