Leak down vs compression test

[Bryan Cotton]

Hey all,
I'll confess my ignorance - I don't really know what a leak down test does. Why is this better than a compression test? My cold compressions at WOT:
119
121
125
119

[13brv3]

Either test will give you a reference number. The differential compression test will be more consistent, since it doesn't rely on how fast the engine is turning during the test. It also give you information about where the leak is if you have one. While holding pressure, you will hear the air leaking at either the carb, exhaust, or crankcase breather. That tells you which valve is leaking, or if the leak is past the rings.

The test units have an orifice that is sized for the amount of leakage that would be considered normal for the piston. For "aircraft" units, there are at least two orifice sizes to choose from depending on whether the piston is larger or smaller than a specific size (5" I think). That's important to make the engine manufacturers specs meaningful. I'm not sure how automotive units chose an orifice size, but I've never seen a spec for those anyway. A larger or smaller orifice just makes your "normal" reading a bit higher or lower.

If you get one, I'd just get an automotive unit, since they're cheaper, and work just as well. I've compared the "aircraft" units to automotive units on the 912 engine, and either works fine.

[Kai]

As already stated by others- a leak down test will tell you where something (valves, guides, rings etc) leaks, whereas a compression test just tells you that something leaks.

To overcome this disasvantage, it is fairly common to pressure each sylinder individually with compressed air after a compression check, and stick a soft rubber tube in your ear. The other end is stuck in the dipstick hole, exhaust outlet, carby throat etc, and listen for the hissing sound of escaping air. That way you could get an idea about the ´where´as well with the comp check.

When doing the comp check, take one plug out of each sylinder before you start, and hook up a car battery to the electrics to warrant consistent readings- otherwise the tiny aircraft battery could very well be drained. And remember: keep cranking the engine until the pointer rises no more, and WOT.

It´s also advantageous to keep some basics in mind. At sea level atmospheric pressure is close to 15 psi. So if your engine is set up with a compression ratio of say 8:1, the instrument reading should not show much more than 120 psi. If it does show much more, you can safely assume that you have carbon deposits on the piston crown (it does not take much).

A lot of engine manufacturers give a maximum allowable difference in comp pressure between the individual cylinders- it is not uncommon to accept 5-10% as within specs.

[daleandee]

I have a dynamic and a differential compression tester. I prefer the differential compression tester as it gives much more information and is quite easy to use. I have an OTC set I bought years ago. This appears to be the newer version of what I have:

https://www.amazon.com/OTC-5609-Cylinde ... 116&sr=1-3

I believe the regs call for an engine compression test for the yearly condition inspection but IIRC they do not specify what type of test has to be used. The old 60/80 numbers are a recommendation and not a requirement as to service limits. I'm quite suspicious any time my readings are in the low 70's over 80. In reality the compression on a given cylinder has to be very low before any loss of power would be noticeable.

[13brv3]

I have that exact same OTC unit, but they must be gold plated now since mine was $30 cheaper when I bought it a couple years ago.
Rusty

[daleandee]

I have that exact same OTC unit, but they must be gold plated now since mine was $30 cheaper when I bought it a couple years ago.
Rusty

LOL! Mine looks exactly like the one in the ad but it must be a new version as the price is noticeably higher than when I bought mine 10 years ago. But they do work well and seem to be quite accurate. Maybe the orifice is gold plated. 8~)