<VV> Vacuum advance/pressure retard

[Padgett]

>The Corvair turbos, in contrast, have FIXED timing from idle to around 
>4000 rpm until boost comes in.  This leaves much to be desired in terms of 
>throttle response as well as poor fuel economy for highway cruising.

Probably bean counters nixed tooling for the composite vacuum 
advance/pressure retard system for the few turbo buyers in the early-mid 
60'sfiguring they were not that concerned about MPG. It always bothered me 
that VW never seemed to bother with a vacuum advance distributor either.

Description is pretty good, flame front spread increases with chamber 
pressure (aka charge density). Flame front propagation also remains 
relatively constant as RPM increases so you need mechanical advance to 
compensate. *Everything* in tuning is to get peak chamber pressure at the 
right place. Is critical for everything that matters power, mpg, and 
controlling engine temperature. Is lots easier with digital since you can 
detect what happened in the last cycle and correct the next. Can come quite 
close with mechanical/vacuum means but need the whole bag of tricks since 
it is all a kludge.

By 1969 the increasing smog control regulations made vacuum 
advance/pressure retard dual inlet advance units available and might look 
at one of them.

Padgett