In a message dated 10/19/04 11:56:43 AM, al_knoll@PACBELL.NET writes:

> In a recent discussion with another listerine, I made the guess that the
> pressure at the rear plastic grilles is negative and would pull air from the
> engine compartment to the outside.� Note the recessed nature of the cavity
> and the location of the grilles and although it may be cosmetic the
> direction of the grille louvers themselves.
> 
> So does it go in or out at "speed"?
> 
> 
Pensioner (with implied unsated curiosity and excess time on palms)

Your question about rear pressure issues is an excellent one, but may impugn 
the VW stylists with more cognitive insight than was the true collage of 
facts. But the details are easily monitored. For example, in the antiquated 
archives (may they last forever) the interested will find long winded soliquies on my 
part about measuring the ram effect of the intake scoops, the distribution of 
pressures and air temperatures in the engine compartment, the two air input 
sides, the area ahead of the transmission between frame members and the cavity 
above the transmission. All experiments relevant to my '82 Diesel Westfalia 
configuration. I did this pressure mapping to find the best place for the 
oilcooler and to design my input air system used for the 1.8 L Digifant conversion. 
These measurements have also helped me locate the air-to-air intercooler (for 
the turbo 3A and TDi projects) and the oil-to-air oil cooler arrays in the 
rear bay. Both of these heat exchangers vent to the rear with moderately 
optimized shapes for the deflector shield covers to minimize the pressure at their 
outlet. As I explained before, the intercooler and oil coolers are bolted to the 
right and left flat bases (now with 8 inch holes) behind the tail lights. The 
airflow from the side intakes is sealed to flow through these channels. Under 
the coolers (intercooler on the driver's side, oil coolers on the right) I 
mount small (8 inch) radiator puller fans to draw the air down. The covering 
shields enclose the structure, direct air to the rear and eliminate road kill 
deposition on the fans from the tire swill. 

The location of the rear heat shield, the sound shield pan, and the muffler 
all effect pressure and temperature. To whit, to measure these pressures you 
need only to procure a manometer calibrated in inches of water. Typical 
pressures and vacuums are in the range of 1 to 10 inches of water, positive or 
negative. The best instruments are made by Drywer. Connection to the measurement 
point is easily made by 1 to 2 mm ID PVC tubing or nylon tubing in the exhaust 
area. Long runs of tubing have little effect, but I shorten the distance and add 
recording capability by enlisting neighborhood individuals once termed urchins 
(often my own progeny) and locating aft of the rear seat back. Entertainment 
for a drive to Texas!

Now cost you might say. These gauges retail in the $500 to $1,500 range at 
times, but can be had for minimal pfennig (I pay $10 to 20 per copy) at C&H 
Surplus in Pasadena, CA. They have a web site and do mail order. I'm sure there 
are many other options for few interested parties. At one point, faced with a 
pocket rebellion of the socially overstimulated offspring, I set up my video 
camera for single operator saturday morning measurements. 

BTW, huge impact on pressures with sheets of Aluminum deployed as bellypans. 
Very interesting measurements with covers over the luggage cell top of the 
cab. Finally, following an article at www.autospeed.com (you will have to pay) I 
often rig these gauges when I want to design intake components, intake and 
exhaust manifolds by assembling a cheap flow bench with an electric leaf blower. 
Probably will set this up again when I do the intake plumbing for the TDi 
install.

Sorry for the rambling, its genetically based,

Frank Grunthaner