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Automotive Diffuser Questions

Automotive Diffuser Questions

Automotive Diffuser Questions

Assuming the rules allow for it is there any advantage in extending the diffuser beyond the rear bumper? My initial thought was "the bigger the better" but I am now thinking that is not the case. My second question is about the diffuser exit. Almost all diffusers for sale for street cars (that are likely mostly for looks) have a rounded exit like the first picture below. Race cars however all seem to have virtually no upward curve at the rear of the diffuser, F1 being the easiest to see but I know ALMS cars are the same way. Based on that alone I am inclined not to have any upward curve in the diffuser. Any help in my design would be greatly appreciated, I have the rest of the car well sorted but have little to no aerodynamic experience and much of the subject is counter intuitive.



RE: Automotive Diffuser Questions

OK, you're comparing something that you've already decided is for looks, with something that the pros are using.  What more would you need?


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RE: Automotive Diffuser Questions

Well even if that were all there was to my second question it doesn't answer my first at all. There are major differences between street cars and the F1 example, the biggest being that there is virtually no flat vertical area (rear bumper, trunk lid tail lights etc). It is possible that the street car versions are curved upward to lessen the drag while not really effecting downforce although assuming the airflow separates once the vertical curve starts I can't see it being much if any different than not having the curve there to begin with.

RE: Automotive Diffuser Questions

My first question is really the biggest though, is there any advantage/disadvantage to a diffuser extending beyond the body work.

RE: Automotive Diffuser Questions

i'd've thought that there was an advantage is creating an afterbody to fill some of the volume behind the vehicle, sort of like a fairing.  but apart from drag reduction ther are probably a host of reasons why not to do this ...  

RE: Automotive Diffuser Questions

The biggest one I could see being interference with the function of the trunk mounted wing. At some point I could see it being possible that the diffuser was actually holding the air down. I'm really hoping someone can provide a definitive answer. Or has seen it done in CFD, or a wind tunnel.

RE: Automotive Diffuser Questions

i may well be wrong but i thought the rear "wing" worked mainly as a deflector rather than as a true lift generating wing ...

as for your 2nd point, if the afterbody is "holding the air down"  then there'd be a significant net load, and a significant over-turning moment ... something i'd've thought was quite destablising ...

RE: Automotive Diffuser Questions

In the case of NASCAR or other similar series where the is no real "wing" and instead just a piece of sheet metal stuck on the truck at an angle you're correct about the "deflector" comment. However in any series that allows it true airfoil wings are used, which is the case on my car and they do generate significant (~300lbf in my case) negative lift.

RE: Automotive Diffuser Questions

I also know that a rear diffuser can aid the function of the wing and allow a higher AOA without stall which is why I wonder if extending it past the rear bumper would lessen the benefit of the whole system.

BTW is there an edit button somewhere I can't find?

RE: Automotive Diffuser Questions

Bensalasko, sorry, but a flat plate at an angle can generate lift in the same way as a nice classical tear drop shaped aerofoil section.  Less efficiently with some 'handling' issues but fundamentally the same principles apply.

(To edit red flag and ask management to make the change - try not to use it too much though.)

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RE: Automotive Diffuser Questions

I wouldn't really say "the same way" since a flat plate cannot generate lift at 0* AOA like a true "wing" can. Either way it's irrelevant to the thread, and RB1957 brought up the deflector vs wing issue. I was just trying to clarify what type of wing I had if it was relevant.

RE: Automotive Diffuser Questions

if you want to edit a post before you submit it "preview post", after you've submitted it follow KENAT's suggestion

RE: Automotive Diffuser Questions

A longer diffuser even if it has no aother advantage will increase the low pressure area of the car.

A longer diffuser allows a more gentle diffuser angle for a given ratio of throat to exit area, so it will give less drag for a given downforce (see Hucho, of course).

The interaction between rear wings and diffuser geometry is exceedingly complex, in the absence of windtunnel or CFD then a DOE on a track may offer some insight, or some experiments with smoke, tufts and oil.

F1 diffusers are very much concerned with vortex control, they may not be a useful guide for a closed wheel car.



Greg Locock

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RE: Automotive Diffuser Questions


You really need to get a copy of Joe Katz, Race Car Aerodynamics.  It answers all your questions plus a bunch more you should be asking.  

You have the wing/diffuser relationship backwards.  Ground effect systems, of which the diffuser is the final piece, are far more efficient than the typical automotive wing.  So the question is, how can you maximize the wing's positive effect on the diffuser by increasing the "pumping" effect of the wing on the diffuser.  i.e. the wing lowers the pressure at the diffusers' exit, increasing the volume of air moved thru it.

