Engine Compression Check

saddleman · Mar 22, 2009

I just finished checking compression on my 04 LT. It has 72,000 miles on it and runs great. # 1 186psi # 2 172psi # 3 179psi # 4 170psi The engine was cold & throttle closed.

I have a custom made 6" H.O.Tererice gage with a Tel-Tale needle that makes it easy to read the highest number to the pound. I know different gages & length of hose gives a little variance in numbers but its the overall average you look for.

How do these numbers look for a LT engine.

dshealey · Mar 22, 2009

saddle-man said:
I just finished checking compression on my 04 LT. It has 72,000 miles on it and runs great. # 1 186psi # 2 172psi # 3 179psi # 4 170psi The engine was cold & throttle closed.

I have a custom made 6" H.O.Tererice gage with a Tel-Tale needle that makes it easy to read the highest number to the pound. I know different gages & length of hose gives a little variance in numbers but its the overall average you look for.

How do these numbers look for a LT engine.

Those numbers are pretty good. It is a fairly high compression engine, and normally if you are lower than 10% variation you are good.

Anytime one suspects a problem, which you do not, it is better to use a leak down tester than a simple compression test. Leak down will pinpoint the area of a problem to either intake valve, exhaust valve, or gasket leak. Much harder to do leak down though, as you have to have the tester, and a way to lock each cylinder at near TDC for the test.

saddleman · Mar 22, 2009

dshealey said:
Those numbers are pretty good. It is a fairly high compression engine, and normally if you are lower than 10% variation you are good.

Anytime one suspects a problem, which you do not, it is better to use a leak down tester than a simple compression test. Leak down will pinpoint the area of a problem to either intake valve, exhaust valve, or gasket leak. Much harder to do leak down though, as you have to have the tester, and a way to lock each cylinder at near TDC for the test.

I also have a leak down tester that I use when the compression test shows a weak hole. I just wanted some numbers so I had something to go by. I,ll do it with the engine warmed up next time.

murray · Mar 22, 2009

Curious as to why did you not perform the test with wide open throttle ? I wonder if the numbers would increase much if wfo.

saddleman · Mar 22, 2009

murray said:
Curious as to why did you not perform the test with wide open throttle ? I wonder if the numbers would increase much if wfo.

I don't know how much fuel with fuel injection would be injected into the cylinders. With a carburetor at wide open throttle at cranking speed very little fuel enters the engine.

With a compression test as long as you do it the same way every time you have numbers to go buy. I now have the numbers while the engine is in good shape.

Voyager · Mar 22, 2009

saddle-man said:
I just finished checking compression on my 04 LT. It has 72,000 miles on it and runs great. # 1 186psi # 2 172psi # 3 179psi # 4 170psi The engine was cold & throttle closed.

I have a custom made 6" H.O.Tererice gage with a Tel-Tale needle that makes it easy to read the highest number to the pound. I know different gages & length of hose gives a little variance in numbers but its the overall average you look for.

How do these numbers look for a LT engine.

They look high, but I suspect that is the result of your technique, which is not correct. The BMW service CD lists three ranges for compression:

Good - 10 bar (145 psi) or greater
Normal - 8.5 to 10 bar
Poor - less than 8.5 bar

I could not find a recommended test technique in the BMW CD, but every vehicle I've had in the last 30 years specified that compression be checked with the engine warm and at WOT, cranked with all spark plugs removed and the value taken after at least 5 compression strokes. The value on the gauge will generally rise on each "hit" on the gauge.

I suspect your high readings are due to the engine being cold, but I haven't checked my LT engine yet to have comparative numbers. However, your numbers are 20% above the bottom of BMWs good range and that seems high unless they have a very conservative lower end on the good range - which certainly is possible.

I would consider repeating your check with the engine closer to operating temperature and at WOT.

Engine Compression Check

Attachments

Top Contributors this Month

Recommended Communities