<VV> Re: Leakdown Test - Yes, but some words of wisdom

[burkhard@rochester.rr.com]

OK, some good stuff here on the merits of leakdown testing. I will offer one very big caveat, though.  ALL LEAKDOWN TESTERS ARE NOT THE SAME.  The "% leakdown" number they give is a just a ratio of pressure drops with respect to a PURELY ARBITRARY reference orifice in the tester. There is no standard for this orifice's effective flow area, so different brands of testers will give completely different leakdown numbers on the same cylinder!  Thus, you CANNOT just pull an absolute number out of the air and say "less than 5, 10, or X% leakage is OK". It all depends on the tester itself. 

A sensor with a small internal orifice will be more sensitive, but will be more likely to change with use over time if a little dirt/oil/crud from the air system starts to accumulate. A larger orifice tester is less sensitive, but less likely to shift over time. Interestingly, as crud accumulates, the testers get *more* sensitive (until they outright plug).

We have a couple of decent professional grade leakdown testers in the engineering facility where I work. One is a Snap-On and the other is a Mac Tools. I also have a third one which I built myself out of commonly available industrial supply parts.

Just for jollies one day, I flowed all three of them on a very accurate and precisely calibrated flowstand.  I've got complete curves, but for brevity I'll just provide a single example point. A certain flow of leak rate (20.0 std. liters per minute or about 0.7 SCFM) on the three testers was considered by the Snap-On to be "11% leakage", my home-built tester "5.3%", and the Mac Tools unit "2.9% leakage". Which is right? They all are... The important thing is that you only compare readings from one tester against itself or a questionable cylinder against a good cylinder. I used a 0.040" drilled internal orifice when I built my leakadown tester, but actually effective area depends on things besides the hole diameter, so even that isn't enough detail to be assured of comparable calibration.

If you buy/build a new tester of unknown history, I would run leakdown tests on a number of "good engines" to develop a feel for the sensitivity of the unit. A measured so-called "5% leakage" could be extremely tight or fairly leaky... it all depends on the tester, so it the value obtained is VERY RELATIVE. Above all, ignore anybody who tries to give you universally absolute "<X%=good / >X%=bad" rules for acceptable leakage. They don't know what they are talking about because they don't understand how the tester works. But such a specific number threshold really is only important when you think an engine is just plain overall worn-out on all cylinders. More often, you are trying to diagnose a bad cylinder and you'll find one at 60% and the rest at 10%...

Have fun and play safe.

Jim Burkhard
CORSA / ACORNs (Rustchester, NY) / NJACE (NJ)

----- Original Message -----
From: UltraMonzaWest@aol.com
Date: Tuesday, April 6, 2004 1:43 pm
Subject: <VV> Re: Leakdown Test

> In a message dated 4/6/2004 10:03:52 AM Pacific Daylight Time, 
> chapmanmd@charter.net writes:
> Not to be disrespectful Matt, but there is a different theory 
> regarding 
> leakdown tests... ESPECIALLY for low compression engines. SNIPPED.
> 
> Everyone remembers how to do a compression test: screw in the 
> tester, flip 
> the kill switch to "off", hold the throttle wide open, and press 
> the starter 
> button. The cylinder takes in air and compresses it, and the 
> tester traps it. The 
> maximum is reached when the gauge holds more pressure than the 
> engine can 
> produce. The weakness of this test is that throttle postion, 
> engine temperature, 
> ambient air temperature, and a host of other factors can make the 
> results vary 
> considerably. What's worse, a compression test checks too many 
> engine 
> components at the same time. A poor reading can indicate so many 
> things, it's hard to 
> tell which engine part is at fault without doing a lot of other tests.
> SNIPPED
> 
> Mark Chapman
> Group Red 66 Monza and too many other cars, motorcycles, and junk 
> to keep up 
> with.
> 
> 
> ******************************************************************************
> *****
> 
> Respect?? for Moi???  gg   Only Hankie "respects" me...everyone 
> else 
> "suspects" me!! LOL!!
> 
> We both agree   Especially for LOW comp.  engines...  but  you've 
> overlooked 
> the "plus's"!
> 
> 
> Irregardless of all those factors....one person doing it at the 
> same 
> times....get the same "Relative"  readings....a low cylinder is a 
> low cylinder....
> 
> Other tests.........  squirt oil in .. retest.....pressure UP??    
> rings, 
> cylinder / piston....I'll pull it apart and I'll have it back on 
> the road before 
> you get setup for "leakdown! gggg
> 
> Maybe that's why I have 5  "turnkey" Vair  vehicles....
> 
> remember "KISS"   ggggg
> 
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