Weird Leak Down Test and Compression Results

[DKneble]

So I finally went and bought a leak down tester the other day and decided to check up on the vitals of my motor. A little background first...

Car has been driving fine lately with good power, minimal oil burning, etc. I always lose a little oil in the catchcan from the pcv and the breather on the valve cover. I'm sure I burn a little too but smoke out the exhaust is pretty minimal even at WOT on 24 psi on the gt35r. Car is making somewhere between 440-470 awhp on 24 psi and pumpgas (12 deg timing) with no meth. Motor is built but has about 30k miles on it; wiseco 8.3:1 pistons, manley rods, si 1mm oversived valves (minimal port work in the head though).

Warmed the car up and did a compression test. Result was almost exactly 160 psi across the board and about 158 psi across the board cold. This is identical to the results I have had since I installed the motor.

Hooked up the leakdown tester I bought from harbor freight with the compressor at 50 psi, and I was getting in excess of 40% leakage. I can hear some air flowing but it does not sound like much. A little coming out the dipstick and a little out the oil cap. None out of the cooling system and I don't think any out of the intake. Not sure about the exhaust. This seems extremely excessive...if this was accurate I would think that the motor would be running like crap.

Also, my rear main seal is leaking as of about a month ago but not horribly (just a little bit under WOT) but I don't think that the crank case pressure is too excessive considering my dipstick does not pop out under wot.

Any ideas? Just a bad tester?

Also, adding oil to the cyclinder did not change the leak results...so, maybe the valves aren't fully seating?

 

[blktalonawd]

Two things to think of is that the harbor freight tester is probably bad and is justfully so as they are not ginger with their stock. The other thing is that if it is leaking minimal air during the test as you stated, that will be normal , but it has to go somewhere. Make sure it is actually leaking through the engine and not through the leak down tester. 40 percent leak down is well, Very bad. I am thinking that given the results of the compression test your motor is fine. You may want to think about bringing the cams to true top dead center by degreeing them as this will give you optimum performance and seal and depending on how much they are off as they are installed right now can up your compression quite a bit more. After you degree them in re-run the leakdown test with a known good unit. Good luck

 

[delta448]

^Both good posts. It's refreshing to see an informed poster asking good questions and a good reply to start off a thread in this section.

I also believe the bottom end is fine. As stated, obviously both sets of rockers being on the cam's base circle is very important for a leakdown test. If you can verify that and the results are still the same, I'd certainly assume the valves are not seating properly, or that your tester is not sealing to the cylinder or within itself. Both of your compression test numbers indicate healthy piston rings and headgasket.

 

[DKneble]

Swapped out for another leakdown tester and just did a "quick test" on one cylinder COLD. Result was about 15% leakdown on cyl 1. Considering the accruacy of the instrument, which as with many things is directly proportional to cost ($30 = just a little better then pulling numbers out of a hat), I am not nearly as worried as before.

I will complete the test at temp sometime this week with the motor at true TDC. Cams are crower's full race 284s so I'll have to do a little research on how to do that.

I agree that the oil test combined with the compression results indicates the rings are healthy...however at 70psi during the leakdown I could definitely hear some air coming from the dipstick. Is there another route for this air to get there without going past the rings? PCV system?

Also, the exhaust valves have some buildup on them (lead from running C12 and C16) so I'm thinking that could be affecting the seal. Yes?

Headgasket is good, valve stem seals I believe are good (no smoke under high vac or startup).

 

[tonster]

Swapped out for another leakdown tester and just did a "quick test" on one cylinder COLD. Result was about 15% leakdown on cyl 1. Considering the accruacy of the instrument, which as with many things is directly proportional to cost ($30 = just a little better then pulling numbers out of a hat), I am not nearly as worried as before.

I will complete the test at temp sometime this week with the motor at true TDC. Cams are crower's full race 284s so I'll have to do a little research on how to do that.

I agree that the oil test combined with the compression results indicates the rings are healthy...however at 70psi during the leakdown I could definitely hear some air coming from the dipstick. Is there another route for this air to get there without going past the rings? PCV system?

Also, the exhaust valves have some buildup on them (lead from running C12 and C16) so I'm thinking that could be affecting the seal. Yes?

Headgasket is good, valve stem seals I believe are good (no smoke under high vac or startup).

The lead will actually help your valves seal better. Also that air is only getting past the rings from contraction due to your motor being cold. Try the test with the motor warm and I am sure you will be satisifed.

 

[DKneble]

Performed the leakdown test at operating temp today.

Result was 2-4% leakdown across the board. In conjunction with the almost perfect 160psi compression on all four cylinders I am extremely satisfied with the health of the motor. I am sure now that the leaking rear main is not due to blowby as well but rather cheap parts (advanced auto in this case).

And I would just like to add that this motor was built by showdown motorsports approx 30k miles ago and has been daily driven in stop and go traffic, highway high rpm, and plenty of spirited driving at the track. A true testament to their quality.

Thanks for the help guys.

Dave