I have to question this, Barney. The manifold and ported setups do use
distributors with different serial numbers, but my understanding is that
this relates to the springs, weights, cam-plate and vacuum capsule - the
distributor type e.g. 25D or 45D is the same for both and the remainder of
the parts are common to many serial numbers both UK and North American
emissions types according to my Lucas parts book. My understanding is also
that both types advance the dynamic timing from the static - the difference
being that manifold gives maximum advance at idle whereas ported gives zero
advance at idle - which is why timing adjustments are done with vacuum
disconnected and plugged. As the throttle is opened the manifold vacuum
reduces while the ported increases and the two types converge quite rapidly.
After that they exhibit the same effects i.e. reducing vacuum (and hence
advance) as the throttle is opened further. The vacuum capsule compensates
for the fact that the tendency for pre-ignition increases as cylinder
pressures increase i.e. throttle opening increases, by reducing the amount
of advance. By running with high advance at idle you get maximum idle speed
for a given throttle opening, and because the idle throttle opening is then
turned down to give the correct idle speed you reduce emissions - and
emissions is what manifold advance was all about. Why we ever had ported in
the first place I don't know - except possibly that the manifold-connected
capsule spends more of its life at higher vacuums and hence had a shorter
life.
PaulH.
----- Original Message -----
From: Barney Gaylord <barneymg@ntsource.com>
To: R. O. Lindsay <rolindsay@dgrc.com>
Cc: <mgs@autox.team.net>
Sent: Thursday, February 01, 2001 7:39 PM
Subject: Re: Vacuum Advance
> The method ported from the manifold uses a different distributor, which
can
> be thought of as "vacuum retard" distributor. The vacuum from the
manifold
> is high at idle and holds the distributor in a retarded position. When
you
> open the throttle the manifold vacuum drops, allowing the timing to
> advance. You need to use whichever port is appropriate for your
> distributor. Or conversely, for those who do not have both ports, you
need
> to use whichever distrubutor is appropriate for your existing port.
>
> Barney Gaylord
> 1958 MGA with an attitude (and no vac Mallory dizzy)
> http://www.ntsource.com/~barneymg
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