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Old 11-13-18, 06:37 AM  
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Default Re: BlueScooby's 06 STI - Redoing the underbody and diffuser. again.

almost finalized the new underbody design. its largely the same, with several small tweaks, and two big tweaks. The big changes are the added width and a diffuser that now expands laterally as well as vertically .

I was reading up on diffuser design and one source was claiming that the upper limit of diffuser design for a road car was an expansion ratio of 1:5 (this seems a bit ambitious really). My current setup is somewhere around 1:2 or 1-2.5. I can't go a whole lot larger due to being limited on the angle of vertical expansion. If I could extend the diffuser forward or rearward more, that would help but thats not going to work. So, a little bit of lateral expansion is the name of the game. I'm only getting up to around 1:3 expansion ratio here (estimates of course, I don't have hard measurements on heights currently).

The underbody tray mating to the diffuser will be 6" wider than previous; and the diffuser exit will be 10" wider than that! In total (including the diffuser), I am adding roughly 575 square inches of underbody. If I can achieve a pressure differential under the car of even a tenth of 1PSI, thats nearly 60lbs of extra downforce! (Obviously to reach a pressure differential that large, I will have to be scootin down the track)

(Do not pay too close attention to the hatched areas locating the tires and the OEM aero panels, they are not dimensioned and are merely approximations for reference. Everything else is scaled properly)
Here is the basic design as it sits:

Here is a graphic showing the added width:

Once I finalize dimensions and such, it'll be time to head back to the sheetmetal shop

As an aside, I finally found a statement regarding "ground effects" that I think explains why adding the skirts like I did previously was not a good idea.

"You are discussing ground effect (low underside pressure due to low volume of air under the car, vs the pressure above the car), and a diffuser (accelerating air into a void to generate low pressure)"

Basically a ground effects (with skirts) car is designed to limit airflow under the car, where as a diffuser design is supposed to increase airflow under the car. They have, literally, OPPOSITE goals... I feel like a bit of a goofball for mixing those up now At least Porsche made the same mistake too (albeit like 30-40 years ago...)

TL;DR - new diffuser design, no skirts
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