Yes, the has SU has variable venturi size. The needle is there to change
the metering jet size as the venturi size changes (thus maintaining the
same ratio at different openings). In this way, the variable venturi carb
becomes many different carbs depending on vacuum.
For this reason, the variable venturi carb approaches fuel injection as
regards to flexibility and driveability.
The fixed venturi carb uses only one metering jet size for all vacuum
settings (albeit with certain power boosters, etc. to take up the slack).
The advantage to fixed venturis is simplicity of tuning for WOT (wide open
throttle).
Marcus A. Pryor
For a new age of reason
> Correct me if I'm wrong, but webers or mikunis use fixed ventury sizes
> whereas the SU needle changes profile depending on amount of vacuum and
> air velocity. How do they differ in maintaining correct air-fuel ratio
> when the fixed venturi has one variable(air velocity) and the SU's have
> 2 (velocity and needle profile)?
>
> Steve
> 70' 1600
> ----- Original Message -----
> From: Marcus A. Pryor
> To: datsun-roadsters@autox.team.net
> Sent: Tuesday, June 06, 2006 12:59 PM
> Subject: Carburetion.
>
>
> The first thing lost is velocity of the air stack. This brings in the
> equation Bernouli's principle; that is, air pressure is inverse to
> speed. Since a carburetor requires a venturi in order to create a
> vacuum and pull fuel in, this creates a natural block to the air
> column. If that venturi is too big, the engine will be severly
> hampered, resulting in erratic fuel ratios.
>
> Ideally, the CFM of a carburetor should take into account the uses of
> the engine and rpm range. There are numerous methods used for this
> calculation, and a simple Google search will find various formulas for
> this calculation.
>
> Marcus A. Pryor
> For a new age of reason
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