Confusion With Leak Down Test

[BassettBritts]

I have used my new leak down gauges on a TR3 and now the 911SC. The issue is, I think the gauges are not working. On the 911 test on a few cylinders I hear faint air leaking but the gauge doesn't move? Shouldn't it move even if it is slightly leaking? It is an OTC Tester.

[yelcab1]

Describe your procedures.

[BassettBritts]

Originally Posted by yelcab1

Describe your procedures.

I get the subject cylinder to top dead center. Hook the air to the gauges and have the other hose screwed in to the spark plug hole. Gradually increase the air and both gauges go up to about 90. Both gauges stay steady but I can hear air escaping, I think out of the exhaust valve. Not much but enough to hear. Yet the gauge doesn't leak down....

And thanks for responding. I'm puzzled.

[Trackrash]

Switch the two gauges and see if it reads the same.

[Solamar]

Originally Posted by BassettBritts

I get the subject cylinder to top dead center. Hook the air to the gauges and have the other hose screwed in to the spark plug hole. Gradually increase the air and both gauges go up to about 90. Both gauges stay steady but I can hear air escaping, I think out of the exhaust valve. Not much but enough to hear. Yet the gauge doesn't leak down....

And thanks for responding. I'm puzzled.

I haven't used a OTC LD set, but with my set you connect to the air source, zero the second gauge, then connect to your cylinder and note the pressure drop (and listen to where the air is leaking from).

Sounds like your are connecting to the cylinder before zeroing.

[yelcab1]

I have the same OTC set up. THis is my procedure.

Calibration: Hook up the gauge to the air line, run it up to 100 psi, you will always see a leakage even with the set up NOT connected to any cylinder. My set up is 3% leakage by itself. That number gets substracted from any measurement afterwards. If you see 100 psi on both sides, your set up is bad.

Get the engine cylinder to TDC compression stroke. Screw in the line to the engine plug hole, attach the line to the OTC set up.

Slowly raise the pressure to 100 psi on the input side. The output side will slowly follow but with a minor drop in pressure. Read the difference, compute the leakage percentage, substract the calibrated value.

It is impossible to get both gauges to read exactly the same value. If that happens, toss it, or you have done something wrong.

[icarp]

It is not recommended to use 100psi
The proper in psi is 80 . then do the math from there
Ian

[BassettBritts]

Originally Posted by icarp

It is not recommended to use 100psi
The proper in psi is 80 . then do the math from there
Ian

The highest I've gone is 90 not 100. Usually I take it to between 80-90 but still the gauge does not leak down even though I hear air escaping.

[al lkosmal]

Originally Posted by icarp

It is not recommended to use 100psi
The proper in psi is 80 . then do the math from there
Ian

Why is 80 psi recommended? What is the reasoning?

regards,
al

[VFR750]

I agree with the question: why not 100psi?

Used 100 psi on mine. Worked fine.

BTW, I took apart my harbor freight version and swapped out a real pressure gauge and closed the orifice down to 0.04”. This is the FAA recommended size for aircooled piston engines leak down testers.

[icarp]

The leak down test was developed by the aviation industry . They set the pressure parameters , if you use more pressure than 80psi you are not being fair to the ring seat on the cylinder walls.
When normal combustion takes place the pressure in the cyl is a rapid expansion of gases . When you inject a constant psi, the rings do not get the benefits of rapid expansion, just a steady pressure downward on the ring , vs the charge getting behind the ring pushing the ring outward and creating a better seal. These are not my words but the words of the Aviation industry, and also what I was taught . The leak down numbers on a nickie Cyl can, and very often are less than one percent , with a proper honing and brake in .
I hope this helps
Ian

[VFR750]

Ok. Aviation settled on 80 psi.

Agreed, can confirm that nicasil with proper honing and broken in rings results in 1% leak down.

[icarp]

This is a link to the tester that I use . Numbers on both gauges, then do the math.
https://www.aircraftspruce.com/catalog/topages/atsdifferentialtesters_orifice.php

I hope this helps
Ian

[VFR750]

https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument

Chapter 8

80 psi is called out. But oddly they expect someone to find TDC by rotating the propellor against 80 psi.

0.04” orifice for 5” or less diameter pistons. ( that’s us)

[ClickClickBoom]

Originally Posted by VFR750

https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument

Chapter 8

80 psi is called out. But oddly they expect someone to find TDC by rotating the propellor against 80 psi.

0.04” orifice for 5” or less diameter pistons. ( that’s us)

As an A&P working for a flying club, I did 5-6 differential compression tests a day. Find TDC by screwing the hose into the spark plug hole and rotating the prop, with your finger over the open hole until no more pressure escapes. Hook the gauge set up and add 30-40 psi. Rock the prop back and forth until there is no pressure, on the prop blade, bump up to 80 psi. If pressure is low, allow the pressure to push the prop slightly(counter rotation) and fiddle with the tip to get best reading. This is to get the rings to seat. Valves are not as fussy. This is primarily a diagnostic trend tool, of limited one time use. Rotating a prop with the piston at TDC is easy, get more than a few degrees off TDC and the work load gets heavy quick.