Is there really any need...

[erlrich]

...to run leak-down tests if the compression numbers look really good?

[Greg Amy]

Yes, they tell you different things.

A compression test is a dynamic, macro test; it checks for basic, general compression, and really only finds the big, bad stuff (broken ring, totally worn-out engine, badly leaking valve, hole in piston, etc).

A leak-down test tells you where and how much in a much finer detail. It'll find things like partially leaking rings, partially worn out cylinder wall, lightly leaking valve, etc. It also gives you SPECIFIC info, since you're putting in 100 psi (for example) and holding 95 psi.

If you wanted a quick-and-dirty check between sessions, then do a leak-down test as a baseline and then a compression test immediately afterward. From that point you can usually get away with a compression test, and when you notice a change investigate it with a leak-down to find out where it's going.

I personally don't use a compression tester, except for "holy jeebus gawd there's no compression" moments.

GA

[joeg]

I use a compression gauge to phase a camshaft. The higher the compression (after inserting an offset key), is good; if low, insert key the other way. Kind of like a cheap dyno run.

Other than that, it just can only tell you general gross condition of an engine, as stated above.

I do not leak-down test much--if ever--because I can tell the general condition of my engines by oil blow-by. In extreme situations, it fills the dump tank between sessions and leaks out of the seals. Time for a fresh motor. The more oil in the dump tank, the more wear on your rings and cylinder walls.

[GKR_17]

The higher the compression (after inserting an offset key), is good; if low, insert key the other way. Kind of like a cheap dyno run.

Is this an IT motor? If so, an offset key is only legal to bring the cam timing back to stock.

[Buford]

I've found that a compression gauge will vary depending on atmospheric conditions. I check my 50hp outboard on my sailboat at the beginning of each season and record it. I also record the temperature and humidity.
You have to be careful not to chase a fantom condition.

For race motors always use a leakdown tester.

[erlrich]

Thanks guys - in the past I've always run both tests at the start of the year as a general health check for the engine. I checked compression yesterday, but it was getting late and I didn't have time to run leak-down. The comp numbers were good, and more importantly (as I understand it) very consistent across all cylinders (within 5 psi). I was just wondering if that told me enough about the condition of the engine, but now I understand there are things the leakdown could find that might not show up on the compression check.

[quadzjr]

I use a compression gauge to phase a camshaft. The higher the compression (after inserting an offset key), is good; if low, insert key the other way. Kind of like a cheap dyno run.


Hopefully not on your IT motor as that would be illegal. Phasing camshafts is illegal, as offset keys are only to be used to bring you back to stock timing.

not a bad idea though.. I have heard of some people doing just this (not in IT or SCCA for that matter) to dial back the compression numbers.

[rhygin]

Ahhh.... figuring out what is up with an engine...

Been on that path for a bit... LMK if you want me to talk through the engine testing process process a bit. No expert as I thingk I need an expert now!
BB

[erlrich]

Ok, so as if to make Greg's point, the leakdown showed a little more variance between cylinders. #1 was around 3%, #2 & #3 were 2%, and #4 was just over 5% (fwiw, comp #s were 179, 181, 184, 181). I've always heard anything over 5% is when you start worrying, but is the variance between the two middle cylinders and the #4 cylinder enough to be concerned?

[JeffYoung]

Not in my book, but see what you can find on your engine. For my car, that would be fine.

[Greg Amy]

(Cue Brit car jokes here...I would, but I'm just feeling lazy today... ;))

I wouldn't worry about it either as long as the compression is not leaking out of an exhaust valve. Did you listen to where it was leaking? You'd hear it leaking into the exhaust (exhaust valve), intake manifold (intake valve), or case/valve cover breather (rings).

It's most likely going out through the rings, though.

[erlrich]

There was nothing coming out any of the exhaust ports (as near as I could tell); all the leakage seemed to be going into the engine. I'll keep an eye on it, but may go ahead and replace the exhaust springs & retainers anyway, since those seem to be a weak link on the KA motors.