Thread: my "TO DO" list
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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,234
like the diagram that i have seen floating around for the vacuum lines. i think it is wrong. it shows the vac advance going to a thermo time valve for engine warm up that is suppose to do what your switch does, raise RPM's. but the vacuum retard should go to that valve. blocking the advance at idle does nothing, blocking the retard at idle will raise the RPM's.
when i got my car, it was connected like the diagram shows, which i think is wrong.
i connected it the way i think is right.


That mechanical thermovalve screwed into the intake manifold under the air flow meter housing you are talking about is origonally in the vacuum advance line - the diagrams you saw are correct.
The reason for that heat transfer operated mechanical thermovalve is to keep timing from advancing any more than centrifical advance alone until the motor heats up.
That way with timing not advancing as far, exhaust temperature is higher and that heats up the catalytic converter faster and lowers emissions sooner after a cold start and that made it so they could sell the car in the USA again starting in 1986.

Retarding timing raises exhaust temperature and advancing timing lowers exhaust temperature.

If you don't have a catalytic converter on the car then bypass that thermovalve and run the vacuum advance line directly to the vacuum advance pot on your distributor.
The advance pot is the outer one farthest from the distributor and the retard pot is the one closer to the distributor.

The way the 1986-1989 930 vacuum pot is designed turbo boost pressure has absolutely no effect on the vacuum retard pot at all! I have checked it and the vaccum advance pot with a mighty vac that can make pressure or vacuum so I know this for fact.

Only the outer side vacuum advance pot is affected by boost pressure and when it does get pressure it moves the diaphram inside the opposite direction and the linkage rotates the vacuum advance plate in the distributor the opposite direction and that retards the timing in the distributor

Only the late eightees 930's have origonal dual pot distributors from the factory with vacuum retard.
The only time vacuum retard is working is during deceleration, idle, and up to around 1500rpms and any rpm higher than that the throttle butterfly moves past the little hole in the throttle body wall that leads to the vacuum retard line and at that rpm point on up vacuum retard is non existant and does nothing.

The reason for vacuum retard at idle, a little above idle, and during closed throttle deceleration is to raise exhaust temperature and that raises the temperature in the catalytic converter and that lowers exhaust emissions.

You mention the metering cone venturi in the 930 compared to a 911.
The 911 CIS metering plate moves upward and the 930 metering plate moves downward so the cone will look quite different.

FWIW, the 930 air flow meter and fuel head assembly is origonally derived from the Mercedes 4.5 liter V8 CIS.
Old 07-19-2013, 09:28 AM
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