Cool! I can argue with Barney without Larry's help!
Barney Gaylord wrote:
<snip>
> 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.
> As I recall, the primary reason for using the manifold vacuum retard
> setup is to retard the timing dramatically on overrun to prevent
> backfire.
Hmmm. I was thinking just the opposite. Perhaps I have my definitions
screwed up. Ported vacuum is usually taken just behind the throttle
plates of normal carbs. Vacuum only present at part throttle openings.
Thus *high* at idle, moderate at cruise, and negligible at full
throttle.
Somebody figured out that, as long as the engine isn't being stressed
(low throttle openings) the engine can benefit from additional advance
thus vacuum advance from a ported source.
Manifold vacuum is taken from the intake manifold, farily close to the
intake valves, thus insuring maximum available vacuum all the time. This
is to the benefit of vacuum operated accesories -- power brake booster
on LBCs.
Someone step in and count votes.
> A hot cam may tolerate up to 36 degrees advance for a racing setup, but
> often a radical cam cannot take much spark advance at low speeds. So, if
> you install a wild cam in your street machine without changing the dizzy,
> you may have to back off the idle timing to zero (TDC) or even a few
> degrees ATDC.
Yep. Agree there. With my Mallory, no vacuum advance, 3/4 cam, and
Webers, I'm running 8 degree at idle and maxes to 36 degrees at 3500
RPM. No ping, plenty of umph.
Then I've heard the DFI (Digitial Fuel Injection) folks check in and say
the more timing advance needed the lousier the carburation. Logic being
that if you just throw the gas in with the air, you have to get the fire
started pretty early to get any bang when the piston is ready to turn
back the other way. My response to them is :P*****************
Bob Allen, Kanas City, '69CGT, '75TR6
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