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RE: More Carb discussion

To: <mgs@Autox.Team.Net>
Subject: RE: More Carb discussion
From: <>
Date: Tue, 17 Jun 1997 17:50:05 -0400
Bill Eastman wrote:

> I love a good technical discussion!
> ... [snip] ...

So do I ...

> ... [snip] ... Concerning the design ingenuity of the SU carb.  I agree
> that anything that does a better job is considerably more complex.
> The SU is a very elegant solution to a very complex problem.  Plus
> they look really cool when compared to the more mundane mixing
> devices.

Agree ...

> However, they still have no way to compensate for engine load.  They
> simply mix a specific amount of fuel for a specific velocity of air.  You
> choose the mixture you want- say 110% and that is the mixture that
> is delivered no matter what (theoretically).

Your loosing me ...

> The signal that they use to position the needle is outside of the throttle
> butterfly and, therefore, does not compensate for changes in manifold
> vacuum.  Manifold vacuum is a primary indicator of engine load. ... [snip] ...
> SU's do not use manifold vacuum as an input signal so they _cannot_
> adjust the mixture for engine load.

Ya just lost me ... the suction of the engine creates a vacuum or pressure
differential between the downstream and upstream sides of the piston ...
the pressure differential is transferred to the piston which rises or falls to
maintain a constant air velocity and pressure differential across the jet.
IOW, the manifold vacuum on the downstream side is transferred to the top
of the piston and atmospheric pressure is applied to the bottom of  the piston
so the piston rises or falls until a balance between the two is achieved. So
either I'm missing something or SU's do use manifold vacuum as an input
signal so they _can_ adjust the mixture for engine load.

> SU's do provide an accelerator pump function with their dashpot dampers. 
> This provides more fuel during a transition between lower airflow and
> higher airflow conditions.  This is a relatively short term enrichment to
> make up for the fuel that runs along the bottom of the manifold taking
> longer to reach the engine so, without an extra boost, the engine would go
> lean and stumble on tip in.  This eliminates the hesitation often
> associated with throttle response but it does _not_ enrich the mixture
> under load.

Agree ... it provides more fuel during a sudden increase in manifold vacuum.

... [snip] ...

> Regards,
> Bill Eastman Can I go home now?  My brain hurts!
> 61 MGA with beautiful, simple, elegant, perfectly acceptable twin SU's

My brain hurts too ... I'm going home ... ;^)

Safety Fast! ...
'61 MGA 1600 MkII

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