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Rear Diffuser

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So I decided to dive into the world of unique aero dynamics. I’ve always been interested in making a rear diffuser and have watched many youtube videos and read many articles on the subject. I finally felt like I was ready to make one and one weekend I made a mock up with cardboard.
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I uploaded it to corner ponies for critique and got some good and bad suggestions. A couple guys have a diffuser made up of ABS plastic; however, I wasn’t sure how strong ABS weld glue would hold up to varying weather and heat when I drive the car. A co-worker then suggested that I just use sheets of aluminum, then the idea came alive. Now many people state that for a rear diffuser to work you “have” to have a flat bottomed car. That is true in a since, but even without one there is still air flowing under the car and it is still under pressure. I do plan on paneling some parts of the underbody in the near future, but our axle cars are not designed well for a fully flat bottom.

Before I installed anything I wanted to go do some testing to see what kind of air flow I was getting behind the car before I installed anything. The video doesn’t show much movement from the spoiler to the rear bumper. I can post that later if anyone really wants to see it. At this point I haven’t conducted my post install test yet.
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So after doing some research on youtube, I made myself a homemade sheet metal bender out of wood, door hinges, and an angle iron. The wood is heavy enough that I didn’t need to attach it to anything, I can also use my body weight to hold it down when bending the metal.
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So my diffuser design is about a 1-yard square design with fins rising to 4.75” at a 7-degree angle. I used diamond plating for the main support because my local Home Depot only have thin aluminum sheets, and the diamond plating was the thickest sheet they had. Maybe in the future I’ll buy some thicker aluminum and replace the diamond plating.
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My sheet metal binder worked very well and was used a lot during this build. I also built two support beams that go on the top of the diffuser to help it keep its shape. Here is the unpainted version.
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I built 6 attachment brackets that attach it to the spare tire well. I used two 5.5” bolts to support the middle of the diffuser and keep it from bowing the in middle.
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I used zip ties to hold up the (evaporator?) module under the wheel well. I originally had it bolted to the diffuser but the aluminum was too thin and bowed under the weight.
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Once I got that fixed and the center of the diffuser supported it looked good. I just need to take it off now and give it a good coat of black spray paint. The entire diffuser weights ~4lbs, so it won't be slowing me down weight wise. The entire build, with building my sheet metal binder, cost me ~$100. Next month I have an event at Hallet, a track I’ve been to multiple times, and since this is the first major mod I’ve done since my last trip there I’m excited to see if it helps lower my lap times.
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-James
 

DEye

You can't get there from here...
Pretty cool, it will be interesting to see if you notice any difference at the track?
How much clearance , gound to lowest edge ?
The idea is the get clean orderly exit of air out from under the car Quickly, accelerating as it moves from the smaller to larger area...
If it works too well, i wonder how much lower will the rear end get ?
Will it rub ?

Keep us up to date on the Results from your track day?

Don
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
2,009
441
Southeast
Jhayes84:

I like it! Here are 2 suggestions for you concerning the leading and upper rear edges of your rear diffuser. Just sharing my thoughts for your consideration, either way.

1. Leading edge: I note it is slopped downward which might force some air above it. Rotate or flip the leading edge upward, thus forcing more air downward and through the diffuser gates.

2. Upper rear edge: Add an additional piece of L shaped aluminum stock along the width of the rear top edge of your diffuser. (1"X3/4" L shaped stock). Mount it across the rear top edge 1.0" side down using the existing 4 bolts used to attach your gates, (fins). This will provide the top edge with enough strength to minimize flexing.

Hope you don't mind me sharing a few thoughts.

Good luck and keep us posted on its performance.
302 Hi Pro
 
302 Hi Pro said:
Jhayes84:

I like it! Here are 2 suggestions for you concerning the leading and upper rear edges of your rear diffuser. Just sharing my thoughts for your consideration, either way.

1. Leading edge: I note it is slopped downward which might force some air above it. Rotate or flip the leading edge upward, thus forcing more air downward and through the diffuser gates.

2. Upper rear edge: Add an additional piece of L shaped aluminum stock along the width of the rear top edge of your diffuser. (1"X3/4" L shaped stock). Mount it across the rear top edge 1.0" side down using the existing 4 bolts used to attach your gates, (fins). This will provide the top edge with enough strength to minimize flexing.

Hope you don't mind me sharing a few thoughts.

Good luck and keep us posted on its performance.
302 Hi Pro

302 Hi Pro,
The front of the diffuser is angled upwards, and I was also thinking that it might not be aggressive enough and might allow air on top of it. I'll evaluate it later this week after driving the car a couple times to see how it looks. Also, thanks for the "L" shaped piece on the back. I've noticed that many guys have that and it would be an easy fab to do before I paint it.

-James
 
VoodooBOSS said:
Nice work. There's a thread here somewhere on how to build your own front splitter and another on front wheel spats. I suspect the front splitter will make more of a difference than the diffuser but they should play nice together.

VoodooBOSS,

I have already made a front splitter out of ABS plastic. It is a 1/4 inch thick and has 8 points of attachment from it to the car.
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I have used it at two track events and a couple auto-x's with great success. I'm working on budgeting for a rear wing for next season to finish things off, but it's just not in the budget for this season.

-James
 
100
0
Looks pretty good. Running a custom diffuser also, mounting a bit more simple, though. My buddy made/makes them. Tucks up in the front behind the sway bar.

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