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aerohead · 12-07-2009, 07:41 PM

The original boat tail for my CRX had an open duct,patterned after the Mustang GT.
At Bonneville,I learned that by completely taping it over I realized a higher top speed and significantly better mpg on the return trip home.
The opening encouraged air to go through a torture chamber,and allowed air that would have otherwise flowed lower and unobstructed,to also collide with the undercarriage.
You''l notice that most modern autos incorporate a very deep valance below the rear bumper,which extends down to intersect the imaginary 2.5-4-degree diffuser angle.
The valance allows air to "pool" against the underside of the car,"drowning" all the under-hanging components in a mass of stagnant air which is dragged along with the car.
All the air passing beneath,essentially skips off this stagnant pool,as if it were a bellypan,passing below all the obstructions,with a straight shot to the rear.
By cutting the bumper open,you destroy what cost millions of dollars to create in the wind tunnel.
SAE journals and transactions and Hucho's book are good places to look for "science" rather than anecdotal heresay.

12-07-2009, 07:41 PM	#45 (permalink)
aerohead Master EcoModder Join Date: Jan 2008 Location: Sanger,Texas,U.S.A. Posts: 16,279 Thanks: 24,401 Thanked 7,368 Times in 4,767 Posts	holes The original boat tail for my CRX had an open duct,patterned after the Mustang GT. At Bonneville,I learned that by completely taping it over I realized a higher top speed and significantly better mpg on the return trip home. The opening encouraged air to go through a torture chamber,and allowed air that would have otherwise flowed lower and unobstructed,to also collide with the undercarriage. You''l notice that most modern autos incorporate a very deep valance below the rear bumper,which extends down to intersect the imaginary 2.5-4-degree diffuser angle. The valance allows air to "pool" against the underside of the car,"drowning" all the under-hanging components in a mass of stagnant air which is dragged along with the car. All the air passing beneath,essentially skips off this stagnant pool,as if it were a bellypan,passing below all the obstructions,with a straight shot to the rear. By cutting the bumper open,you destroy what cost millions of dollars to create in the wind tunnel. SAE journals and transactions and Hucho's book are good places to look for "science" rather than anecdotal heresay.