The ring end gap on a cold engine will be greater than a hot engine.

Setting a regulator under static conditions will produce a false reading. You will find the regulated pressure under "load" or flowing gas will be lower than the "setpoint".

 

I have read this thread with much interest. I have been an A&P (Airframe and Powerplant) mechanic for 25 years or so and have only recently seen the auto/ truck people finally discover this technique.
It is also called differential compression testing. We in aviation are required to use it on piston engines at annual and 100hr inspections. We commonly use 80psi as the inlet pressure and the differential pressure is specified by the particular engine manufacturer. Most Lycomings anything above 65/80 is good.
I once had an auto mechanic (?) tell me that he could use it to test valve guide condition. I had to explain to him that that was just not possible. The comments on doing it on a warm engine are spot on. You do get your best reading then. Another thing is to wiggle the crank a little with your breaker bar ( on the socket) which will set the rings a little also. We of course have the great big handle to use (a propeller). You can also get thunked on the head with a prop. If you have a valve that is leaking by try to stake the valve...tap the valve stem with a soft face hammer (with the air off) sometimes a little carbon will get under the seat and cause a little leakage. All and all it is a great indicator of engine condition.
Steve

 

Thanks steve! -Turn your PM system on

80 psi is a lot easier to maintain as an inlet pressure than 100 psi. Do you know what the orifice size is in your tester? That seems to be the major bugaboo in calibrating one of these things. I assume here you have a set of number and letter drills or gages to check the size of the orifice (just don't bugger your tester up as the entry and exit angles into the orifice are critical also). If you can determine those angles and the approx length of the orifice it will help these guys out also. It alsways helps to have a level of confidence in your test instruments

If the books spec a % leakdown you can always convert the 80# readings with a little math.

((reading)/80) x 100 = % leakdown

Example in the post above:

60/80 x100 = 75% leakdown

-or possibly expressed another way:

100% - 75% = 25% leakdown

I don't know which way your manuals would spec it since I haven't seen that many specs... Any info?

 

Snap On sells a 2 gauge leak down tester that is very good and is about $100.00. With 2 gauges you can tell very quickly the % of leakage per cyl. Coupled with a compression test gives a very good health report on an engine! I looked in to buildind a tester and found that by the time I found ALL the parts and fittings I needed I would be close to a good comercial unit!
Also the 3 Factory Ford Mechanics in our grage at work told me, on an older motor to stay around 40-50 psi. Could blow seals in the motor is very bad!