Quote:
Originally Posted by aerohead
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.
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I found my timing slips from Bonneville.
The boat tail with Mustang-style open duct added 1.21 mph to the top speed.
When I blocked the opening with cardboard and duct-tape I saw a 5.51 mph increase with the tail.
If you stand far to the rear of an automobile and can see any of the undercarriage components,it's valence is NOT LOW ENOUGH.
Drop it down even with the floor of the trunk at least.It will NOT act like a parachute.The air in front of it is stagnant.It has very little kinetic energy.