Cyrille:
Vacuum advance is applied only during cruise (moderate throttle)
and serves to improve fuel economy (only). A typical amount of advance
would be 12-14 degrees IN ADDITION TO whatever mechanical advance is
already built in to the distributor.
During cruise, the manifold vacuum is very high, and the
cylinders are only partially filled, effectively lowering the
compression ratio. The reduced air/fuel density in the cylinder burns
more slowly, and lost efficiency is the penalty. By advancing the timing
over and above the mechanical advance, some of that lost efficiency can
be regained, and mileage will improve. Vacuum advance cannot be applied
during heavy acceleration because the engine will ping.
There is no performance advantage to vacuum advance, it is
strictly an economy device.
Vance
-----Original Message-----
From: owner-spridgets@autox.team.net
[mailto:owner-spridgets@autox.team.net] On Behalf Of de Brebisson,
Cyrille (Calculator Division)
Sent: Wednesday, April 19, 2006 10:20 AM
To: David Lieb; Spridget List
Subject: RE: Distributor and Carburetor problems...
Hello,
So what you are saying is that the vacuum 'timing tinkering' is designed
to reduce the effect of the mechanical advance under heavy load to avoid
pinging?
My understanding was that it was to increase the advance in order to
provide more power when the load increase...
Or maybe it depends unpon the amount of mechanical advance provided by
the distributor...
One thing is sure, is that you always want to maximize the advance (just
bellow knock zone), so if your mechanical advance is 'sloopy', you will
want to use a vacuum advance to increase the advance under load to
maximize power
If your mechanical advance is too aggressive, then you will want to
decrease the advance under load to minimize pinging...
I guess what I need to see is a "normal" advance curve (let us say at
every 500rpm) and compare it with what my car is doing...
Does anyone have advance curves data around?
Thanks, cyrille
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