Effectiveness of RAM induction?

[Craig Dotson]

> Now here is what I'm planning to do...
> From the TB, I'll go through a hot-wire MAP.  I'll have to build a circuit
> to make the MAP look like the AFM for the ECU.  From the outside side of
the
> MAP, it's going to trumpet out to about 8"x6" and bend up toward the top
of
> the engine compartment.  This 8x6 intake box will come up through the
engine
> hood, where the air filter will mount.  Then I'm going to have a duct,
which
> is hinged over the rear window.  This duct will continue from the air
> filter, up and over the rear window.  I'm toying with the idea of having
the
> duct trumpet out yet some more to about 6" height and 36" wide, right
above
> the passenger compartment.

You want a "scoop" of sorts just above the roofline, right above the rear
window?  I don't think that will really gain you a whole lot.  Your best bet
is to measure air pressure at speed at various point where you could place
an intake scoop.

A friend of mine at school here has an MR2 with a sunroof.  If he un-latches
the sunroof, at 55 mph it will intermittently pop-up in back, due to low air
pressure above the roof of the car.  If that's the case, you might not get
ANY "ram-air" effect with a roof-mount scoop due to decreased air pressure
at speed.

I'll try to find the link, but I think it was the Australian AutoSpeed
website that recently did an article on placing a pressure switch at various
points on a vehicle and measuring the speed at which pressure was reached to
turn the switch on.  Choosing the point on the car that turned the switch on
at the lowest vehicle speed is the point where most pressure would be
generated at higher speed (assuming a bunch of things, but for drivable
speeds it's likely true).

Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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