Re: Airbox Mod

["Jouko Haapanen" <joukoh@vtoy.fi>]

-----Original Message-----
From: Gerard <gerard@poboxes.com>
To: Jeffrey J. Goggin <audidudi@mindspring.com>


  --snip--

>So, the mod of putting on a intake duct on the top of the hood would
>mean ramming air, right? Even if the duct is a flush mount multi-
>slotted thing which just breathes off the air flowing over the hood?
>Isn't mounting the duct at the front of the car same as ramming air
>into the system?
>


One should be careful in the placement of air intakes, as the pressure on
the surface of the hood may very well be negative due to boundary layer flow
at the surface...

My experience with the design and placement of air intakes tells me to place
the intake around the grill/bumper/headlamp area, and use the hood
(generally) for exhaust purposes.  There are exceptions (the 924 Carrera
comes to mind), but the placement and the design of the duct are important.
The Z34 certainly has the grilles for aesthetic purposes, but many cars have
the hood grilles for purposes such as intercooler exhaust flow.  A low scoop
certainly (I hypothesize now...) would not be the answer, but a scoop (hate
that word, sounds so hot-rod) elevated from the (hypothetically)
low-pressure area at the surface.  Maybe a NACA duct would do the job,
albeit at high speeds.  I recall there are equations one could use for a
NACA duct, although one would have to make a number of assumptions.  Even
when I worked at GM, as well as in our racing days, trial-and-error is what
it comes down to.  Arm yourself with a number of setups, measure air
pressure and temperature in the manifold under stead-state conditions and
make your conclusions.  Hell, I recall it being kinda fun.

Audi used the headlamp bucket of the Gruppe B Audi quattro, and as we all
know, this will do the job.  They were disqualified at Acropolis (if my
memory serves me correctly) for this solution after Mikkola (memory??) won
with Stig and Michelle taking the enxt two positions.  Hell, I might have it
all wrong.

Not Audi related, but Paul Frere writes about the testing of the Porsche 904
back a few decades ago, and the aerodynmic/chassis setup was done by the
driver(!!!) recording readings on what were essentially moving graduated
sticks mounted on all four corners.  No telemetry back then!


Jouko Haapanen
Pori, Finland