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356-930 356-930 is offline
Acceleration Junkie
Join Date: Aug 2007
Location: Laguna Beach, CA
Posts: 263
Originally Posted by JFairman View Post
If there is still a small vacuum solenoid installed in the vacuum advance hose line then maybe you still have some boost retard as it was from the factory.
I've been told that solenoid makes it so the vacuum advance and boost retard don't fight each other at full throttle.
Vacuum retard and vacuum advance change places or something like that when boost comes on since manifold vacuum becomes pressure then.

I think the vacuum advance signal from the throttle body is disconnected by that solenoid under boost possibly by the throttle position switch on the throttle body at full throttle, but I'm not sure, it may be the other way around...
The term "Vacuum Advance" is misleading. With the conventional mechanical 'vacuum advance' distributor, the harder the vacuum, the greater the retard. Timing "advance" occurs with loss of vacuum.
I tested my dual diaphragm/dual port '79 930 distributor on a Sun distributor machine. The machine's vacuum retarded timing -20 degrees. Applying up to +15-psi on the vacuum port had no effect on timing. With +5 psi applied to the boost retard port, timing was retarded -8 degrees. Increasing pressure on the boost retard port up to 15 psi had no further effect on timing.
Not having the specified solenoids may not affect performance but they may affect longevity of the diaphragms. Porsche must have had a good reason to use em.
Old 04-06-2008, 04:30 PM
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