On Mar 3, 12:09*pm, John Henderson > wrote:
> Bill wrote:
> >> Note that there will be greater vacuum at partial throttle than
> >> while accelerating.
> > *John
>
> > Explain?
>
> For a given RPM, the inlet manifold vacuum is greatest with the
> throttle butterfly closed (foot off the pedal) and least with the
> butterfly fully open (foot to the floor). *That's because the
> butterfly directly closes and opens the air path into the
> manifold
Understood, however since the vacuum advance port is above the
butterfly It's the transition from high vacuum (intake manifold ) to
atmospheric pressure which allows a momentary vacuum signal to the
advance port in the throttle bore. The key to remember is that it's
momentary, not sustained as would be the case using an actual intake
manifold take-off (i.e. like the vacuum brake port on a later bus.) On
a dual vacuum dist. the advance port is above the throttle and the
retard port below the throttle valve.
> The vacuum tube to the distributor is taken from a point which
> reflects manifold vacuum. *To get maximum power and economy for a
> given amount of fuel, you want maximum ignition timing advance
> without the risk of pre-ignition/pinging/pinking. *And that means
> less advance at full throttle.
>
> The alternative to vacuum advance is centrifugal advance, where
> the degree of advance is governed by engine speed alone. In
> that case, the advance curve is designed for full-throttle
> operation, and economy at partial throttle suffers slightly.

>
> VW industrial (stationary) engines were always fitted with
> centrifugal-advance distributors, and these distributors were
> much sought after by performance tuners whose modifications
> were likely to upset the vacuum advance curve.
> John

SA type distributors are best suited to the industrial motors which
tend to spend most of their time at constant RPM or competition
engines which are usually in the high rpm range. This is why the 009
is best suited to competition use and can't provide the initial
advance at low rpm need for smooth acceleration. Also, aggressive
camshafts often don't produce enough vacuum for SVDA type distributors
to properly function at low rpm.>