Not too many people have the robispec rear piece to compare to the APR right?

unless they are pretty good sized holes the air turbulence will still cause a lot of drag. might be a small reduction from stock if the holes are as large as on that honda.

the stock 8 bumper cuts very easily, you could approximate the look of the jdm 9 bumper by cutting away that section.
I have made that cut on my bumper and then placed a ramp I made that directs the air from the underbody to the cut I made on the bumper.
Basically fits to the spare wheel well and then up to the bumper at a height about an inch above the license plate bottom.

Quote:
the stock 8 bumper cuts very easily, you could approximate the look of the jdm 9 bumper by cutting away that section.
I have made that cut on my bumper and then placed a ramp I made that directs the air from the underbody to the cut I made on the bumper.
Basically fits to the spare wheel well and then up to the bumper at a height about an inch above the license plate bottom.

Pics? Or maybe we could meet up sometime and you could show me, I see you are in Bellevue, I'm in Redmond. I know you hit PR I've been meaning to get out sometime this summer.

What he's talking about is really quite easy. Take a look at this picture for an example of the plate you need to make. Can be done out of sheet aluminum pretty easily:



To attach it, you basically just want to cut away enough of the bottom of the rear bumper cover that you could lay a yardstick flat against the bottom of the spare wheel well and have it extend out behind the car with no interference from the bumper skin.

You'd want it to sit inside of the bumper something like this:



Here's a really dramatic (and probably inefficient) example of this:



Cutting away part of the bumper, having the diffuser angled upward more, and generally providing a larger rear opening for airflow under the car all should accelerate airflow and reduce drag significantly. That said, all of this extra work over a simple flat plate on the rear isn't going to mean much if you don't address the rest of the underside of the car.

thanks peter,
thats the info and type of pics i'm looking for.

I dont think many... hand cut piece of carbon....

Here's mine:

red,
that looks well fabricated, i like the sheet aluminum. do you have any other underbody panels?

Looks pretty good. Can you show us more pics (leading edge, attachment to the bumper sides, etc.

How did you fab this? Do the work yourself or pay a fabricator?

I do not have other underbody panels.

I paid Calculated Risk (http://www.calc-risk.com/) to fab this for me. They did all the fab for GST time attack Subie: http://www.gstmotorsports.com/home/i...29&Itemid=28It

The diffuser uses sheet aluminum and is bolted into the spare-tire well. The leading edge is not angled upward and probably should be. The other mod I'd add would be to extend the steep section of the diffuser a little bit wider. I originally asked them to extend the diffuser to the end of the gas tank but that would make rear-suspension adjustments a huge pain. For more pics: http://www.norcalevo.net/gallery/thu....php?album=226

I have noticed a slight increase in top speed (especially running Thunderhill backwards into a gusting 40mph headwind. I've also been able to dial out a bit of the APR GTC200 - I compared back-to-back runs at Infineon.

As others have said, diffusers don't contribute much downforce unless the car is *really* low, like 4in or less. However, a diffuser can help reduce drag and help front aero be more effective - front aero is my next plan of attack.

This is great topic ! keep it going ! like to learn more about aero.

Bump for info on the CBRD rear diffuser. Its pretty close to the end of summer.

It's the APR one. I ordered mine last week

Sorry for the late response Jason but I was under the impression you could not add/change any undertray is the car. That's why the APR front splitter is not legal for STU.

you are not allowed to change the undertray in front as the rules are written. But you are allowed to remove and or re-enforce the stock undertray in order to mount a splitter on the front of the car.

This is true in SM, not 100% certain in ST, but many of the rules in this case overlap in this area.