Sergio wrote:
>take care not to
>connect manifold vacuum to the distributor. What you want is "ported
>vacuum",and must be routed from the proper port at the carburetor butterfly
>valve location, never from the manifold itself.This port vacuum has a
>rising characteristic with rpm, precisely the opposite of the manifold
>vacuum that decreases with engine speed. Your ignition settings would be
>completely wrong with manifold vacuum.
Excuse me, but manifold vacuum is exactly what you want for a vacuum
ADVANCE distributor. Simple explaination to follow:
The vacuum ADVANCE distributor is advanced mechanically as a function
of RPM, and manifold vacuum is used to provide additional advance,
depending on the load (as measured by manifold vaccum). The vacuum is
higher when the load is lower i.e. crusing, or idling, and lower for
heavly load, i.e. accelerating, long uphill climbs. This allows the
vacuum to advance the timing when there is little load (for maximum
fuel efficiency), and to retard (actually just not advance) the timing
when the engine is under load to reduce the "pining".
Some emmision control equiped cars have vacuum RETARD distributors
(yes some have both...). These distributors DO act on "ported vacuum"
(usually from the carb venturi), i.e. higher LOAD (carb butterfly wide
open) causes a higher vaccum. These distributors usually have a more
aggressive mechanical advance curve, since the timing gets retarded
under load by the higher "carb vacuum". The reason for going to
"ported/carb vacuum" is for more precise control of the advance for
emmisions purposes. For example, decelerating creates high manifold
vacuum but not high carb vacuum - we don't care about the advance
under these conditions for performance purposes, but we do for
emmision control purposes - less advance while decelerating reduces
the output of "bad stuff" (don't remember which one, NOx, I think).
This is the reason for the change to the more expensive (and less
simple) "ported vacuum" for later emmision controlled cars.
Standing by for corrections...
Tim Mullen mullent@ymv5.ymp.gov Chantilly, VA
72 Elan Sprint "For the few who know the difference"
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