FEEDBACK ===> Compression vs. Leak Down.....

[Car Guy]

Is there good (useful) information that can come from both of these tests, or is the compression test just a 'preliminary' before a leak down test...???

Reason I ask this question: if I'm pulling a motor that I know I will be using in something else, I always do a compression test and then a leak down just to get an idea of it's condition. HOWEVER, I pulled a motor recently and failed to do either test :chair: . I know I can't do the compression test anymore but will be doing the leak down, which is why I asked the above question.....






:3gears: :3gears: :3gears:

[BAD LS1]

A compression test is completley subjective... it will give a rough idea of general condition. Its results can also be affected by how fast the engine spins, oil/fuel in the cylinder and if the engin is hot or cold. It will also hide flaws in valve sealing or head gasket or ring/cyl issues unless they are major. Leakdown is good all around, it will tell u the integrity of just about all the compression components. You can tell if the valves are leaking by air noise out the ports, or major air/noise out the oil fill if its past the rings, bubbles/air noise in the cooling system if its a head gasket etc... but like anything its not 100% gauranteed there isnt a issue. When in doubt, take it apart.

2-cycles are the worst for hiding compression component issues using a compression test. The thing could pull down 120psi, but the cyl, piston and rings could be scored to **** and you wont find it till you pull the muffler. but the engine wont start or will start hard and really lack power, even with the high compression reading.

[Prince Valiant]

Compression test can do a couple of things...1)quick indicator as to the general condition of all the cylinders relative to each other. So it'll give you an idea if one reads low (or high even) relative to the rest of the cylinders, and the potential of it being ring or valve related (IE, one has 30psi less pressure, throw a little 20-40W in the cylinder...if it spikes up to the others rating or higher, it's ring seal problem, if no to very little change valve problem. if ones unusually high, good idea that you've got carbon build-up. As tom points out, unless it's a major HG failure, it might not show it (but if you've got 140-90-90-135 on a cylinder bank, you can bet there's a HG failure b/w the two middle cylinders).

Compression test are also satisfactory for giving you an idea of where you're at if planning performance mods...IE, I run a compression test on a stock 318 v8 and get 120psi across the board, I shouldn't be planning on sticking a HUGE cam in it....however, I get one that gives me 160psi, I have a little leeway there.

The leakdown test is the better pure diagnostic test...it'll give you a better idea of the overall condition, more helpful dx'ing if it's ring vs valves vs HG.

As with anything, each test is a tool...it's only useful when used correctly for the correct reasons.