Short answer:  keep the diffuser "roof" flat, don't exceed 7-10° or it will stall.  Make it as long as you can w.o. stepping on it every time you raise the trunk lid.  Install the wing behind the diffuser exit (I know you can't do that), or as far back as you can.

RE: Automotive Diffuser Questions



Short answer:  keep the diffuser "roof" flat, don't exceed 7-10° or it will stall.

It's currently 7*

[quote]Make it as long as you can w.o. stepping on it every time you raise the trunk lid.  Install the wing behind the diffuser exit (I know you can't do that), or as far back as you can.[quote]

This is really what I'm wondering about. The wing can realistically only be so far back. So is it worth making the diffuser end at the bumper to increase the pumping effect of the wing. Or will the diffuser be more effective with the extra length regardless of the reduced effect of the wing.  

RE: Automotive Diffuser Questions

1.   Yes, you need to make the diffuser as long as possible & practical.  Katz's data shows that a wing perched completely on top of a street car body still makes a surprising amount of improvement in diffuser downforce generation.  Of course, much will depend on whether your wing is actually getting any airflow, living as it does in the wind shadow of the cab.

2.    What ground clearance are you running at the diffuser leading edge?  Closed wheel race cars typically run 1.0 to 2.0"  at that point  Assuming parallel walls on the diffuser(s) sides, the trailing edge height of the diffuser roof should be about 5 times the leading edge ht. in order to achieve the expansion ratio needed.  If you're running 5" at the leading edge, you can see you've got a problem (a 25" trailing edge!)

3.    The diffuser is only part of a ground effects system and is useless by itself.  The flat floor in front of the diffuser and diffuser feeding is critical.  If a flat floor in front of the diffuser can't be installed due to mufflers, differential, suspension components and the like, you may be looking at a mostly cosmetic exercisel Unlike open wheel cars, (and for reasons too complicated to explain here), closed wheel car floors/diffusers are primarily fed with flow coming in diagonally  under the sides of the car.   And yes, vortex generation due to the exact diffuser shape is crucial in making the whole thing work.     

RE: Automotive Diffuser Questions

I should probably add some pictures to give some background on my car. The wing most definitely gets significant airflow. It's an APR GTC-300 wing mounted about 16" off the trunk lid. I also have the hardtop below so the airflow probably stays attached for the most part.

As for the flat bottom, I have the front splitter about 2" off the ground and about halfway back it gets down to about 1" off the ground before sloping back up around 7* all the way past the trunk.

Your comment about being fed from diagonal flow has me concerned though, I have just finished building side skirts that for all intensive purposes touch the ground. Should I not run these?

RE: Automotive Diffuser Questions

There's a wing in that photo?

RE: Automotive Diffuser Questions

I couldn't see the wing either.

Mike Halloran
Pembroke Pines, FL, USA

RE: Automotive Diffuser Questions

No wing, the picture was just of the top to show that the airflow would likely still be attached. However I did find a picture of a car with my top and wing if it helps:


My intention is not to generate lift on the front but to generate as much downforce as possible around the center of the car. Based on a lot of race cars I have looked at it seems the front is usually a bit higher. I could be wrong or there could be another reason for that though.

RE: Automotive Diffuser Questions

So, we're still not looking at photos of _your_ car, the one you want help with?


Mike Halloran
Pembroke Pines, FL, USA

RE: Automotive Diffuser Questions

Thank you.

RE: Automotive Diffuser Questions

Yes, your wing looks fine, although mounted a bit close to the rear deck.  

Perhaps you've duplicated the raised front floor inlet as seen on recent Le Mans cars.  That was a rule introduced by the organizers to REDUCE downforce and improve safety at very high speeds.  If you go from a 2" high floor at the front to a 1" high floor mid chassis, you will develop a positive pressure zone under the car.  That's why everyone runs the front as low as practical and then runs approx. 1° rise the rest of the way back to the diffuser entry.  That way there is a small neg press under much of the floor because the increasing ground clearance helps compensate for the floor's gradually thickening  boundary layer.  

    Once your diffuser starts working, it will demand more volumetric flow than the tired flow coming from the front end can supply.  That additional volume of flow will come in from the sides, and a good thing too.  This side feed flow will have a thin, energetic boundary layer and because it is freshly attached, it will be able to cling to the diffuser roof as it turns the corner and heads up the hill.  You can check this behavior by putting drops of thin tracer oil on the bottom of the floor and seeing where the air flow is carrying it.


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