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Old 03-06-2009, 07:12 AM   #20 (permalink)
Frank Lee
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Blue - '93 Ford Tempo
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Originally Posted by MetroMPG View Post
On that car, not necessarily. Just guessing (I know that's dangerous when it comes to aero), but I don't think that car has reattached flow on the end of the decklid. It's too short, and the rear glass is too steep. So effectively raising the decklid as that "spoiler" does, may be helping a small amount.
Agreed, "eyeball aero" is not good science. Buuuut...

I think dragging a spoiler through turbulent air is still dragging something that otherwise isn't there.

I intuitively agree with Daox' assessment (# 25 of this: ) :

"Spoilers work by increasing the pressure between the back of the roof and the spoiler itself, so there is less of a low pressure zone at the bottom of the winshield. If the slope from the back of the roof to the back edge of the trunk is more than 15 degrees, you can benefit from a properly designed spoiler as long as the slope from the back of the roof to the top of the spoiler is around 12.5 degrees, or as close to it as possible. Also, if the air has someplace to go below the spoiler the aerodynamic purpose is somewhat defeated."

I recall reading somewhere that Fieros with factory spoilers gained a few counts of drag, and spoilered Countaches lost a noteable amount of top end speed.

And from 60+ Vehicle modifications for better fuel economy - :

Raised wing type rear spoilers
Sure, the dealer or that go-fast kid on the corner might've convinced you that spoilers are good for downforce, but in reality, most are merely a cosmetic addition - one that is more likely to add drag than it is to do anything meaningful for handling.

Take it off and your car will not only look stock, but it'll have better aerodynamics.

It isn't always obvious whether a particular spoiler style is an aero help or hindrance.
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