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 BLOW
HARD!
Darren Mayer
answers your questions about supercharging
10/9/03
I was wondering if you can help me understand something about
superchargers. I have on many occasions read that a fixed displacement
king of supercharger (i.e. roots, twin screw) needs the throttle body
placed before the supercharger. Why is this so? Why can't one place the
throttle body after the compressor and have the intake side of the
supercharger suck air right for the air intake (filter)?
I would think that this would work better, as you would not starve the
supercharger for any air.
Shane
Dear Shane,
At first it would seem that
one could place the throttle blade after the
air pump, however, when one considers the working
principle of the positive displacement type
supercharger, it becomes clear that this would
not be the case.
The positive displacement air
pump will displace the units swept volume per
unit revolution as long as there is a path for
air delivery. The
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control
of this displacement must be the throttle blade
(air path limiter), as full airflow is not needed
at particular engine conditions.
If one did place the throttle
blades after the positive displacement pump,
one would find full unit capacity at the throttle
blade. This would lead to many problems, however.
The first would be the ability to open the throttle
blades. As the blades will become pressurized
against the throttle's idle stop, the driver
would find it impossible to open the throttle
and also difficult to control once opened.
Other problems after the throttle
will arise as you consider part throttle air
control and other impossibilities that would
happen if the throttle blades were mounted on
the discharge side of the supercharger.
The non-positive style superchargers
work with post throttle blade positions because
the superchargers are not positive displacement
in design. Centrifugal superchargers/turbo-chargers
use diffusion priciples as a means of air movement
and this theory requires turbine rpm as an ingredient
to promote air movement and pressurization.
Hope this doesn't confuse you
more.
Darren Mayer
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