Subaru BRZ Aero
 

Hey all,

Several years ago I bought an MR2 Spyder, and made a thread about improving its horrible aerodynamics, which never happened because the engine blew, and all the aftermarket hardtops weren't very aero friendly anyways.

Now I've graduated and got the trigger finger on standby for a Subaru BRZ as my commuting car for the next few years. This car has way better aerodynamics and engine efficiency, hooray.

But obviously, there's no way I can just not modify my car :)

I'll probably keep the wheels as is for resale value, but partially covering them is something to consider for the future.

I'm probably getting a Limited model since most BRZs are Limited not Premium, so the car will come with the factory spoiler which is claimed to cut Cd from 0.29 to 0.28. I have a feeling that a ducktail spoiler would do a tiny bit better because it doesn't have fins coming out the side, and I like the look better, but I still have to think about whether I want to waste money on that.

The area to look for improvement on this car then seems to be the underbody. I can make a diffuser out of sheet aluminum or steel and attach to the exhaust with reasonably low effort using U bolts. 2 bends at the rear wheels to "seal" off the turbulence behind the wheels.

However the underbody in front of it is still messy...it's said that the factory underbody plastic covers drop the Cd by another 0.01. I don't see any easy mounting points for a DIY coroplast underbody so I'm tempted to buy them, but it's 300 bucks shipped, aka probably 10k+ dollars in gas to recoup the cost, which is a hell of a lot of years of driving.

The other possibility is a small airdam. Porsches use this, don't give a damn about the underbody, and are reasonably successful in producing more aerodynamic than average cars.

So what do you think I should do? Mini airdam, full underbody, or just diffuser? I would really love to be convinced that underbody panels are easy to DIY but I'm not really seeing it. My optimistic projection is that if underbody panels get the car to 0.27, then adding diffuser will get it to 0.26 which is pretty good. Some guy did a CFD simulation with sealed front grill and smooth underbody and got 0.262 Cd.

that car and its nose is already low I would say smooth the underbelly. airdam will just get scraped all over

Haha well, Porsche airdams are very flexible half inch plastic or rubber (I can't really tell) pieces. I imagine a rubber strip would get scraped but would still be functional for some time.

BRZ technical drawing
 

Here's an image adapted from ROAD & TRACK Magazine.
http://i1271.photobucket.com/albums/...titled19-1.jpg
The rear spoiler looks like it was thought out pretty well.
Same for front airdam.
Completely smoothing the belly and adding a diffuser wouldn't hurt.

Toyota says their spoiler drops cd by 0.02.

Are you saying no, do not add a front airdam?

What about diffuser without belly pan? Belly pan is a more difficult install :/

Where? If that's actually true I'm switching the Subie spoiler out.

Toyota (or at least our local press) claims a 0.02 CD (the internets seems to say 0.01) reduction with the spoiler vs without. I don't think it's any different to the Subaru Spoiler.

A full or partial bellypan will reportedly improve the efficiency of the difusser.

I vote for full under belly (front and middle) tray and rear diffuser.

It gave my STI a solid 3mpg increase

front airdam
 

If you're gonna do 'track days' with the Subie,then go all the way down with a flexible airdam and expect it to scrub all the time,like Porsche's new CAYMAN GT4.
You'll gain 0.5% mpg according to one researcher.
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As to the diffuser,their function is predicated upon a 'perfect' belly pan upstream.There cannot be any turbulence leading to the diffuser or it won't perform.
The longer you stare at the underside,the better you'll discover opportunities for attachment points.
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Also,there may be some MPGs in your side mirrors.Take a look at Ford's 2017 $400,000 GT.
Minimum mirror frontal area,on a very thin (strong) mounting stalk,moved significantly outboard into slower air.Until camera systems are approved by the DOT we're gonna have to cheat all we can.:p