Leakdown and Compression test, or just leakdown

[Grinder34]

I did a brief search on the forums and didn't find a definitive answer, so sorry if i missed something obvious...

If you do a leakdown test, will it "miss" anything a compression test might "find"? Or is a leakdown enough?

Follw-up based on responses below:
Would there ever be a reason the cylinder can hold pressure but doesn't create it?

 

[ZachTTLM]

A compression test tells you your engine's ability to create cylinder pressure.

A leak down test tells you about it's ability to hold pressure.

Another way to say this would be, with a compression test you are seeing how much pressure the engine creates, and with a leak-down test you are seeing how much pressure it loses.

They arent really the same thing so it is best to do them both.

 

[Grinder34]

with a compression test you are seeing how much pressure the engine creates, and with a leak-down test you are seeing how much pressure it loses.

I understand that, but I guess the question then becomes:

Would there ever be a reason the cylinder can hold pressure but doesn't create it? Or are just the higher pressures in the compression test (~170 psi) vs the leakdown (~100 psi) the real issue? I can't think of a reason other than that where a motor would no longer create pressure but hold it just fine.

 

[Grinder34]

Im going to bump this up to the top. Just saw another thread where a leakdown AND compression test were recommended. When i do one, I'll probably do both, but the engineer in me would like to know why a compression test is necessary if you also do a leakdown.

 

[Grinder34]

Zach explained that earlier in a post. They show different things as far as the compression test shows the cylinders ability to build pressure and the leaks own shows it's ability to hold that pressure

I saw that and asked the follow-up:
Would there ever be a reason the cylinder can hold pressure but doesn't create it?
(I mean, realistically...with a running car)

 

[ZachTTLM]

Would there ever be a reason the cylinder can hold pressure but doesn't create it?
(I mean, realistically...with a running car)

I have never ran into this or ever heard of an engine holding pressure but not creating it..

BUT you can have an engine create compression but not hold it..make sense??

 

[Grinder34]

Yeah, it makes sense. Consequently if i were forced to do just one test, i would make it a leakdown. I think that would be 99% effective (making up numbers here). A compression test DOES use ~2x the pressure so it's possible it would show something the leakdown missed, although it would be unlikely. It also might show something like gouges in the cylinder walls below the cylinder at TDC, allowing air to escape on the up-stroke. Maybe i'm making things up.

Anyways, thanks for the answer.