Leakdown Test Technique

In the process of searching for my missing torque I made up a test plug to bolt on the block and do leakdown tests of the piston rings. I was getting some very flakey, unpredictable, and unrepeatable results. I would check a cylinder and get 7% then come back around later and get 14%. I finally found that the direction that the direction of piston motion prior to locking the crank and charging the cylinder can make a huge difference in readings. If the piston was approaching TDC the readings are far better than if the piston was on its way down. I guess it all has to do with the rings seating on the lands or not. I always assumed that a shot of 100 psi air would always seat the rings against the lands! Bottom line, leakdown tests need to be carried out in a controlled manner or the results can be misleading. Any one else come across this?

BTW my leak downs were mostly 5% to 8% through the rings on a cold motor. I would assume that would get a shade better if the motor were hot. I don't think much of my torque is sneaking by the rings so I'll have to look elsewhere.

Steve

Bottom line, leakdown tests need to be carried out in a controlled manner or the results can be misleading. Any one else come across this?

BTW my leak downs were mostly 5% to 8% through the rings on a cold motor. I would assume that would get a shade better if the motor were hot. I don't think much of my torque is sneaking by the rings so I'll have to look elsewhere.

Steve

Exactly! 5-8% isin't showing any huge problems. It's not race car good but it's not OMFG this thing is junk either.

Bret

Home made leakdown testers are based off using 100 psi air pressure since that's the easiest way to calculate the percentage. Aftermarket leakdown testers use less than 40 psi so the chance of spinning the crank from air pressure is greatly reduced. This way you can do a leakdown on different parts of the cylinder walls.

The only real way to use a 100 psi tester is to have the piston directly at TDC. Without a degree'd balancer, you're only going to be able to get cylinders 1 ands 6 at TDC using the timing marks.

Aftermarket leakdown testers use less than 40 psi

Really? That's the first time I've heard of that. My leak down tester has an adjustable pressure regulator on it so it would be no problem to regulate it down to 40. I have noticed, however, that the calculated percentage leakdown it slightly different at 70,80,90,and 100 psi test pressure. My Compressible Fluid Dynamics brain cells are long dead so I'm not even going to theorize why. Let's call it an "empirical observation"

Bottom line is, no matter what, you cant always just slap a tester on one hole and get confirming or condemming results. Every thing ha to be taken in context WRT the other cylinders, test pressures, test technique, etc. Heck, I was within minutes of yanking my motor and going back to square 1.

Steve