Compression Test Results

[mikeshoup]

I just did a compression test on the EA82T I just put together and dropped in my XT. Since I put it in, the car stumbled a bit, and I wasn't sure if the head gaskets sealed properly.

 

Cyl 1 - 100

Cyl 2 - 105

Cyl 3 - 95

Cyl 4 - 105

 

What do these results show me? Should I do a wet test now as well?

 

Also, I didn't do this with the engine warm. I've heard I need to do it with the engine warm, and I've heard I don't. Is the engine supposed to be warm?

[nipper]

they are fairly even. it shows you a tired engine, but still relativly balanced. you should do a wet test if you want to see if the lower compression is from valves or rings.

I have always done it cold on an aluminum engine, as you dont want to mess with plugs on a hot engine (can damage threads). Also do it with the throttle wide open, and a fully charged battery (or a jump from a running car).

 

 

nipper

[DrKrazy]

Did the test at least twice on each cylinder also right?

According to my FSM that is at lower limits. I would do a wet test and report back.

 

Also remember that engine sat for a few years so I would also run some seafoam/oil treatment through it and give it some miles...did have a possible 180k on it though. How did the cylinder walls look when you had it apart?

 

Oh and here is a link to the 2 pages from the FSM about compression testing:

http://subaru.kacsh.com/compressiontest.pdf

[GeneralDisorder]

Turbo engines don't show as high on the values due to their lower compression. Those numbers don't look that bad. They are even and that's all that matters as different test methods can affect the results to quite a large degree.

 

GD

[Alexx]

Is there a direct correlation beween compression and miles per gallon?

 

If so, and I assume there is, anybody want to take an educated guess?

 

For instance, when a car is new, the compression number is probably around x, and the mpg is usually y,

 

When a car 10 years old with 150,000 milies on it, the compression number is probably going to be around x, and the mpg is usually y...

 

When a car is 20 years old, with 250,000 miles on it, the compression number is probably around x, and the mpg is usually around y....

[mikeshoup]

Not that this has anything to do with my engine's compression...

 

But yeah, milage would decrease if compression decreased. The higher compression the engine has, the more efficient it is. The lower the efficiency, the lower the gas milage.

[GeneralDisorder]

While that might seem to be true, you have to realize that *most* problems that result in slight decreases in compression at 200 RPM (testing speed) do almost nothing at actual engine running speeds of thousands of RPM's. This is why we keep repeating that the testing NUMBER's do not matter. It's the percentage difference that matters. Anything above about 75 psi per cylinder should run fine. Lower compression ratios, testing insturmentation, testing parameters, starter speed, etc all work against comparing your numbers with anyone else. A proper leakdown test would be needed to tell ANYTHING about the engine's state of wear. The fact that your numbers are all pretty close would tend to indicate that your engine is in GOOD general mechanical shape. You simply cannot draw any other conclusions from a simple compression other than that. You have 4 cylinders, and any problems that would cause loss in mileage would tend to NOT occur in all 4. You would generally have 1 or two cylinders showing bad readings - if that were the case you would then go ahead with a leak-down to verify the exact nature of the problem.

 

GD