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spuggy spuggy is offline
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Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 2,826
Disclaimer: I don't know what I'm talking about, I found this all pretty confusing too - and almost no documentation. But here's what I think happens - and I'd appreciate anyone who knows more/better chiming in.

The 78-79 RoW 930 distributors have one nipple, for vacuum advance.

The 80 up RoW 930 distributors have two nipples, one for vacuum advance and one for boost retard.

(The vacuum nipples are plumbed below the throttle plate because they won't see vacuum otherwise, the boost nipple should be plumbed above the throttle plate).

Both distributor types also have a mechanical (centrifugal) advance mechanism.

As RPM's increase off-idle, the mech advance brings the timing up. As vacuum increases in the advance can, it also advances the timing, until the timing is fully-advanced with both vacuum and mechanical advance in-play.

At a certain point, boost enters the picture.

As the vacuum drops with boost building, the advance which was imparted due to vacuum will go away (leaving only the mechanical advance). On the 78-79 dizzy's (without the boost retard can), this is the only ignition retard due to boost available.

On the 80 up dizzys, as boost continues to build, the boost retard will overcome some of the advance dialed in by the mechanical advance.

(Warning! This last statement is purely supposition on my part - but I can see no other reason for a separate can, and it fits the assertion that 80-up dizzy's provide more retard on boost.)

I believe the reason that applying pressure to the dizzy when it's at rest does nothing visible, is because the mechanical advance is already "on the stops" - e.g. because the dizzy is sitting motionless and fully-retarded already, it cannot move any further retarded.

You can, however, see the vacuum advance rotate the plate by applying vacuum to the advance nipple.

How'd I do?

And does anyone know if the later, two-nipple advance/retard can will retro-fit to the earlier distributor? Sure looks like it will from photographs of the two cans...
'77 S with '78 930 power and a few other things.
Old 09-11-2007, 05:46 AM
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