Barney, Are you sure?...
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Ported vacuum is low at idle, comes on strong on acceleration at low
speeds, then increases slightly with engine speed. Manifold vacuum is
high at idle, drops dramatically under hard acceleration, climbs again at
higher RPMs, and hits the highest when you close the throttle. Ported
vacuum with the vac-advance dizzy and manifold vacuum with the vac-retard
dizzy have roughly the same effect. Spark advances when you hit the gas.
**spark should _retard_ when you hit the gas WOT due to the increase in
turbidity and density of charge. As the name implies... vacuum advance
units, advance spark timing when there is a vacuum. At WOT vacuum is low and
therefore there will be less advance, retarded.
**
As I recall, the primary reason for using the manifold vacuum retard
setup is to retard the timing dramatically on overrun to prevent
backfire.
Vacuum spark timing modulation is usually limited to less than 10 degrees
advance and maxes out before 2500 RPM. Mechanical spark timing advance
is nearly linear, starts just above idle, and maxes out around 3600 rpm.
Total spark advance should be limited to no more than 32 degrees on a
stock MG 4-cylinder engine.
**On an 80 B the vacuum advance unit can bring in a max of 25 deg advance at
10-15 Hg(book spec). The amount of advance depends upon the amount of vacuum
applied to the unit, not the rpm of the engine. While rpm affects the amount
of vacuum applied to the unit, the main factor is throttle position, WOT :
low vacuum, closed : high vacuum.
Mechanical advance is not linear. More advance is added at low rpms than at
high where the advance tapers off to 0. According to the book, an 80 B with
all the emissions crap can have a total of 63 deg advance at 4500 that's
combining vacuum and mechanical.
Chris Reichle
**
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