Anthony Rhodes asked:
> I still do not understand the physical and operational differences of the
> vacuum port for advance and retard.
Ah, that's easy. One goes forward, the other backwards. :-)
I don't know exactly where your vacuum lines tap into the manifold or carb.
I assume you're talking about a TR6; my Spitfire has only the retard.
However I can explain the principles behind each one. Perhaps that will
help. (I hope the implementation is at least close to the theory. :-)
The reason for having a vacuum advance is this. The more you open the
throttle the higher will be the density of the air entering the cylinders,
and the flame front spreads faster with this higher density. So if you had
optimized your timing for part-throttle you'd get pinging under full-
throttle. The solution is to set your timing for full-throttle operation,
which means that at part-throttle it isn't as early as it could be.
Now, this isn't a serious power loss since you aren't interested in power
generation at part-throttle anyway, but it might cost you fuel mileage. To
recover some of that lost power the vacuum advance pushes the timing forward
under part-throttle, and allows it to relax under full throttle. The vacuum
used for this is taken downstream from the venturi and throttle plate, and
theoretically can be anywhere in the intake manifold that isn't subject to
ram-pipe reasonances. It would respond more or less to the same reading as
shown on a manifold vacuum gauge: Open the throttle more and the manifold
vacuum goes down, and so does the "extra" advance that the vacuum unit had
added in.
(This, by the way, is the reason why smaller cars often dispensed with the
vacuum advance unit. When you're selling a car that weighs only 1700 lbs
and gets 37 mpg already, no one cares whether you've optimized your part-
throttle operation. But when the mileage is down because the car is a
monster V-8, or even a 2.5 liter fire breather with an air pump and EGR,
then you try to extract all the extra mileage you can.)
> I'd expect that the CD principle would cause the vacuum at the port to
> have a plateau...
This vacuum is not the same as the CD you mentioned. That CD is the air
pressure depression through the venturi. The vacuum in the manifold
downstream from the carb can't be "constant". For any given rpm the engine
cranks out the same displacement per time regardless of your throttle
opening. If you want to slow down by running less air through it then that
air has to be at a lower density, hence lower pressure, hence higher vacuum.
By comparison, the vacuum retard unit was meant to act at idle only. The
vacuum tap for it would be immediately downstream of the inside edge (i.e.
the downstream edge) of the throttle plate. This location causes it to see
vacuum only when the throttle is fully closed. As soon as the throttle
opens up to anything resembling normal running, that tap gets uncovered and
the vacuum disappears. The spring unit in the vacuum unit would be chosen
so that the retard action would be fully on with whatever level of vacuum it
experiences at idle, but then fall to fully off when that vacuum is removed.
The exact location of the tap is fairly important but a soft spring rate in
the vacuum unit lets it accomodate a wide range.
So the two vacuum units were more or less independent. The retard acted at
idle only, and went away quickly under power. Its purpose was strictly
emissions control. The advance would be fully on at idle, and gradually go
away as the throttle was opened up more and more. Its main purpose would be
increased fuel mileage, and possibly some response immediacy at part-
throttle.
Hope this helps. If you knew all this already, then just forget it.
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
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