differential compression testers

larry@oma · 12-21-2007, 09:32 PM

aircraft differential compression tester acceptable to use on my 911sc for a leakdown test? or must i use an auto tester ? what size orifice in the tester for using on my 911sc? thank you for any answer. .... larry....

OldTee · 12-22-2007, 04:50 AM

I believe that Aircraft Spruce kind of testers include an automotive fixture. On mine I welded a piece of brass pipe to the fixture on one end and a compressor quick disconnect (male) end to the other. It greatly helps the find-it-and-cross thread-it game you play with one on the rubber hose.

hcoles · 12-22-2007, 07:01 AM

IMHO - this is an interesting question namely - What is the specification for making a leak down tester? The last time I googled this to death I could find no specification e.g. orfice size or orifice specification. There could also be a pressure and flow rate curve with tolerance that an automotive leakdown tester should fall within.... but there isn't. I expect you could get 10 leakdown testers and you would get 10 different results. That being said, as most will agree the important thing is that the cylinders are leaking the same amount and from approximately the places. If anyone can find a spec. for an automotive or Porsche leak down tester, please post. I'm pretty sure the airplane industry has a spec.

304065 · 12-22-2007, 10:38 AM

There is a bunch of information about making a roll-your own leakdown tester, and the orifice size is specified. http://www.xs11.com/tips/misc/misc3.shtml

One builder says use a 1mm orifice. You just want something to isolate the input air pressure from the residual pressure on the cylinder side of the gauge.

The only thing specific about an aircraft tester is that they always use 80 PSI as the standard, and any A/C leakdown test is expressed in a log book as those numbers, not the percent leakdown, e.g. 76, 75, 73, 72 for a four-cylinder lycoming. Other than that the tester is the same- but airplanes don't use a 14mm spark plug, the typical REM40 plug is 18mm, so be sure to get one that can accomodate an automotive plug.

hcoles · 12-22-2007, 11:46 AM

John,
yes, I've seen that type of DIY info. But it is still interesting to me that there is still no "official" spec. I'll have to look again but I think there is a spec. for A/C. E.g. if you lost your first tester you could buy another, from the same or different mfgr., and continue taking readings.... not true with automotive testers.
I guess not a real big deal but curious, to me at least.

larry@oma · 12-22-2007, 02:13 PM

looking around online at various info: looks like Lycoming aircraft engines are to use a tester with a specific orifice size. Continental aircraft engines also specify an orifice size. i suspect all factory Porsche testers also all use the same orifice size? a larger orifice size will make your test numbers look a lot better? (less leakdown) anyone know what the orifice size is in a porsche factory tester is? thanks for the interesting replys. ....larry...

OldTee · 12-22-2007, 06:37 PM

For what we do, ie uncertified work, and aircraft tester is fine. Set it at 80 lbs and do the math to reach the number based on 100. Or, take it up to 100 lbs and read accordingly. It is leak down you are after. This is the amount of air escaping from the cylinder. Tune your ear to know what you are hearing. Air through the exhaust is an exhaust valve leaking. Air in the intake is a intake valve leaking. Air from the oil tank is rings. Some air and high compression number are OK, lots of leaking air and low leak down numbers make your mechanic happy. Number way off on that cylinder is cause for further looking. 80/80 is the highest. 80/60 is a cylinder at 75% 80/78 is 97.5% efficiency. Prices on these testers from Aircraft Too Supply (www.aircraft-tool.com) run from $60 to 110.