Rob't Allen-
Mr. Schooler is absolutely right. Listen to him.
Principal Bernouli
On Wed, 09 Apr 1997 19:29:33 -0700 Bill Schooler <schooler@erols.com>
writes:
>Robert Allen wrote:
>> (snip)
>>
>> By golly, you're sure right. MGs use manifold vacuum for their
>> distributor and the carbs are CD -- constant depression (or
>continuosly
>> drips) so there will always be vacuum on each side of the throttle
>> plate. Curious. That would imply that vacuum advance contributes to
>> total advance even under periods of moederate acceleration. From a
>> traditonal carb perspective, that is pretty strange.
>
>Bob, I've just gotta chime in here because your description of the SU
>carb as a
>constant depression device is incorrect. The ONLY place in the SU
>where there is a
>constant depression is right on top of the jet, where the venturi is
>formed by the jet
>bridge and the top of the piston. On the upstream side of that point
>is atmospheric
>pressure and on the downstream (engine) side is a varying degree of
>vacuum, depending
>upon the position of the throttle plate. If you take a real close
>look at an SU HS4
>carb, with the vacuum port on the rear carb, you will note that this
>port is just
>barely on the upstream side of the throttle disk, at idle. IOW, at
>idle, there is
>very little if manifold vacuum available at the takeoff for the
>distributor vacuum
>advance. (That's also why the piston rides low at idle - there's no
>manifold vacuum
>available upstream from the throttle plate to lift it.) Now, when you
>open the
>throttle, manifold vacuum is available to the distributor vacuum port
>on the carb and
>you get a bit of advance. Wider throttle opening, such as during
>acceleration, will
>increase the level of manifold vacuum to that port. This increased
>vacuum is also
>serving to lift the piston. Yes, the air flow has increased, but the
>cross-sectional
>area of the venturi has increased also, thereby preserving the
>"constant depression"
>at the jet bridge. The SU type carb is also known as a Constant
>Velocity device -
>again the reference point is the jet bridge. The air stream over that
>point has a
>constant velocity, but a varying size.
>
>
>Bill Schooler
>69 B/GT
>
|