Hi Paul,
You are pretty much correct on all accounts. The main purpose of
vacuum advance is to improve part throttle fuel economy for the reasons
that you state. It may also improve throttle response at lighter
throttle settings.
Regards,
Dave Russell
PG wrote:
> For all you weber guys...
>
> * I am fitting triple webers on my 3litre engine. * The distributor
> is a Mallory dual point.
> My problem is that with the isolated runner design of Weber, any
> vacuum I pull off of one of the manifolds is going to be pulsating.
> I know that I can fix this by designing some form of collector to
> combine the vacuums from the three manifolds and reduce the pulses.
> I also know that could put some form of damper on the single
> manifold line.
>
> However, when I was talking to a very knowledgeable person, he
> suggested that I simply not use a vacuum advance...that most weber
> conversions do not use it.
>
> Now, the vacuum advance is only going to kick in when vacuum exists
> which is going to be when the throttle is closed (or partially).
> At WOT or in a racing environment, Vacuum is likely to always be
> low and thus, I think he is right, the lack of a vacuum advance
> would not be noticed in such an environment.
>
> So I started thinking when I would actually notice this myself in a
> Highly Modified 3.2 litre Healey engine (overbored 200k) that is
> used on the street (as well as hillclimb/rally).
>
> The idea of vacuum advance is to advance the ignition when the
> flame front is likely to slow..this happens at high vacuum because
> vacuum is strongest when the throttle is closed in which case, the
> engine is not getting acoustic or inertial ramming and thus, the
> density of the fuel mixture is lower which creates a slower moving
> flame front.hence, more advance.
>
> I am coming to the conclusion that at the end of the day, I
> probably would not notice the absence of vacuum advance in a street
> environment except that I might burn a little rich at idle and
> cruise as some fuel might be left unburnt. That being said, with a
> long duration cam with a relatively small Lobe Separation Angle
> (lots of overlap), the engine is going to behave this way anyways.
> They really don't run efficiently until the come "on cam" at which
> time the vacuum advance is likely not engaged.
>
> Does anybody have any thoughts on this or experience that they can
> provide?
>
> Thanks
>
> Paul
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