last night i bought my own compression tester and re-tested the beast. this time was slightly different then the last in that i did the test by myself and, due to not being able to see the gauge while cranking the motor, cranked the motor for a longer period of time then before. last time i got 170-100-140-180 twice on the dry test. this time, i got 210 across the board. i experiemented a little and found that if i cut the cranking time in half the numbers were a little closer to before, but with repeatability problems.

now, 210psi doesn't sound that bad, but why so different from before? and why was this test showing all 4 within 5% of each other, while the other one showed such a huge spread? i know that since i didn't actually see the gauge before (i was turning the key) that the first test may not have had enough time to get the pressure up in the cylinder, but the test results were repeatable.

furthermore, at 210psi one would have to say the motor doesn't sound like it's too unhealthy. however, doing a wet test showed a 50-60psi jump across the board, strongly indicating bad rings. in thinking about this, the number the gauge reads doesn't seem very accurate from gauge to gauge. as i understand it, the compression gauge measures how well the piston can compress the fixed volume inside the cylinder. change the volume that the gauge contributes though and you change the reading. does the gauge contribute such a small amount to the total final volume that it doesn't affect the results? did i just crank the thing too damn long?

nate

 

What is the wet test? I recently heard a high school shop teacher mention it but I don't recall it from my formative years like the compression and leakdown tests.

 

wet test - pour a tablespoon full of motor oil into the cylinder and retest.

Nate - were both tests done the same? Engine hot, fuel injector fuse pulled, throttle wide open? Same guage?

 

My thinking is that it may either mean there was something keeping a valve from seating properly when you tested before, or the rings are worn and it took some of the cylinders longer to build peak pressure. As asked before, was the original test done with the engine hot?

Based on the power numbers you posted after the dyno run, I would say the compression you have sounds reasonable. Whenever I do a compression test, I always turn it over enough times to make sure a given cylinder has peaked out. Can you do the test again where you can watch the guage? A smooth increase in compression on the guage is more important than how many cycles it takes, I think.

 

i talked to rodney on the phone last night and got myself edumacated.

the wet test: yes, you poor a small amount of oil in the cylinder and retest. the idea is that the oil will fill in the gaps around the rings and make a better seal then a worn ring would. i now think that this test method is bogus (at least for my motor) and for one simple reason. the pistons in the z6 are dished. the oil only fills the dish, doesn't go down past the rings. fill the dish, compression ratio goes up, and so will the reading on the gauge.

the first test was carried out in the same manner as this last one i did. warm motor, throttle open, and ecu fuse pulled. however, the first test was done with an old gauge and it would also seem that i didn't crank the motor over enough times. i got some history on the gauge i first used and it was 20 yrs. old and had been sitting in a toolbox for most of that time. needless to say, it's accuracy is questionable. my new gauge is a craftsman. i'm going to be redoing the compression test again this weekend when i can get an extra set of hands to turn the key and i'll be looking for how the pressure comes up in the cylinder. thanks guys!

nate

 

BTW RJ. - 140-145 is well above the GTI "your engine sucks" threshold of 109psi. The "new engine" range is 145-189psi

Thanks again for your help. The cage goes in starting this weekend, the seat is here, and I have a deal with the local VW place for parts...

K

 

uhm. even at 250psi, i'd be willing to bet that engine still sucks

 

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The cage goes in starting this weekend, the seat is here, and I have a deal with the local VW place for parts...
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Let the fun begin, I love building racecars. I'm contemplating building another one in the Spring and that'll be my sixth or seventh build. Exquisite agony.

Thanks for the definition of the wet test. Sounds like an old school test that was more effective in the days of flat tops and hemis but information gathering nonetheless.

 

Yeah, it sucks. And blows, And sucks, and blows, sucks, blows, sucks, blows - about 24,000 times per minute.

Nyah.

K