Getting started with Aerodynamics

[MadBill]

Putting last things first, I've done a fair amount of testing re engine air intakes, often as a result of race team debates on cowl induction Vs. ram air. Depending on the windshield slope, details of the ram system, etc., the former has usually shown only 45-55% as much pressure as the latter. BUT ram air fed through through long, corrugated and convoluted hoses can suffer a lot of pressure drop, not to mention possibly picking up hotter air from ground level or during the trip to the air box.
Rather than trying to measure the actual pressure at the potential air sources (complicated by varying reference pressure in the cabin, unless you use a correctly-positioned static pressure pick up), it's easiest to use something like a Magnehelic gauge: http://www.dwyer-inst.com/Products/P...eName=Ordering (10" W.C. is plenty for comparisons, 20 for total ram pressure) to compare any two candidates.

The higher a test speed you can use the better, but the results should be representative even at highway speeds. One trick I have found very useful for an existing ram air system is to compare the air box pressure at WOT Vs closed throttle at the same speed. If the pressure spikes substantially when the throttle is closed, it indicates excessive pressure drop in the system.

A couple of other points: Rear spoilers do not universally increase drag. In fact on most cars, some modest height of same (usually ~1-2") can increase downforce and reduce drag (check the rear edge trunk contour on various high-end sedans) At greater heights the drag starts to increase again, but downforce goes up substantially.



As far as front air dam height, it is usually a matter of how rough the underbody is (very, for 99% of vehicles) and how low the worst bits hang. As a rule of thumb, a dam ~1" lower than the worst lumps is about the best compromise, absent extensive coast-down tests.

As Ray says, for most cars the turbulent flow under the car means the effective frontal area extends right to the ground, so lowering the car reduces the frontal area and drag.

Another item worth investigating: Vortex generators (e.g. from aircraft supply houses or: http://www.airtab.com/ ) Similar devices are used on the roof of some Mitsubishi EVOs: http://www.mitsubishi-motors.com/cor...004/16E_03.pdf and tested on a race car, we found they had a pronounced effect re 'steering' the air off the roof onto the rear wing.

One last note. Ecomodder: http://ecomodder.com/ has an astounding array of info on drag reduction.