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Re: Carb/Emission Control--Follow Up

To: "Trevor Boicey" <tboicey@brit.ca>, "David Massey" <105671.471@compuserve.com>
Subject: Re: Carb/Emission Control--Follow Up
From: Paul Tegler <ptegler@cablespeed.com>
Date: Tue, 25 Sep 2001 19:24:36 -0400
Cc: <Harrymague@aol.com>, "[unknown]" <triumphs@autox.team.net>
References: <200109251520_MC3-E10C-25C0@compuserve.com> <3BB0EEE8.4F575B32@brit.ca>
...probably one of the better worded descriptions I've seen
describing the true and correct 'physics' of what is going
on.
But to the huddled masses.... the terminology used
previously
is good enough.

Just remember... everyone here doesn't necessarily feel
comfortable with a tool in their hand, let alone really know
the 'inner' workings of our LBC's.   :-)

Paul Tegler      ptegler@cablespeed.com
www.teglerizer.com

----- Original Message -----
From: "Trevor Boicey" <tboicey@brit.ca>
To: "David Massey" <105671.471@compuserve.com>
Cc: <Harrymague@aol.com>; "[unknown]"
<triumphs@autox.team.net>
Sent: Tuesday, September 25, 2001 4:54 PM
Subject: Re: Carb/Emission Control--Follow Up


> David Massey wrote:
> > The level of vacuum at this port is the depression at
the venturi.  This is
> > the vacuum that draws fuel through the metering jet into
the air stream.
>
>   I am afraid this is very much NOT true.
>
>   It is not the intake vacuum that pulls fuel through
> the jet. The intake vacuum pulls air over the top of the
> jet, and the venturi effect pulls fuel in from the
> jet. (Bernouilli's principle)
>
>   The intake manifold depression is created by the
> pistons and held between them and the throttle
> butterfly. The jet is on the OUTSIDE of this
> area, before the throttle. Other than the restriction
> caused by the air filter and air trunking, the
> air pressure at the jet is atmospheric.
>
>   Therefore, the "vacuum" at the jet is only there
> because of the air flowing over it, not from any
> vacuum caused by the mechanical action of the engine
> itself.
>
>   Case in point, the intake manifold depression is
> highest at idle, and lowest at full throttle. Obviously
> the fuel flow is LOWEST at idle and highest at full
> power, which would not be the case if fuel was pulled
> by this vacuum.
>
>   Fuel flow is kept roughly proportional to air
> flow by the venturi, with two exceptions. The oil
> in the dashpot piston causes the venturi to stay
> smaller-than-usual during acceleration, which raises
> the venturi effect and increases fuel flow.
>
>   As well, the taper of the needle compensates for
> non-linearities in the venturi effect over different
> airflow rates by effectively increasing the jet size
> for higher airflow rates.
>
> --

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