<VV> Vacuum advance/pressure retard
pp2 at 6007.us
Wed Oct 12 12:19:48 EDT 2005
>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.
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