Weird compression test results

[zdude1967]

I did a compression test on my recently rebuilt 260z engine. It has about 1500 miles on it. On the dry test with the carb fully open and the coil wire off I get the following results # 1 thru #5 came in at about 72 lbs but the # 6 came it at around 90lbs On the wet test all came in at 90 lbs including the # 6.

I am rather new to this and this is the first time I have done a compression test, so my questions are- why would the # 6 not change from a dry to wet test.

The warranty expires next week on this engine and I wanted to do this test prior to the warranty expiring.Is this something to be concerned about?

Thnaks

zdude1967

[240260280z]

they all seem low... here are a few "gottchas" that get most first timers (including me):

did you remove all 6 plugs before starting

did you open throttle valve for each test

did you crank engine over a few times for each test

is your battery strong enough to crank the same

is the test gauge calibrated or cross-calibrated

are the valves seating well (check valve lash)

[Walter Moore]

I recently discovered that my ancient compression test gauge is off by something like 25% low. You might have a similar problem.

[Zed Head]

Was the "dry" test on a recently run, warmed up engine? That's what you want. You might have been too dry and too cold.

If you used the long open tube adapter that comes with some gauges, that will drop the numbers. The adapter adds its volume to the combustion chamber, resulting in lower pressure. I had some 180s drop to about 120, with the adapter (if I recall correctly, I've posted the numbers in the past).

Edited November 13, 2011 by Zed Head

[zdude1967]

I did the test cold with a tool that I rented from Checker Auto. If the numbers I took were real I would guess the engine would run like sh*t,correct? The car runs well. In any case I will have to try it again. How long should I run it before I do the test, just ubtil normal operating temp?

Thanks

zdude1967

[FastWoman]

I have an old Chrysler Marine 318 that measures about 95 psi compression on all 8 cylinders, using the same compression gauge that measures my Z at 180. The 318 runs fine, except for an exhaust riser that I can't seem to get to seal correctly against the head. It might be a combination of low compression and a bad gauge.

[beermanpete]

Check the cam timing. Late timing will lower the compression evenly on all cylinders.

[zdude1967]

Well at this point the warranty has expired....so the hell with it. I am just going to drive and enjoy it. It may be best for me to stop opening pandoras box as what is inside always sucks.

zdude1967