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.

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