Building a rear diffuser?

[Motorcitydak]

Have any of you guys ever built a rear diffuser for a project? I have a '68 Charger that I am working(don't worry Im used to having no one make parts for it) that may benefit from one. I do not want to get into a debate of wether I need one or not. This is just about how to make one.

Obvioiusly Id smooth out the under body of the car and put a large flat plate under the car with the leading edge aimed at the rear axle and tilted up as high as possible. Then I would toss a few vertical fins on it and be done. What I am curious about is how could I do it? I can mock it up out of card board easily enough but how could I go from that to a piece I can fix onto the car? I cannot weld aluminum and I would prefer to do this myself. Im thinking maybe if I could get a big flat piece of fiberglass, I could make it from that but where could I get that? I could epoxy the pieces together. Any other ways I could do this that would be relatively easy to do?

[Force Fed Mopar]

You could probably use flat sheetmetal and bend it to fit. It's my understanding that a diffuser should have a slight curve to it for it to work most effectively. My understanding could be flawed though :)

[Motorcitydak]

I am a little worried about using sheet metal. I just imagine the thin metal vertical fins shaking and vibrating really bad. I could use a bead roller to give em some more strength but that may cause some turbulance. Using steel in a thickness that I would feel comfortable with would cause it to be way too heavy I think, something like 3/32 or 1/8. I think 1/8 aluminum would be better, but again I cannot weld aluminum. I could take it to a local hot rod shop to have them do it. They may charge around 75 bucks to do it Id think.

You may be rite about it needing a curve in the horizontal piece. Nothing I have read about them has said a thing about it being flat or not. Should it be curved up in the center or down?

PS, I see that you have a GLHS, that car must be awesome!

[wellis77]

I have thought about doing one myself but am struggling with the fins as well. Not sure how to attach them to the horizontal piece as well as how thick they should be.

[daredvl22]

It would be really cool if Anvil would just create a rear diffuser like the concept drawings off their site....hint hint..:) With that said, I'll take one for a '67!

[Rod]

I'll take one for 68 camaro :cool:

[KillerWolf]

I'm going to do one for a 71 Cuda (down the road) in aluminum. If you really don't want to do alum, try phenolic sheet. You can get it at McMaster or maybe local. Just you a good strong epoxy & drill/screw (drywall) the vertical fins. Work it like wood, but wear a respirator.

[Motorcitydak]

That is perfect! Thanks a lot

[Bjkadron]

With aluminum you could just use a break to fold a 90* angle in it. Then rivet it to the flat part. you don't need really thick aluminum... Just sandwich a piece of foam in between two thin sheets. Violla! you have a thick stiff lightweight piece.

And Phenolic would be pretty heavy.

[Damn True]

Look here and on CC.com for Preston's mustang.

[Force Fed Mopar]

PS, I see that you have a GLHS, that car must be awesome!

It was before I wrecked it :hammer: Lol, I'm in the process of rebuilding it, so hopefully it will be again. It will also be getting a rear diffuser.

[Motorcitydak]

Heavy? This site says 4oz/sq yard
http://www.usplastic.com/catalog/item.aspx?itemid=23654&catid=679

Well it says sp yard, am I correct in assuming that they meant to write sq?

edit:sorry I think I understand it better now. They use fabric weighing that in the making of this stuff. Does anyone know what the finished product weighs? Maybe 3/16 thick 12"x12"?

[Bjkadron]

Heavy? This site says 4oz/sq yard
http://www.usplastic.com/catalog/item.aspx?itemid=23654&catid=679

Well it says sp yard, am I correct in assuming that they meant to write sq?

edit:sorry I think I understand it better now. They use fabric weighing that in the making of this stuff. Does anyone know what the finished product weighs? Maybe 3/16 thick 12"x12"?

Well it isn't Really heavy... But it is heavier than a foam-core/alum composite. For example:

Have you ever checked out that "craftboard" stuff? all it is is cardstock and foam glued together. Really stiff and light! I think that is the way to go.

I'm building one for my Duster racecar. But I Have to finalize the position of the gas tank before I start. Have you done any concept sketches? or are you just diving in blind?

