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fastflyer 08-05-2019 10:57 AM

New Diffusers
For several days I have been seeing cars with this type of "Diffuser". And I would like to know if someone understands its operation or knows of any study. Well, I've only seen it in modern German cars.

Pics from

Vman455 08-05-2019 11:45 AM

Everything you can see there is cosmetic. A diffuser is a panel under the car angled so that it forms one side of a divergent duct with the road surface the other. There's no way of knowing if this car has a diffuser without seeing underneath it....

Snax 08-05-2019 01:08 PM

Diffusers are also largely considered ineffective if airflow under the car is also not smoothly managed. I.e., turbulence from under the car remains turbulence behind it, but if smooth flow can be maintained, not only can these create downforce, but reduce wake size. In most production cars as Vman455 already stated however, they are cosmetic, and even most people adding them onto their street cars are really just playing dressup without the underbody work to go with it.

Snax 08-05-2019 01:14 PM

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Looking at the rear of my i3, you can see a legitimate effort at an OEM diffuser. The tray extends all the way up to the rear axle and is preceded by relatively smooth body work ahead of it.

I'd hesitate to say that down force of any significance is created or intended by it, but the extra hole mine has for the gas generator cooling has a 0.01 hit on the drag coefficient of the car. Pure battery i3s don't have that hole.

Vman455 08-05-2019 07:02 PM

Interestingly enough, the only car on sale outside of exotics that has a diffuser panel that looks the way most people think diffusers look--that is, one with a row of straightening vanes--is this:

That would be the underside of the current Nissan Leaf.

freebeard 08-06-2019 02:56 AM

An aerodynamically efficient difusser will look like this:
The production vehicles have been worn off to the point were the average customer won't be constantly high-centering everywhere.

rob.e 08-07-2019 05:55 AM

has anyone fitted a DIY diffuser and seen improvements?

its something i'm considering for my car. it has pretty good /flat floor already but some parts at the rear bumper could be better if there was a smooth panel there.

that part of the underfloor already slopes upwards so easy to fit a "boy racer" ebay flat panel with straighteners. Do you think it'll make a noticable difference?

I'm thinking about something like this:

freebeard 08-07-2019 12:47 PM

It depends on the car. The entire undercarriage is maybe 25% of the total. The delta in overall Cd is twice the delta in fuel consumption IIRC. IOW 10% better drag is 5% better mileage (kilometerage?).

Other areas that might see improvement could include rear-end lift, but it's going to depend on the vehicle.

euromodder 08-08-2019 05:16 PM


Originally Posted by fastflyer (Post 603913)
For several days I have been seeing cars with this type of "Diffuser". And I would like to know if someone understands its operation or knows of any study.

Hucho ...
And from his studies, that is far too steep an angle to be beneficial.

For it to work in reducing drag on regular cars, a diffuser would need to angle up only a few degrees, say like 5

Also depends on length, height of the underbody, and shape of the rear of the vehicle - see the drag rise on an estate ...

fastflyer 08-08-2019 05:25 PM

Yes, with a smooth undertray it works.
But with a normal car is there any benefit?

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