Rear Diffuser Experience, Advice and/or Input??!!

[Feffman]

Hi All:

Does anyone have experience and benefit input with a true under body rear diffuser? Not the "diffuser" on the rear bumper, but a true rear diffuser that goes to at least the center line of the rear axles/diff.

I've removed the charcoal canister, canister plumbing and plastic cover and I'd like to close off that rear area from the axle/diff to the rear bumper. I'm developing a template for light gauge aluminum but thought it may be easier just to buy a diffuser. Below is the list I've compiled so far.

Bimmerworld ($1,000)
https://www.bimmerworld.com/Body-Aer...-Diffuser.html

Aeroflow Dynamics
Version One ($800) https://www.turnermotorsport.com/p-5...r-diffuser-v1/
Version Two ($800) https://www.turnermotorsport.com/p-5...r-diffuser-v2/

Mkah Motorsports ($400) http://mkahmotorsports.com/e46-m3-diffuser-new-item/

Fancywide ($400)
https://thebuildjournal.com/shop/bmw...-diffuser-e46/

AGT($885+) https://agtstyling.com/index.php/onl...iffuser-detail

VRS
System 1 ($1,895) https://varisna.com/product/varis-re...6-m3-vab-4608/
System 2 ($1,195) https://varisna.com/product/varis-re...6-m3-vab-4609/
Combined ($2,795) https://varisna.com/product/varis-re...em-1-2-vsdc-2/


Any input is appreciated.

Feff

[eacmen]

The car we bought has the bw rear diffuser with the race exhaust. Dont have any wind tunnel data on it but the fitment is spot on and seems to be a nice piece.

The only upgrade from here would be full flat underbody from splitter to diffuser.

[thundermoose]

AJ Hartman. I don’t have a flat bottom but this diffuser extends to the rear of diff. I measured the pressure at the leading edge of the diffuser and it drops 0.2 psi >110 mph.




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[thundermoose]

AJ Hartman. I don’t have a flat bottom but this diffuser extends to the rear of diff. I measured the pressure at the leading edge of the diffuser and it drops 0.2 psi >110 mph.




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Another pic


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[Feffman]

AJ Hartman. I don’t have a flat bottom but this diffuser extends to the rear of diff. I measured the pressure at the leading edge of the diffuser and it drops 0.2 psi >110 mph.

No doubt AJ's diffuser, like all his work, would do the trick, but, and I should have clarified this, the SCCA rule (purple highlights in note below) are somewhat tolerant. Hoping we can sneak one of the more moderate appearing rear diffusers by. Easier to ask for forgiveness than permission perhaps.

January 2021 SCCA GCR page 624, #22.
An underbody close-out panel(s) may be used in the area behind the rear axle. These panels shall not alter the external appearance of the car when looking from the rear and sides of the car (i.e. we want to have to lay on the ground to see them). If the production car uses underbody trim pieces, the OEM trim pieces may be removed or replaced, but any close-out panel(s) used may not visually hide any more of the mechanical components, when looking from the rear and sides of the car, than the OEM trim pieces do. The close-out panels shall not completely bridge the gap between the rear floor pan area and the rear axle centerline. On rear engine cars, any close-out panels shall not extend any further forward than the rear axle centerline. Cars with a fuel cell, engine, etc. that extend down into external visual range shall fit the close-out panel(s) around the component in such a way that it does not alter the external appearance of the car.

[Fresh1179]

I would love to see some data on the CF one that Turner just came out with. I can’t imagine that it is effective as the aforementioned ones but who knows. I need to add some aero to car mid-season because around 118-123 the front starts to lift and I go into pucker mode.


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[Onefastsicilian]

the VRS one looks beautiful!

[eacmen]

I would love to see some data on the CF one that Turner just came out with. I can’t imagine that it is effective as the aforementioned ones but who knows. I need to add some aero to car mid-season because around 118-123 the front starts to lift and I go into pucker mode.


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You should not be getting lift at that speed with stock aero. I suspect something else might be causing the lift. The plastic underbody panels are actually functional on the m3, if you removed them (like I did once because I was lazy) - then you might experience some lift.

[Feffman]

I would love to see some data on the CF one that Turner just came out with. I can’t imagine that it is effective as the aforementioned ones but who knows. I need to add some aero to car mid-season because around 118-123 the front starts to lift and I go into pucker mode.

Fresh:

This one from Turner? https://www.turnermotorsport.com/p-5...ffuser-e46-m3/

Feff

[bmw m3 s50]

Honestly, I have looked into this because I am into aero and love how certain cars like Ferrari use it to gain stability at higher speeds. I have done tons of research in trying to figure out an effective diffuser to generate actual downforce. In a nutshell, slapping a diffuser to the back of your car will not create the downforce your looking for. The diffuser is the end piece of a comprehensive system designed on car. The diffuser is used to evacuate low pressure air that is under the car. In order to generate the low pressure you will need a perfectly smooth underbody surface with air vanes creating a low pressure zone. Additionally, you will need a front splitter up front to start forming the air stream. The e46 makes it hard due to the central passage of exhaust, driveshaft, transmission and diff as well as the generate shape of the car. This areas require cooling and can not be really covered. You can take a look at Lang Racing. They have a full underbody setup for the m3 e46 but is not advised to be used on the street due to the problems of heat dissipation. I have been looking into other ways around this by creating two separate low pressure tunnels on either side of the center line of the car but have not really researched if this would be effective or not in creating downforce.

[bmw m3 s50]

Also just wanted to add, as someone already said speeds of 113-125 should not be creating lift situations. These cars with the suspension geometry and setup feel pretty planted even at higher speeds past 125. Generally, you do not really start to feel the effects of aero until you beyond 120-130 or higher.

As the other poster mentioned, I will would look into your suspension if your car is feeling unstable at 113-125.

[Feffman]

My thought is to rid the M3 of the parachute effect the rear bumper creates with the open area in front of the bumper, thus the simple idea of the flat aluminum under panel. Only hitch may be heat but some NACA ducts with rear venting should handle that.

[eacmen]

My thought is to rid the M3 of the parachute effect the rear bumper creates with the open area in front of the bumper, thus the simple idea of the flat aluminum under panel. Only hitch may be heat but some NACA ducts with rear venting should handle that.

What did you end up going with?

[Feffman]

What did you end up going with?

Flat aluminum panel just aft of the centerline of the rear axle (meets SCCA rules) to the rear edge of the bottom of the rear bumper. Closed off the driver's side wheel well area with a vertical section as well. Fabricated two vertical mounts which tie the front of the panel to the rear roll bar mounting point. No aerodynamic data to determine if it helps but rules don't allow any visible vertical pieces so most available diffusers are no go.

Feff

[Cronenberged]

Lang Racing has a design, I'm sure Andrew could modify slightly for your needs.