Leak-down Test: Best Tester and method?

[Walt Fricke]

I have a Tavia. Has worked fine for me for some years. But I have no way to compare it with any other brand. So, like others, I can't really answer Dr. J's question.

I'd say you want a double gauge one - one tells you what your regulated input pressure is. If you can't get 100 psi from your source (which is convenient mainly because it simplifies the math), you can use a lower number. The leakdown is the percentage difference between the readings on the guage closer to the air supply, and the one closest to the engine. I used to use 50 psi, so a reading of 49 on the second gauge meant a 2% leakdown. You could use any psi and a calculator if you chose. These things are inexpensive enough with two gauges. If Tavia is out of business, the Summit guage would do I expect.

I was a bit disconcerted to find that my two gauges did not read quite the same when I had not connected the line to the spark plug hole. This was pretty clearly a question of gauge inaccuracy, but I just took this into account. Eventually I replaced the gauges to remove this irritant - they are standard and not that expensive.

But James Brown points out an inherent issue. In many discussions of this topic I have yet to see anyone claim that there is a recognized orfice size standard for automotive work generally, or Porsche (much less 911) work more specifically. I gather that aircraft engine makers do some kind of sophisticated testing, which allows them to specify an orfice, and also, I suspect, to specify acceptable limits.

The rest of us just use some guesswork, sophisticated in some cases and based on tearing down a lot of engines, or seeing how much changes from 100,000 miles to 200,000 miles, or something. Otherwise by old mechanic's tales. One told me, when considering purchase of a well worn 2.7 street motor, that 12% was probably fine - that the motor was somewhat tired, but would probably run fine for quite a while.

My race motors with Total Seal rings normally show 0 leakdown, or close to it. Does that mean they are in superb condition? And I should worry if it climbs to 1? Don't think so.

But I carry mine in my toolbox to the track. Miss a shift or have some kind of problem (most recently a rocker broke), do a leakdown all around. If all is well, replace rocker and carry on.

As to the difference between ring leakage at various points on the stroke, you can, if you wish, test for that by turning the engine from TDC and figuring out a way of locking it there, and cover most of the stroke to ferret out odd cylinder wear patterns. You'd expect those to be somewhere toward mid-stroke, or wherever the piston speed is highest? For that matter, you can examine those parts with a bore scope. Something you can't do for rings. And the tester will tell you about valve seating.

Yes, the low buck solution is to make up something like Grady showed with old spark plugs, tubing, and an old air fitting, and just listen to where the air is coming out. Even at my nominal 0% readings I have always heard some leakage past the rings, but you can easily pick out a very damaged valve, and by comparing sounds cylinder to cylinder, really bad rings.

But I have never regretted this modest purchase. Same with the CIS tester even if I don't use it much.

Walt

[James Brown]

Walt, that is the reason I have never checked my diff. pressure on mine. i don't know if the orifice is correct. It would work if the rings were new and you used it for a baseline number. Then any deviation from that would indicate a problem and/or normal ware. One of the best uses I get is finding worn intake or exhaust valves, just listen to the tailpipe or the intake to here the "hiss". You still have to change the valve but at least you know.
Also, a normal compression tester is a important tool to use for overall engine diagnosis.

[cgfen]

Originally Posted by Steve@Rennsport

One must be VERY sure that each piston is at TDC and your wrench is not left parked on the crank pully bolt; remove the tool each time.

OUCH!
Something tells me that that lesson was hard-earned.

Craig

[al lkosmal]

Gunter,
I'd stay away from the Harbor freight units. Mine lasted a year or so. I replaced it with a unit from Aircraft Spruce. Very nice, differential tester units and still very affordable. Get on their website and search on 'tester" and you will find a couple to choose from.

regards,
al