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Old 06-03-2009, 06:57 PM   #1
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Default Attention aerodynamics people..need opinion

OK, so i've been contemplating making a front undertray for my car...and i've got a few ideas that need some insight.

I know that less air under the car creates downforce because of the low pressure, I also understand that a protruding front splitter helps "catch" air from going under the car nd increasing the low pressure under the nose. My "problem" is the Garage Var bumper. You see, the leading edge is already much lower than a stock bumper, so it's already blocking air from going under the car. It also has a nice airguide bother on the upper side of the mouth and the lower part of the mouth..it actually goes all the way up to the intercooler and there is about a 1/4" gap between the guide and the intercooler...although a piece of rubber/foam weather stripping would seal this off... a future plan.

What I am wondering is if i'm already getting a good low pressure area because of the void created by the lower edge of the bumper and the relatively flat area between the airguide and the OEM belly pan. Or would it actually be better to smooth out the air from the bumper all the way back under the engine (I am limited to going as far back as the center of the front wheels per SCCA SSM class rules..which I am not trying to break). OR - should I just a protruding splitter to the bumper and NOT go all the way back?
The problem I see with having a protruding splitter and going all the way back is that the bottom edge of the bumper kinda points up... or if you want to look at it the other way, the edges angle down slightly as the get to the side. I'm wondering if that would have much effect on air pressure because a protruding splitter would have a slight angle up....but then again, I don't know how it would end up because as you can see in the pics there is a flat lip on the bottom of the bumper that would probably point any attached splitter straight out.

So.... what do we think here?

Area of "void" created by low bumper edge and air guide.



From the front edge to the belly pan is pretty even. Don't mind the angle aliminum bracket..it's pushing up on the airguide to help keep it from flexing...I plan on making a 'cleaner' version of this.



And the angle of the bottom edge of the bumper...you can see how it gets lower at the outsides (closest to the wheels)

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Old 06-04-2009, 12:54 AM   #2
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the further back, the lower the pressure under the hood. Just use some alumalite and make a bend if you have to. I've seen a ton of splitter set-up and they don't all go that far back because if you duct, and to a tray roughly 6-9" back you'll have enough cooling.

Leave what you have and put a splitter on it and it will move air around the car. Air that goes through the heat exchangers creates lift...its a trade-off.
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Old 06-04-2009, 01:16 AM   #3
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Wouldn't creating more low pressure under the car help pull air out the back/bottom of the engine bay? I ca understand the high pressure area right in front of the intercooler as air backs up...but once through i don't see why it wouldn't have a easy time escaping out the back.

Luckily there is a sign shop next to my neighborhood that has alumalite on hand. $140 for a 4x8 sheet...not too shabby.
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Old 06-04-2009, 01:57 AM   #4
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yes, we're on the same page.
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Old 06-05-2009, 12:53 AM   #5
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Anyone else care to chime in?
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Old 06-05-2009, 01:49 AM   #6
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You can buy a 0-5" Magnehelic differential pressure gauge and do experiments and measure the actual pressure at different points under the car.
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