Scott Tilton asks:
> Why would I want some advance at a partially open throttle and none at wide
> open throttle?
This is the basic question of "Why vacuum advance at all?" After all, if
the timing depended not at all on the level of vacuum, you could just
advance the timing to its supposedly-ideal position and that would be the
end of it.
The reason timing depends on vacuum is this. The more you open the
throttle, the greater the density of the air entering the cylinders. This
higher density causes the flame front to spread faster. Now, you normally
want maximum power at wide-open throttle, right? (Duh, of course!) And
open throttle generally means the minimum vacuum, higher air density. So
you set the non-vacuum-influenced timing to be as early as your engine can
handle. The trouble with this as a general operating point is that when you
close the throttle the air density goes down, so the flame velocity
decreases, which means you could actually tolerate a bit more advance. Most
of your running is done at part-throttle anyway, so with no vacuum input
you'd spend most of your driving with the timing later than ideal for those
particular engine conditions. The solution is a vacuum module that advances
the timing when the vacuum goes up, just enough to compensate for the lower
air density. Or conversely, it retards the timing when the vacuum goes
down, just enough to compensate for the higher density. This lets you set
the no-vacuum timing for best open-throttle power, and also gives you more
power, better throttle response, and higher gas mileage at part-throttle.
The smaller, cheaper cars might even dispense with the vacuum advance
completely on the grounds that with a light car, you can tolerate slightly
retarded timing most of the time and still optimize the timing for full
throttle.
Hope this answers your question. Perhaps I misread your note. If so, sorry
to waste the bandwidth.
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
'70 GT6+ (Nigel)
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