Assuming the vacuum advance unit is working correctly and the timing is
set correctly in the first place, vacuum advance is not normally a bad
thing. It advances the timing only when there is high vacuum in the
intake manifold, which means only when the engine is lightly loaded. The
engine can stand more advance under light load (cruising, decelerating)
than under heavy load (accelerating). The most obvious result is better
fuel economy, not lower emissions. In fact low emissions is more often
associated with retarded timing, although that varies according to engine
design.
If adding vacuum advance hurt your performance, then the vacuum unit isn't
the right one for your car, or it leaks, or the static timing is wrong
(probably too advanced).
High-performance cars from the BritCar era usually did without vacuum
advance because they were not designed to run under conditions that
benefit from it. On the other hand vacuum advance is normally a good
thing on a street-driven car and won't hurt performance if it is working
correctly.
On Tue, 5 Jan 1999, MSLishego wrote:
> ...And what does the vacuum advance do?
>
> I've been wondering this for a while. This summer, I found a NOS
> vacuum advance unit for my 45DE dizzy. I figured that, paired with my
> electronic ignition, I ought to get some good results. I checked for
> leaks and found none, then put the unit in the car.
>
> After driving the car for a while, I realized that this unit was
> costing my car power! Even though it was hooked up to factory specs
> and there were no air leaks, it still caused my car to suck. My theory
> was the it was an emissions-related change that made the car run
> cleaner, but with less guts.
>
> So, since we've got so many great experts on all things automotive, I
> was hoping to get a dissertation from one of you about what a vacuum
> advance does, how it affects car performance, and why they were fitted
> to the car.
>
> And of course, I'll be willing to listen to arguments as to why I
> should hook mine back up... 8-)
-----------------------------------------------------------------------
Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO
fold@bcpl.net
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