Being a computer scientist, which makes me not a scientist but a logical
thinker, I suggest we consider both points right in that there is a tipping
point.
Normally configured air cleaners impose resistance on the airflow
trying to get to the Venturi. Consider that cold air boxes and venting from in
front of the grill lessens the resistance on the airflow allowing the Venturi
to operate at peak efficiency, increasing the vacuum in the Dash Pot and
raising the needle.
Let's also consider increasing the configuration to push
more and more air into the carb will reach a tipping point where 0 resistance
tips over to positive pressure. At this point the vacuum drops as does dash
pot leaning out the mixture.
Again, as a computer "scientist", I am not bound
by imperial data.
Greg
--- On Sat, 1/21/12, Bob Spidell
<bspidell@comcast.net> wrote:
> From: Bob Spidell <bspidell@comcast.net>
>
Subject: Re: [Healeys] 100M cold air boxes and flow
> To: "Greg Mandas"
<gmandas@yahoo.com>
> Cc: "Greg Lemon" <glemon@neb.rr.com>,
"<healeys@autox.team.net>" <healeys@autox.team.net>, "Adam Nolde"
<adamnolde@yahoo.com>
> Date: Saturday, January 21, 2012, 11:00 PM
> re: ' ...
wouldn't the vacuum in fact
> be less, because it wouldn't be sucking from the
engine,
> well it still would but it would be pushing air through as
> well,
and maybe a push doesn't suck the gas up as well as
> regular manifold vacuum,
therefore...'
>
> Remember the jet of an SU is located upstream of the
>
throttle plates, so there is no engine or manifold vacuum to
> speak of around
the jet. The vacuum--or 'depression'
> as the Brits call it--is caused by
Venturi Effect of air
> flowing between the bottom of the vacuum piston and
the jet
> bridge. As the airflow increases, the 'port
> vacuum' formed as
air passes over the bridges and under the
> pistons--similar to the port
vacuum that changes distributor
> advance--causes the pistons to rise, in turn
mediating the
> lower pressure in this area. This is why SUs are
> referred
to as 'constant depression' or 'variable venturi'
> carburettors, as opposed
to 'American' style carbs which are
> 'fixed venturi' type (that's why they
need several jets
> instead of just one and usually need other mixture
>
augmentation schemes).
>
> Note that there are two holes in the bottom of an
SU vacuum
> piston, and--usually, I think some carbs don't--there is a
>
machined channel downstream of these ports so as to avoid
> any extra
turbulence or pressurization in this area.
>
> I suspect--only
speculation--that increasing pressure and
> hence airflow in this region
disrupts this balance, quite
> possibly resulting in the lean-running Chris D.
describes.
>
> Bob
>
>
> On 1/21/2012 7:11 PM, Greg Mandas wrote:
> > Ah.
That makes sense.
> >
> > Greg
> > 65BJ8
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