Review of the Ford Fiesta WRC new rear diffuser

The new Ford Fiesta WRC aero kit introduced in the Rally Finland, and that probably will be used again by Seb Ogier in the coming Rally Catalunya, consisted mainly of significant bigger rear wheel air vents as well as in a new rear bumper with a new diffuser design. It is the first time that any of the WRC teams modify their original diffuser design since they appeared in early 2017, for what a review is required, especially because the new Fiesta’s diffuser is somehow different to the rest.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Catalunya Pre-event tests, Rojals, October 2018

The new diffuser was first seen during the Rally Finland pre-event tests last August. The first difference with the original one was the increase of the diffuser exit area, due to a significant increase of the diffuser angle.

diffuser comparative.jpg

Original (up) and new (down) Ford Fiesta WRC diffuser design

The increase in exit area allows removing more air from under the car into the rear end of the car. Due to the area increase the air flows at a lower velocity, thus at a higher pressure. The result is a bigger pressure difference between the rear of the car and the undercar. The higher the pressure difference, the more the air is accelerated under the car, resulting in an even lower pressure there, what brings to a higher pressure difference with respect to the car overbody. The final consequence is an increase of generated downforce, due to the area increase.

side comparative.jpg

Original (left) and new (right) Ford Fiesta WRC diffuser design – pictures by Javier Blanco (left) and llluis555 (right)

The number of strakes has increased from 7 to 8 in the new design, which also includes two additional small strakes in the central channel (only one in the pictures from Rojals test, due to breakage of the other one, as they are located in the space used to insert the car elevator during repairs). The use of strakes (converting the diffuser into a multiple-channel diffuser) allows channeling the air towards the car rear, reducing side turbulence and removing the effect from air coming from the sides of the car (especially from the rear wheels). The result is that a higher amount of downforce is generated with a minimal increase in drag.

rear diffuser detail.jpg

The main differences between the Fiesta diffuser and the rest of the cars remain in the shape of the dividers and the diffuser width. As shown in the picture below, all the other car’s diffusers included vertical dividers, which means airflow is directed straight to the car rear.

rest of diffusers comparative.jpg

But the new Ford diffuser introduced curved dividers, in order to send the air to the sides of the car. The new diffuser then splits the air coming from under the car in three different directions, as shown in the picture below.

difusor fletxes

The big central space still sends the air straight backward, while the four strakes on each side, together with the diffuser ends, channel the air to the car sides, behind the wheels. In these areas, the amount of air has been reduced, thanks to the new rear wheel arch extensions that are sending upward a much higher amount of air from the wheel space.

The result is a smaller and less turbulent wheel wake, which, in addition, is partially blocked by the presence of the ends of the same diffuser. This is because the Fiesta’s diffuser is the widest of all, even wider than that of the C3, while those of the Yaris and the i20 are significantly narrower.

In the end, the goal of the curved dividers and wider diffuser would be to improve the pressure distribution in the rear of the car, generating a more homogeneous (less turbulent) wake. The less turbulent the wake, the lower the wake dragging effect, so, in the end, what they get is a net drag reduction, which translates into higher velocity.

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Catalunya Pre-event tests, Rojals, October 2018

In summary, the new diffuser contributes to a higher downforce generation and a net drag reduction, which will convert the Ford Fiesta WRC in an even faster car in smooth terrains, as we will probably see in the Rally Catalunya. The question that remains is how all this will perform on rough terrains, with higher car elevation and changing surface conditions. But the answer probably won’t arrive until next year, when M-Sport will be forced to install the new aero kit in all cars.

car top.jpg

S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Catalunya Pre-event tests, Rojals, October 2018

This is the third post we have devoted to the new rear aero package of the Ford Fiesta WRC introduced in Rally Finland 2018, you can read the other two here:

New rear aero design for the Ford Fiesta WRC 2018 for Rally Finland

Why the new Ford Fiesta WRC 2018 have such big rear wheel arch extensions?

This post includes the opinions of the author, which are certainly not always right. If you detect any error or you disagree, do not hesitate to contact us, by sending an email to We love to learn!

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