Although I don't have a map of the Advance Curves like Richard Atwell
has for the Air Cooled variety on his site, I do have the specs shown
my site for this distributor:

http://www.oldvolkshome.com/ignition.htm#V8384

Other answers follow below in your text:

On 4/26/05, Damon Campbell <damoncampbellvw@yahoo.com> wrote:
> Hello list,
> 
> I am trying to understand the various mechanisms that
> control the timing on the early digijet.  There is:
> 
> * Static setting (just the twist of the distributor -
> sets the baseline for centrifugal and vacuum)
> * Centrifugal advance (rpm based advance... seems to
> kick in around 1600 rpm or so.  Where does it max out
> at?)
> * Vacuum advance (one half of the vacuum canister)
> * Vacuum retard (the other half of the vacuum
> canister)
> 
> Static and Centrifugal are pretty straight forward.
> The vacuum advance/retard is a little more confusing
> to me.  First off, does the vacuum canister actually
> ever retard the timing?

Yes - 9 to 11 degrees at idle speed as per original factory spec
(which would mean ideally when you disconnect & plug both vacuum
lines, the "mark would move to about 6 or 7 degrees BTDC at idle (or
even set at "static").

Secondly, the vacuum signal
> adjusts the timing for engine load (i.e. lugging up a
> hill at low rpms), right?

Yes, but not significantly.  The purpose of the vacuum advance is to
help prevent hesitation from a stop.

> 
> Is idle the max advance condition (as far as vacuum
> goes), and then it just drops proportionately with a
> lower vacuum signal, or...?  On a standard 1.9L wbx,
> how much of an effect of the timing does the vacuum
> signal have (5deg...50 deg)?  I guess a more direct
> question is: at idle, if you unplug the vacuum line to
> the distributor (and plug it), how much does the
> timing change by?

About 5 to 7 degrees depending on the age/mileage of the distributor. 
At least it does on mine.  I find that my engine idles and runs better
set at about 2 to 3 degrees ATDC, so my distributor is showing it's
age.

I hope this helps.

-- 
Jim Thompson
84 Westfalia 2.1 "Ole Putt"
oldvolkshome@gmail.com
http://www.oldvolkshome.com