Compression Tester Orifice

[cub builder]

I'm not making myself clear. For years we had compression testers without calibration orifices. With this tester any cylinder that had leakage to below 60 psi "failed". Then someone ( I think Continental ) decided that the number was not that important, cylinder condition was. At about this time a new compression tester appeared. This tester had included a calibrated orifice and slightly different testing protocol. So my questions are who decided to put the orifice in the tester? Who decided the orifice should be .040"? Who decided on the testing protocol?

There was never an automatic failure at <60 psi compression differential. Many mechanics would call it a failure, and often times cylinders were failing at that pressure, but not necessarily so. I have had engines that were less than 60 psi on a compression diff chk, but were still acceptable and flown for a number of hours yet before overhaul. The "master orifice" added to the gauge sets gives a comparison leakdown for large bore Continentals and was a recommendation from Continental. I never knew or cared what size the reference port was.

[bigdog]

Continental specifies the .060 master orifice for the big bore engines like my IO-550. That typically results in ~45psi and controls where expanded inspection kicks in. The orifice size is in the manual so I don't question the number any more than I do other limits. I just bought a test set with that size. Most people start to squirm when the leakage gets into the 40's so I suspect the size was chosen using TLAR (that looks about right).

[cardo0]

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[Eric Page]

...crank up the supply pressure to 80psi or 85psi or whatever you use.

Just a note that Rotax specifies using a pressure of 6 bar for testing, which equates to 87 psi. This is often missed by mechanics/operators who are used to the Lycoming and Continental world where 80 psi is the norm.

[cardo0]

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[cardo0]

X

[exftrplt]

Gentlemen, please accept my apologies for my lack of clarity. There are orifices and there are orifices. There is the orifice that is in the flow when the compression test is being performed. The size of this orifice is either .040" or .060" depending on bore size. These sizes were determined shortly after WW2 and not the subject of my question. Although how these sizes were determined would be interesting to know. My question concerned the orifice that Continental calls a "Master Orifice". Continental calls the first or in use orifice a "restrictor orifice". The little that I can find says the master orifice size also is .040",but I am not sure what orifice they are talking about. Incidentally as I understand it the use of master orifice to determine "allowable" leakage is only applicable to Continental engines. In any event, the question is, how was the size determined.