Project Porsche 991 GT3 RS- Improving the Aero with a 911R Rear Diffuser

by Mike Kojima


How do you improve a supercar? It is not easy, but leave it to us to give it a try. We recently got to add a Porsche GT3 RS to our project car fleet. If you don't know much about Porsches, a GT3 RS is a limited production model, part of the current 991 platform family and the last of the great naturally aspirated 911's. 

Basically, the RS is built very close to the 911 cup cars that are raced in IMSA, for instance, with some concessions being added for street use like emission controls and a dual clutch PDK semi-automatic transmission. 

With a chassis packed full of exotic lightweight materials, huge carbon ceramic brakes, wall to wall rubber, 500 hp of naturally aspirated, super fast close ratio shifted fury, and best of all production Porsche aero, the GT3 RS punches above its weight class and delivers amazing performance.

It all comes with a pretty price tag, which is probably why it is the rawest, most engaging, most responsive and most fun to drive car we have ever gotten behind the wheel of. This includes our most well-built project cars and even other exotics. There is something magical about the car and all the engineering behind it. So can we alpha nerds at MotoIQ make the RS better or are we gonna mess it up?

One thing to remember is that the RS has a lot of potential as a collector being a pretty limited production car and being the last of the naturally aspirated Porsches. We have to limit ourselves to things that do not alter the car and allow it to be returned to stock with no damage. However, we accept the challenge and we are ready to do some limited things.   


Strangely enough, unlike most exotics, the GT3 RS does not come with a rear diffuser. It does have a rear tail pan with a slight kick up that probably slightly acts like a diffuser, but it is not like the ones in most supercars for sure. 

So we decided to see if the rear diffuser for the 911R would fit. The 911R is like a GT3 RS, with its wide body and powerful 4-liter NA engine but without the rear wing and with an old-fashioned manual transmission instead of the PDK. The 911R also has more interior amenities and is a more refined version of the race-oriented RS. The 911R was built to be the ultimate Porsche purist machine.

We think that since the 911R has no rear wing, Porsche added a rear diffuser as a low key sleeper way to make up some of the lost downforce that the RS's rear wing provides. The 911R rear diffuser has a proper kick up to smooth airflow under the car, being fed by the RS's nearly all flat bottom. It also has vortex generating strakes, as shown below, to help keep the airflow attached to the diffuser. This helps it generate downforce at lower speeds and making it more effective in general. 


The 911R diffuser is ready to bolt on and even comes with the mounting grommets and bolt sleeves ready to install. It is priced very reasonably as well. We got our for $559.95 at Sun Coast Porsche, a dealer with a pretty decent web sales department and great prices on parts and genuine Porsche accessories. 

Hey Nissan, why is the GT-R rear diffuser $5175 bucks? It does not even have vortex generators!


We removed the stock rear pan by simply unbolting it, undoing the many large flanged allen bolts that hold it in place. 

The pan is off in about 5 minutes of super easy work! We wish everything could be this easy.


Page 1 of 3 Next Page
Bookmark and Share
Thursday, February 15, 2018 3:13 PM
What possible reason could Porsche have for not making that a stock part on the RS? I mean, I understand the base model Carrera.
Mike Kojima
Mike Kojimalink
Thursday, February 15, 2018 3:21 PM
Maybe because the 911R was developed after the RS? The 991.2 GT3 has a rear diffuser, possibly this same part.
Thursday, February 15, 2018 9:08 PM
Great oem mod.
Would be cool extend the vortex generators out on the muffler and past it in stainless and weld it on. Make it look like oem as well.
Thursday, February 15, 2018 9:29 PM
This article is a great illustration as to why Porsche went with a mid-engine layout for their newest 911 racing machines. The rear diffuser you installed is a lovely part, but it is so dinky in its proportions. The angled portion is not very tall or deep and just looks anemic compared to functional after-market diffusers I have seen on other front-engine and mid-engine cars. That rear-engine layout just doesn't leave much room for a big, bad diffuser.
Mike Kojima
Mike Kojimalink
Thursday, February 15, 2018 11:32 PM
KKOG, what a great idea!
Mike Kojima
Mike Kojimalink
Thursday, February 15, 2018 11:33 PM
StrangeLiform, you are 100% correct.
Saturday, February 17, 2018 4:01 AM
The hole in the plate is not for the PDK, the PDK is in front of the engine!
Mike Kojima
Mike Kojimalink
Saturday, February 17, 2018 6:16 PM
you are right, I don't know what I was thinking! I was staring right at it too. A Porsche "expert" told me it was the pan for the PDK and I turned off my brain I guess.
Tuesday, February 20, 2018 10:34 AM

The 2018 GT3 comes with a proper diffuser now!

Post Comment Login or register to post a comment.

MotoIQ Proudly Presents Our Partners:

© 2018 MotoIQ.com