Question on distributor Vacuum advance?
 

Is the vacuum advance on the distributor completely independent of the centrifugal advance?

I think I can see a way to make a little adjustable stop cam to extend the reverse (retard) action when my vacuum advance line goes over to boost. It is a clever little way to retard the ignition. The goal would be about 8 to 10 degrees of retard under full boost. Not totally sure, I have to do some more investigation.

My car does not use the retard vacuum port on the back side of the distributor...anybody know how that one operated and on which years it was used?

I'm off to the garage now to fix my scavenger pump system.

Assuming that the Bosch dist. is the same as everyone else's, the mech. adv. turns the upper part of the dist shaft (rotor) in relation to the lower(geared to cam). The vac. adv. swings the plate which holds the timing pick-up in relation to the trigger wheel. So yes they are independant if the Bosch is built in the standard way.

The way I understand the retard side was that it was only active under closed-throttle operation;idle and de-cell. The advance line is ported in near the throttle plate and uses the pressure diff. over the plate to make it's vac; it's not the same as manifold vac. I would think that with the retard-equipped dist with the adv. side only hooked up and the retard port not capped, it would retard under boost on it's own. Another thing to check is that most vac. cans are slightly adjustable by way of an allen-head down inside advance port; again not sure if Bosch did it this way but it's worth a check and if there may be useful for tuning.

That is what I thought as well...

You are correct, it does retard on its own.... only slightly though. There is a fixed stop which stops the retard function. I can take this round stop out and put a cam in, which can be rotated to a desired position. Theoretically it could go from 0 retard to max with infinite resolution. I guess it becomes a function of the diaphram geometry and the link arm. I think I can make it work.

Right now it looks like there may only be a couple of degrees of retard....like 3. The maps in tyhe factory manual stink.

If I modify it, I think I can get 8 or more degrees, but I have to see.

The goal is to get an ignition curve which is totally stock off boost, which this would be, but then have the capability to retard ignition timing out under boost. I can do this modification, then measure it with a light.

Do you know which port on the throttle body is slated for the vacuum advance?

W/O going out and looking, I believe the adv. port is at the front of the TB. It will have no very little or no vac at idle, and the vac will climb and maintain with throttle position. For an example; as you slowly open the throttle you can get the vac. reading to maintain say 5" vac., most other ports will bounce around then immediatly upon maintaining a steady throttle position will jump up to manifold vac(~18-20")

Sound as if you'll be able to tune it for what you need. The adv./ret. charts in the FSM are at the distributor both RPM and degrees; you'll double these for crank specs.

I'm assuming that you are blowing thru the TB?
Also due to the location of the adv. port on the TB you may not get full boost PSI there at low part-throttle regardless of RPM because the port works on the pressure differential over the throttle blade, heavy or WOT it will see the same as manifold.

So it looks like I may be able to get 8 to 10 or retard on it. I need to do some more investigation.

Well you are in uncharted territory, so now ya gotta tinker!

The rule is to keep as much advance in as possible without getting into detonation!