I believe that the only difference between ported vacuum and manifold vacuum
is at idle.
With ported vacumm, since the throttle plate is closed, there is little or no
vacuum
at the carb although there will be 15-20 inches at the manifold. However, at
higher revs,
the amount of vacuum is constant regardless of source as the throttle plate
is no longer
blocking it. This ported vacuum was purely an emissions strategy so using
the manifold
source will be fine (make sure that you have a vacuum advance dizzy and not a
retard unit
and never share vacuum source with the PCV).
Assuming you have a stock engine, you should set your initial timing (with
vacuum
disconnected), to give you around 35-38 degrees total mechancial advance at
3500 rpms. The vacuum advance on top of that at high vacuum conditions
(cruise)
will give you better fuel economy. The vacuum advance will drop out at low
vacuum
(acceleration) preventing detonation.
Best!
Greg Rowe> Date: Mon, 26 May 2008 08:56:48 +0100> From:
rbgosling@googlemail.com> To: bill@gingerich.us> CC: spitfires@autox.team.net>
Subject: Re: [Spits] Vacuum port> > Bill wrote: "...While I'm not an expert in
vacuum dynamics, I would think in> this case that> vacuum is vacuum..."> >
With the proviso that I am not an expert on this area either, I do think>
there is more to it than that. Based on vaguely recollected forum messages>
I've read over the years, you get a different vacuum at the carb as you do>
from the manifold - different strengths of vacuum at different combinations>
of revs and throttle. Some distributor set-ups are designed to work from> carb
vacuum, others from manifold, it's quite possibly different for the> UK-spec
SU-fed engines and the US-spec emission controlled engines. Some> US-spec
engines I think even had a vacuum retard rather than vacuum advance.> > The
long and short of it is, I don't know the answer to your question, only> that
I don't think it's as easy as plugging your distributor to any old> place
where there will be a vacuum.> > Richard>
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