>Steve Maher recommends opening the engine hatch to let the engine run 
cooler.
>I'll buy that at low speeds, but I have to picture that at highway speeds
>the area of the breadloaf engine hatch or Vgon license plate door is a low
>pressure area that would actually make your engine work harder to breathe.
>Certainly the amount of schmutz that a van's tail wears when it's running
>rich is an indication that the area doesn't flow a lot of air.
>
>Anyone got engineering input as opposed to my guesswork?

No engineering input from me, I'm afraid, Bill... even though I are one.
Just the experience at both low (<30) and high (<55) speeds, that
my engine started pinging and smelling hot while towing (mostly on freeways)
with the hatch closed, but stopped fairly quickly once I opened the
engine hatch.

My opinion on why (yes, guesswork now), is that an overworked engine
starts heating up the air in the engine compartment, and doesn't inhale
it into the cooling fan and out the bottom of the bus, fast enough to
get it replaced thru the upper vents with cooler air. So you get warmer
air flowing over the cylinders, which causes the engine to run even
warmer, and around and around in a vicious cycle that results in
significant overheating. Opening the hatch scavenges that hot air out,
in my opinion.

Yes, this isn't proof. But it works for me.

 -Steve Maher