Aftermarket exhaust and downforce
 

Here is an interesting post from another thread:

"scion/toyota/subaru really did a lot of work on the aerodynamics on this car. They use the stock rear muffler as a defacto diffuser (Mitsubishi did this on the Evo X design work also and they were awarded an international aero award for back in 08). Diffusers act BOTH to reduce drag and increase down force. This effect can favor either side of this equation to produce the results you want. The twins are VERY efficient on fuel this is due in part to the DI injectors and also to careful aero management of the air streaming around the car (and some complain that we didnt get all the cool plastic panels under the car that are part of this package). There is also a track day benefit as the car works well at the track without the addition of any kind of aftermarket aero devices(wings lips spoilers). THIS LACK OF REAR END LIFT IS VERY HARD TO ACHIEVE in a stock vehicle and the OEM designers should be thanked for their hard work. A real world example is the S2000 which NEEDS a wing just to be safe on the track above a certain speed due to rear end destabilizing lift. Sound clip aside, I think Borla should be given credit for trying to maintain the stock aero package. Their diffuser is designed to work as a replacement for the muffler/diffuser used by the OEM designers and this is shown to be needed by many who have changed their exhaust and noticed at the track a increase in rear end dynamics at higher speeds (gets light and more unsettled the faster you go). Oh the borla diffused has slightly more down force and drag due to their vertical elements which promote more efficient emptying of the air under the car (and the faster it leaves the space the lower the air pressure and the higher the down force)."

From here:http://www.ft86club.com/forums/showthread.php?t=23229

My questions are:

1. Is he correct in saying that the exhaust system acts as a diffuser?
2. Has anyone noticed any loss of high speed stability sine changing out their exhaust?
3. Is there a system on the market that takes all this into account?

I plan on tracking my car so this is a concern to me. Also, it could effect freeway driving.

I have the greddy evo 3.. I had to take this one hand on the steering wheel, twice as the first time the flash was on.. So I wasn't nervous or anything.. Felt pretty stable to me.. Btw, it is a closed road, slight downhill

http://i305.photobucket.com/albums/n...224730_193.jpg

The lift effect won't be noticed unless you are doing high speed braking or high speed higher load sweepers. AKA real track driving.

Or, high-speed freeway interchanges :thumbup:

We certainly noticed a difference when we were using a muffler delete... we went back to a full axleback rather quickly.

Haha forgot about the highway on and off ramps!

Did you go back to stock? Or to another aftermarket exhaust? If so, which one?

We're currently utilizing a prototype Berk axleback that incorporates a "diffuser" into the design :)

Any chance we can get a picture?

Also, how do you feel about the borla diffuser on their exhaust?

I can't say, since I've never actually seen the Borla in person. Show me the windtunnel testing and I'll be a believer!

Borla has always been an exhaust company, not an aero company, but it's also plainly obvious that their concept should work.

Would like to see a pic of that Berk also

I find it interesting that exhaust companies are doing the diffusers versus more exterior part body kit companies.

Are there any aftermarket exhaust systems that act as a diffuser like the stock unit?

I may go with the Borla unit... But I'm sure it has never seen a wind tunnel so its function will be questionable.

I doubt any exhaust manufacturer will wind tunnel their exhaust; it's WAY out of budget for R&D.

That being said, I personally recommend an exhaust with a layout similar to the OEM, like the Berk and Borla.

The Berk will be released when it's ready; no effort is being wasted in trying to rush a product. :)

I would say NO......the stock exhaust setup would not benefit a diffuser....diffusers must be engineered correctly to work....typically they start at the centerline of the rear axle and rise UP at around a 15' angle with some vertical slats towards the bumper.....if the angle is much more or less they don't really work....most diffusers you see on cars don't work....they just look cool

Adding an exhaust outleft to the diffuser IS a great way to increase airflow over it....BUT at one drawback, it increases downforce WHILE on throttle, then when off throttle the flow stops and you lose that downforce out the back...which can be compensated for....personally I like a car thats slightly loose on turn in....but it took me years to become comfortable driving this way