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Unread 08-02-2019, 08:32 PM   #211
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Default Leak down percentages

Got the leak down numbers which are paired with the compression number:

Cyl PSI LD %
1 140 10
2 140 12
3 130 16
4 150 10
5 145 11
6 145 8
7 140 12
8 130 18

The correlation on the graph is really clear. The leak down percentages don't mean anything as far as leakage except for relative amounts of leaking.

I had a craptastic "Pittsburgh" tester kit on the shelf and it was a mess as many Harbor Freight tools are. I couldn't make sense of the reading which would snap to new values on each test of a single cylinder. It took a while to get something repeatable out of it. The first step was to open the "percentage" gauge and re-adjust the scale.

Inside the gauge, which is pretty standard, there's a Bourdon tube that's connected by an arm and gear to the needle. The calibration is by bending a little arm to read higher/lower. (There's no zero and span, just zero.) I twisted it so the gauge read 90% at atmospheric pressure and that pulled the bottom up, too.

The real problem is the pressure regulator. On each use it'd pick a new applied pressure and wouldn't be repeatable even under identical conditions. It also did a craptastic job of regulating the pressure at all. The more flow; the lower the pressure even with a much higher input pressure than output.

So... What I'd do is to readjust the regulator on each cylinder until the measured percentage was stable and the applied pressure was 1 Bar. And tap on the gauges so they weren't stuck. This gave repeatable results going back to prior holes for a check.

If you haven't checked one out, a leak down tester is pretty simple. There's a regulator with its output measured by a gauge and then the air goes through a tiny orifice. After the orifice is the "percentage gauge." Then the air runs through a hose into the cylinder. What you're supposed to do is run the pressure with the outlet capped and adjust the regulator to make that read zero percent leakage. (Makes sense.) Then the second gauge shows the lower pressure proportionate to how much air is flowing through the orifice. But! The regulator is crap so each run would have a different zero and a different reading. Sometimes by 10 to 20 percent. Useless!

1 bar was often close to zero and seemed usable so I adjusted the applied pressure to at least be consistent. While hard numbers are appealing, relative numbers and consistency are good enough.

The final analysis is that #3 and #8 are worse but not terrible. I'm not convinced the rings are broken although that's a likely problem.

I really wish I'd have checked the gaps of the top rings after installation. I sized the rings and bores and put rings into specific holes to compensate but didn't measure the gaps on each hole. It'd be interesting to see if that top ring gap correlated at all to the leakage and compression numbers.
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File Type: jpg LDvsComp.jpg (25.1 KB, 15 views)
1980 928S Euro
1987 944 N/A

Last edited by GlenL; 08-03-2019 at 09:48 AM..
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