Grill Block question?

[wyatt]

Ok, my bad... I shouldn't have tried to post anything in as little as 2 minutes. I think the graphic you guys are looking for is this one.


I will make my argument again, but this time I will try to make more sence (although I am posting quickly)
I will equate #20 to a grill block blocked from behind; Cd 2.30. Is this right? No. #20 is taking into account the separation around the edges which pushes up the Cd, but we will say this is conservative.
I will equate #18 to a grill block blocked from the front; Cd 1.98. Once again it's taking separation effects into account that are not there on a car, but once again, it's conservative.
I will equate #14 (or #13) to Aerohead's redesigned truck nose or the Aerocivic's redesigned car nose, where the shape just gets redefined; Cd 1.16 (or 1.20). This is where you see the big gains.

Between #20 without edge effects and #18 without edge effects, you will be hard pressed to see a difference, since you are talking about a 15% difference on maybe 1 or 2 square feet on the front of a car. Reshaping the nose will make a big difference, but some don't like the idea of losing the auto's original sporty looks.

This is what I was trying to say. I chose a bad analogy, saying that a flat plate like #7 Cd 1.17 would slow you down less than #9 Cd 1.42, but would slow you down all the same. Here the difference is about 20%, and in the overall size of the openings we have on our autos vs. everything else that's going on, it's going to be hard to see the difference.

[davidgrey50]

Winkosmosis - Yes, racing cars commonly vent a front-mounted radiator through a hood vent - check out a Ford GT40 or most any racing car. If you look at the flow diagrams here and there on EM you'll notice that most cars have a huge high-pressure area right at the nose, and a strong low-pressure area just aft of the leading edge of the hood. Put the intake in the high-pressure area and the outlet at the low-pressure area, and seal up that underbody!

The low pressure over the front of the car was more-or-less discovered by Bruce McLaren in the early 60's when he was testing a racing car. Someone hadn't fastened a flap and, to his surprise, rather than being held down by the airstream, the flap lifted more and more as speed increased.