Throttle Plate Control

Fri May 12 14:24:57 GMT 1995

>Ed Lansinger <elansi01 at> writes:
>At the Detroit Auto Show in January I observed the Lexus 4.0l motor had
>two distinct throttle plates of identical size in series in the throttle
>body, the second apparently actuated by a servo, the first mechanically.
>It appears that engine has a true secondary throttle plate.  I have no
>idea what it is for ...

Ford has similar setup on large displacement engines (5.8l and 7.6l). I was
told the reason is because the air flow through a butterfly valve is non-linear
to the angle of the butterfly, and the situation gets worse as the diameter of
the butterfly gets larger.  In laymens terms, for a large bore throttle, 70-90%
of the air flow through the throttle may be achieved at less than 50% of the
throttle travel and going from part throttle to WOT has very little effect.

By using two smaller bore throttles that are staged (ie. have a progressive
linkage), then a more linear throttle to air flow correlation may be achieved.

As for distributors, it seems the only good reason to have one (not two) these
days is to drive the oil pump !


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