[s2man]

A very good read. I was surprised by the negative X velocities. Especially beneath the truck, where I would expect the high pressures in front of the truck to force air through the engine bay. and maintain some positive velocity. Never mind, he didn't model that - the front of the truck is solid. I noticed the air velocity beneath the truck was about 0 when he raised the dam 4" off the ground. I suspect a full dam with air exiting the engine compartment beneath the truck would also equate to minimal air velocity beneath the truck.

I was also surprised, at first, the air dam had a much greater affect than the bed cap. But then I realized the air was already flowing in our beloved tear-drop shape over the dead air spaces created by the cab and tail gate. And the real issue with the 4wd is all that exposed running gear underneath, which the air dam fixed quite handily. And it was quite interesting that while the cap "provides a negligible benefit on its own", adding it to the air dam raised his FE increase from 7% to 21%. So the whole of our aero mods is greater that the sum of the individual mods.

This has encouraged me to try a full air dam this spring. It seems easier to construct than a full belly pan. But I note, the optimized NASA box truck has a full belly pan.