Porsche 991 GT3 RS 911R Rear Diffuser


The 911R diffuser in place. You can see the vortex generating strakes here. We think the GT3 RS should come with this part stock!

Here is a bottom view of the 911R diffuser in place. You can see how the exhaust for the rear-mounted engine interferes with the ability to build an even more effective diffuser. You can also see the plastic gap fillers that make the GT3 RS virtually a flat bottom.

The rear diffuser is pretty impressive looking with the GT3 RS’s wide body and wall to wall rubber. The 21×12″ wheels have some of the widest rubber found on any production car, a 325/30-21!

So the 911R rear diffuser was a neat looking, hopefully effective, easy to install and even a relatively inexpensive part. We still think a car like this should have come with one from the factory. It is a genuine Porsche part, and it is easily reversible, so we feel that it cannot devalue the car.

This part will also fit on a 991.1 GT3, the 991.2 GT3 has a diffuser already.

Stay tuned as we dream up other things to do with Project GT3 RS.



Suncoast Porsche Parts


  1. Hi, really love this clean and simple mod, but I’d like to add some technical clarification on one point.

    In response to: “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.” Diffusers increase *front* downforce bias (counterintuitive because its a rear-mounted aero device, I know. This used to confuse me for years), by further increasing air velocity underneath the whole car, but especially towards the nose where there is a sort of “suction peak,” or point of minimized pressure and maximized velocity. The airflow is actually *increasing* pressure (slowing down) as it passes the diffuser itself, at the rear of the car. If you look at pressure distributions across an (upside-down) airplane wing, you’ll see this same “suction peak” at about 30-40% of the way down the length of the airfoil, and then the pressure steadily increases towards the back. Race cars with flat floors can be like this. The father of the Porsche 958 spoke about discovering this in an interview with Chris Harris several years back.

    This is the first time commenting on one of your articles, Mr.Kojima, but I’ve been a fan for years! Thanks for reading!

    1. Your assumptions seem correct but that’s not how cars work. Aerodynamics are not intuitive. A car with a diffuser isn’t simply an upside-down airplane wing. You are forgetting ground effects and the moving ground plane. If you look at published wind tunnel studies of race cars you can see that changing the diffuser and or the rake of does affect downforce toward the rear of the car, sometimes reducing the front down force.

  2. Thank you for great post!!
    I have 991 GT3, and keep considering to install this diffuser or not.
    Mr. Kojima, do you think this will be only positive results?
    According to suncoast “consult a race specialist to ensure aerodynamics are balanced for high speed or race applications”
    I’m just concerning, if current GT3’s aerodynamic design is enough, I don’t need to add this.
    Please advise.

  3. As a follow up to Koo’s question on a .1 GT3 and Mike’s response. The .2 comes standard with the rear diffuser, but it also has different front inlets and front diffuser that according to Porsche creates more front downforce, hence the need for the changed diffuser and slightly elevated wing. So question remains, will adding the diffuser to a .1 GT3 negatively impact the balance of the car on the track from what Porsche intended?

  4. It seems that diffuser is attached to hangers/frame in those pictures and frame again into engine block (??) or where (. There seems to be attachment point also in 997.2 block (?). Do you have Porsche part number for those frames and can you give maybe the measurement of the frame upper part holes i.e how much attachment holes for bolts are apart

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