Clive writes: " .. but if you have read ALL previous you'll know this -
UNLESS THE EXIT VENT IS AT A LOWER PRESSURE THAN THE INLET THEN THE FLOW
RATE WILL BE COMPROMISED OR NEGATED."

This is certainly true in the water-cooled version, in which the "exit
vent" is a conglomeration of spaces around the motor though which the
air meanders.

In the air-cooled (my primary reason for discussing this subject) the
cooling fan is creating negative pressure (vacuum) even at low speeds,
and forcing the air through a sealed system of ducts, and out the rear
of the van. I don't have any data on whether the Vanagon side ducts keep
up with the cooling fan or there is an imbalance (pressure or vacuum) in
the sealed area above the motor during highway speeds.

I know the output volume of the cooling fan is gargantuan, but have no
hard data on amount of flow at speed, versus incoming air
(pressure/vacuum) in the ducts. So I'm dropping the duct subject.
Alleluia. Amen.

Developing business and guiding change since 1996,

G. Matthew Bulley
Bulley-Hewlett
Marketing & Communications
Business: www.bulley-hewlett.com
AIM = IExplain4u
Phone: +1.919.658.1278