Velox Motorsports – FR-S / BRZ Rear Diffuser Development


Diffuser Ahmed Models CFD:

Velox simulated the addition of a diffuser by removing material from the lower rear portion of the Ahmed model and ran the CFD analysis again. They then added a radius to the rear-most portion of the diffuser to represent a similar up-turned lip to their diffusers. Below are some of the CFD photos.

The velocity contours between diffuser and diffuser with the up-turned end are shown here. There is a marked difference between the velocity distribution of the two in the flow region behind the body and to some degree underneath it. 

The coefficient of pressure plot between the plain diffuser and the diffuser with the rear up-turn are shown here. This represents the pressure distribution on the surface of the Ahmed model. Negative numbers (light blue, dark blue, and purple) signify negative pressure, also known as reduction in lift or downforce. It is clear to see here that the radius of the rear lip increased the magnitude and multitude of low pressure on the diffuser surface. This is without any strakes, however. With properly placed strakes, this could be further improved upon! Strakes act as vortex generators, and thus help the flow stay attached to the bottom of the diffuser, particularly at the end of the diffuser. 

Ahmed Diffuser Model Numerical Data:

The numerical data speaks volumes. The diffuser with a radius at the exit end is more efficient. Not only does the radius'd diffuser make more downforce, but it also has less drag. 

The plot above is a pressure plot along the Ahmed body. The red (radius diffuser) can be seen to have lower pressure points across the surface of the body.

Real-world Tuft Testing

Tuft testing uses yard taped at one end to the surface of the body to indicate flow direction and characteristics. Tufts can show if the flow is laminar or separating. Tufts are a low cost way to visualize how the aero parts are working. 

Here we have the finished Velox product! Notice the detail of the front diffuser extensions that allow full articulation of the suspension components and also follows the exhaust. You can see a full list of compatible exhausts on the Velox Motorsports website and also a list of which exhausts will fit with a little bit of trimming. 


For this analysis, the radius on the rear of the diffuser makes the vehicle more efficient by adding downforce and reducing drag. This might not always be the case, but this analysis does indicate that the radius on the rear diffuser is not only structurally beneficial but aerodynamically as well. Regardless, Velox Motorsports always suggests that all setups should be analyzed and tested before implementation. This is why they CFD test every model they design for optimization.

The detailed analysis also shows that the Velox diffuser is a functional part that both increases downforce and reduces drag. This is more then what can be said with other diffusers on the market that are purely designed for looks. You can guess which part we want for our project car!



Velox Motorsports


  1. Hi, i am looking for a aerodynamic race tested rear diffuser for a 2016 Ford Mustang GT running 700 rear wheel horsepower. I already have front splitters, cannards and rear spoiler. I do track sprint racing

  2. Hi: I’m working being an amateur aerodynamist with my FRS. Your tuft testing was very good but you should have it without a diffuser to give an overall comparison of the benefit of a diffuser.

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