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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