[mellowyellowCJ7]

This site has a lot of information about aero tricks: http://www.mulsannescorner.com/

The shape is important, but one of the keys to a good diffuser is the transittion point. This will determine the location of the downforce. Also feeding the diffuser is not to be ignored. I've wondered why more sedans don't take advantage of under car aero. There is lots of room for improvement.

[Motorcitydak]

Your idea of the aluminum and foam seems do-able. It would make everything easier to build. I will have to get a brake tho to make nice bends. What thickness would you suggest? More importantly, what kind of foam and glue?

You could say I am sort of flying blind rite now. I am trying to work out how to build it before I go about actually designing it. My plan is to put a slightly curved ceiling and 4 fins total with the ends curving to match the contour of the rear valance.

[Bjkadron]

Here is a site with a bunch of info on foam cores. It is geared towards boat building but the same principles apply... Hope this helps.


http://boatdesign.net/articles/working-with-foam-cores/index.htm


Benjamin

[jackfrost]

this sounds like exactly the right occasion for using fiberglass, or even carbon fiber if you can swing it. just build your plug out of cardboard or aluminum and then cast the part right from that. you could put the fins directly in the plug, or epoxy them on later. it's light, cheap, easy to work with, and you can change it later if you want.

[gonein67bird]

my rear diff. is alluminum bent around the gas tank carbon fiber rivited on with L angle. there are pics in my pt garage
https://www.pro-touring.com/forum/vbpicgallery.php?do=view&g=566

[brans72]

your link is not working gonein67bird.

[Motorcitydak]

His link works fine for me. If its not for you, just go to his public profile. You can get to his garage from there


Yours looks pretty good and looks very straight forward to build.I am going for a more invasive approach. Ill be hacking up into the rear valance. The rear bumper will be getting modified to fit the shape better. I am still not sure about where to mount the license plate tho. That area will be taken up by the diffuser

[CRead01]

my rear diff. is alluminum bent around the gas tank carbon fiber rivited on with L angle. there are pics in my pt garage
https://www.pro-touring.com/forum/vbpicgallery.php?do=view&g=566


this car is sick...

[Bjkadron]

gonein67bird: What does it attach to in the front? I can't really tell.

[MCMLXIX]

https://www.pro-touring.com/forum/vbpgimage.php?do=full&p=7675

[Procharmo]

Rear diffusers work , there's no doubt about that. But if you look under a Porsche 911 996, 997 or Ferrari 430 you will see that the smooth under body and front air dam create more down force. If a diffuser is used it's the finishing piece. That is to say it may be inefficient on its own but will gain efficiency when combined with the under body tray and air dam...

The aim is to have low pressure under the car so if your muscle car has headers, sump, gearbox, mufflers and pipes you will have high pressure.... Just a thought.....

[Motorcitydak]

I want to smooth out the entire under side of my car, leaving the frame open to use for jack access. I am planning on mounting the panels flush with that. The exhaust may be a problem here with heat build up, I am not sure what to do about that but its a long way off. For material, I am still unsure too, maybe .25 UHMW?

[CamaroAJ]

build it like a vette, exhaust down the middle. oval pipe welded together right under the drive shaft. i plan on building removabele panels with louvers to help pull heat out when driving but leaving the exhaust open in the middle. with a front splitter and rear defuser.

[Bjkadron]

https://www.pro-touring.com/forum/vbpgimage.php?do=full&p=7675

Yes... But what does it bolt to? A fuel tank strap?

[Rod]

what is that at the top of the tank?? between the tank and trunk floor?? spray foam??

[Motorcitydak]

Looks like the 'great stuff' expanding foam. He probably had problems with the gas tank rattling and squirted that stuff in there to make it a little bit more solid and stop making noise

[MCMLXIX]

That foam hold in moisture and will promote rust... please find another solution....

[gonein67bird]

drilled a hole in the L-angle slid it over the bolt that the gas tank strap attaches to and ran another nut over it.Yes that is spray foam, had a horible thud sound when I hit a big bump so I tried the foam. It was the exhaust hitting the rear end though, I nedd to dig that out of there.

[Bjkadron]

drilled a hole in the L-angle slid it over the bolt that the gas tank strap attaches to and ran another nut over it.Yes that is spray foam, had a horible thud sound when I hit a big bump so I tried the foam. It was the exhaust hitting the rear end though, I nedd to dig that out of there.

Ahhh! I see, thanks.