Today's Research Topic, Leakdown Testers Options

[McMark]

I inherited a Harbor Freight Leakdown Tester (PN: 94190) from someone and was playing around with it lately, checking out a 2.7 CIS 911 engine that will be going up for sale soon ( (IMG:style_emoticons/default/shades.gif) ).

I learned a few things that I feel should be common knowledge when looking at leakdown results and leakdown testers.

Backstory
I hooked up the HF gauge out of the box and immediately ran into confusion. The instructions indicated that you should pressurize the tester and adjust it to 0% leakage, then hook it to the cylinder and read the leakdown percentage. Well that was a bust, right off the bat. Setting up the gauge as instructed resulted in an inlet pressure of around 35psi. Most people on the internet recommend running a 100psi inlet pressure. So naturally, you would increase the inlet pressure to that recommended level, but the HF then drops to 0% leakdown, as pressure increases.

Initially I thought this was a gauge issue, so I ordered a couple quality pressure gauges from McMaster car and replaced the HF gauges with the McMaster units. This allowed me to compare inlet pressure with cylinder pressure, which is what the leakdown tester is supposed to do. But alas, these numbers still didn't make sense.

What I Learned
It turns out that a two gauge leakdown tester is showing you inlet pressure on one gauge, which is essentially your compressor pressure, regulated to 100psi (or whatever you choose). The other gauge shows you the pressure inside the cylinder. But between those two gauges there is an orifice (precisely measured small hole). The size of this orifice is EXTREMELY IMPORTANT.

What a leakdown tester is actually showing you is a comparison between the amount of air that can squeeze through the orifice COMPARED to the amount of air that can squeeze through the engine leak path (ring gaps, burned valves, etc). It's more like a ratio than a percentage. So if you think of it as a ratio (Orifice : Leakage), then it's easy to realize that if you change the orifice size you change the ratio, even though the leaks on the engine haven't changed.

Bottom Line
I completely disassembled the HF tester and the orifice in that unit is >2.5mm. While there is no particular standard for leakdown testers MOST units and most people are using a 0.040"/1mm orifice. This is referenced in aircraft leakdown standards, and has basically been adopted by the automotive industry. Posts I found on this subject confirm that expensive testers (such as MAC) come set up with a 1mm orifice.

The HF tester should not be used without modification. My next step is to try plugging the HF orifice and redrilling it to 1mm. This should make my HF gauge work like it should. I'll then reinstall the HF gauges and see how they compare to my quality gauges.

Be wary of leakdown numbers. I'm posting this because it's imperative that anyone who's shopping for 911 engines be aware that it's not enough to simply know the leakdown numbers. You need to know the inlet pressure that was used and the brand of gauge that was used.

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