No bother, You've just made me have to think, that's all.
It's not the volume of vacuum (i.e.how many cc's of air pulled
through per unit time) but the differential in pressure relative to ambient.
by placing the vacuum take-off ahead of the throttle plate and perpendicular
to it, Bernoulli's law applies the vacuum (a volume of gas/fluid moving at a
specific rate across an opening will pull gas/fluid through said opening at
a rate proportional to the relative diameters).Thus while the manifold
vacuum is near 0 at full throttle, the vacuum APPLIED to a perpendicular
hole at the throttle plate (the place of greatest restriction/highest air
velocity)is as high as possible relative to the engines' air demands. Since
the vacuum system isn't open ended, it doesn't change the airflow to the
engine, it just applies a differential signal to the membrane in the vacuum
advance mechanism, which is hooked (mechanically) to the advance plate in
the dizzy. If this is too technical, I can try again, but I think it's
pretty understandable
Does anyone understand differently? Am I missing something?
Mark Haynes
> -----Original Message-----
> From: Richard Shipman [SMTP:mbelect@mindspring.com]
> Sent: Wednesday, August 04, 1999 8:42 AM
> To: Haynes, Mark
> Subject: RE: Timing the 1275 ? (vac. advance with a sidedraft)
>
> Hi Mark,
>
> I'm really trying to get a good handle on this...
>
> Isn't the vacuum level exactly the same inside the manifold and just in
> front of the throttle plate at full throttle, since there's no
> restriction?
>
> If so, then how can placement of the vacuum pick-up effect full-throttle
> advance?
>
> Also, is the (manifold) vacuum pick-up placement on my '74 Midget just
> plain
> bad engineering? Does it work backwards?
>
> Sorry to be a bother...
>
> Richard
>
>
> At 07:47 AM 8/4/99 -0600, you wrote:
> >Manifold vacuum decreases as your throttle plate opens. The dizzy wants
> full
> >advance at full throttle. if you use carb vacuum,the dizzy gets an
> >appropriate indication of the engines running demands otherwise, you get
> >less advance at full throttle, and more at idle . just the reverse of
> what
> >you relly want.
> >Mark Haynes
> >'62 Sprite XSP motor
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