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Walt Fricke Walt Fricke is offline
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Join Date: May 2004
Location: Boulder, Colorado
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One thing to look out for with oil pressure readings is the match of the sender to the gauge. 911s started out (when a pressure gauge was added) with a 10 bar/150 psi gauge and a sender to match. Sometime in the SC era (I think the SC's and 3.2s inherently operate at a slightly lower oil pressure, but I could be wrong there) this was changed to a 5 bar/75 psi gauge. I think the gauges are the same except for the faceplates (the same resistance puts the needle in the same position), and the senders (which are marked) had different operating characteristics, with their resistance dropping down to the 100% of gauge resistance at a lower pressure for the later senders. So a 5 bar sender with a 10 bar gauge gives readings 2x high.

Another thing which could affect readings would be the spring used. The '77 and later (oil modification) cases (and I think that the pre '77 930 cases also had this since the case I have does, but I don't know this for sure) use the same left side spring. The older cases used the same spring for the right side, vertical, spring. But after the oil mod, with the different piston (Wayne's 911 book has a good explanation of this, and Anderson covers it as well), the right side spring is longer (hence the guide tube inside), and appears to be of thinner wire. Both mean it has a lower spring rate. Using the short spring in the right side position is going to affect oil pressure. Using the wrong (old) pistons is also going to cause problems.

Sudden jumps or changes in pressure suggest something sticking, or perhaps this is just what one sees when the secondary or real relief valve opens to prevent overpressures.

I think these oil pistons run in a steel tube which is inserted into or cast into the case. I don't know what effect that might have on their sticking versus case and oil temperatures. One might think this would prevent differential expansion.

Walt Fricke
Old 08-30-2005, 12:17 PM
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