Spark timing,points/Hall effect

[Mario Tito]

>
>It took me a while to think this one up, but given enough
>time I might be able to come up with something:-)  What's
>the question?
>
>Jim Boughton
>boughton at bignet.net

   I have a question in stock spark timing. Looking through the Bently 
manual for my 2.0L VW air-cooled, compression 8.5:1, 125psi(looks low, 
compared to others on the list), and there are a few different 
distributors over the years of engine changes, like size change, 
different enviroment laws, etc., with different advance curves, and idle 
timing, which all make some sense. BUT in the California engine, they 
used a Hall effect sensor ignition, the ignition timing was changed, what 
seems to me, drastically. My point- and-condenser system works best at 
stock 7.5deg BTDC at idle, ported vacuum advance (I'm not sure of this 
term- the vacuum port is blocked by the throttle plate at idle position, 
and uncovers just off idle. Right?). The earliest timing at WOT (no 
vacuum), 3400 RPM, is 28-32 degrees BTDC. Ported vacuum starts at 80-130 
mmHg, ends at 200mmHg, advancing 8.5-11 deg.
   The Hall Effect Ignition is set at 5 deg ATDC at idle, with vacuum 
advance and retard diaphrams. Total advance at WOT (no vacuum), at 3600 
RPM is only 16-20 degees BTDC. The Bently manual is suprisingly lacking 
in where these vacuum signals come from. But WOT has no vacuum, right? 
Vacuum advance and retard both start at 60-120mmHg, ends at 240mmHg, and 
both change timing 11-13 deg. The Advance has a 'cut- off' which enables 
it only in fourth gear or if air temp is below 54F. This cut- off system 
is described only in the older dual- vacuum point system, but I guess it 
applies to this as well.
   The year prior to mine, with points and two vacuum chambers, is 
roughly the same as the Hall- Effect system. 
   Does anyone have a clue as to why in a mostly equal engines (the cali 
does have a cat converter), this difference exists? Am I missing 
something?
  Perhaps a discussion will help Tom C. as well.
  Thanks

Mario T.
mailto:Freshmar at aol.com-----'76 VW Camper FI A/T,,'79 Fiat X1/9