View Full Version : DIY Rear Diffuser

05-10-2013, 04:35 AM
I was admiring Konig_Feather's Fit at the last HellaFit Meet, and noticed that the rear bumper of the "base" Fit looks quite nice without a Sport Lip. After studying Henry's Fit for a several minutes, an idea hit me! With a little bit of imagination, I envisioned my GD3 with a Rear Diffuser on it!

It took me a day or so, but I eventually figured out a simple way to fabricate some "strakes" to give my Fit, a decent looking Rear Diffuser.

First I made a pattern out of cardstock, then traced it onto some plexiglass.
http://imageshack.us/a/img822/3232/img0060dj.th.jpg (http://imageshack.us/photo/my-images/822/img0060dj.jpg/)

After cutting the plexiglass, I riveted it to the bumper.
http://imageshack.us/a/img221/2673/img0061xt.th.jpg (http://imageshack.us/photo/my-images/221/img0061xt.jpg/)

After riveting them all in place, I masked the bumper and sprayed it flat black.
http://imageshack.us/a/img341/8149/img0064fq.th.jpg (http://imageshack.us/photo/my-images/341/img0064fq.jpg/)

Here's what it looked like after pulling off the mask
http://imageshack.us/a/img839/4261/img0069rg.th.jpg (http://imageshack.us/photo/my-images/839/img0069rg.jpg/)

Here is the result
http://imageshack.us/a/img705/6428/img0073mgj.th.jpg (http://imageshack.us/photo/my-images/705/img0073mgj.jpg/) http://imageshack.us/a/img708/8829/img0085il.th.jpg (http://imageshack.us/photo/my-images/708/img0085il.jpg/) http://imageshack.us/a/img694/5814/img0089op.th.jpg (http://imageshack.us/photo/my-images/694/img0089op.jpg/)

My next project is to fabricate a bellypan to cover that open space just ahead of the diffuser.

05-10-2013, 07:15 AM
AWESOME job. I love DIYs like this and it looks GOOD. Every time I wash my jazz I think "I have got to buy some sheet aluminum and make a filler for that space".

05-11-2013, 11:21 PM
DRAT now you got me thinking LOL. I have been looking around for some of the "plates" or whatever you call them that mount on the back bumper cover and are about the same size as real license plates.

They have a series of three or so big holes in them and just add some bling around holes cut into the rear bumper cover to allow trapped air to escape. I have seen some nice CF ones but have never found where around here they sell them.

So now you got me thinking it wouldn't be TOO hard to make some out of plexiglass..... and a little vendor lady down the road sells cheapo CF vinyl in small squares that would be the right size for like $3.00

05-12-2013, 02:30 AM
I had a similar thought, but with a different look. However, back to your idea, what about this... forget the Plexiglass! Why not just put the CF material right onto your bumper, around the holes you made? Or, how about just putting holes right into the bumper...period?
Before you get too far into your idea, tell me what you think of this idea. I envision cutting a "window" into the bumper, let's say for example, a 24" x 3" hole. I would then place a piece of aluminum mesh behind that void to give it a finished look. I was debating whether to paint it flat black, or SSM, the matching body color.
I too feel that doing something like this would help vent the trapped air caught behind that area.
Hey, how about you do it first, and let me know if there is any benefit to this.

05-12-2013, 03:32 AM
Were you thinking something like this?

I was thinking something similar to this...

05-12-2013, 07:29 AM
Similar to the first set but much smaller. The ones I saw were about a foot long and maybe 4-5 inches tall with three holes in them. I think they were in photos on the freak site where everybody was laughing at them UNTIL DSM put them on the back of his race box then they became cool again.

They have to sell them around here as I have seen them on a lot of cars but have never been able to talk to the owners to find out where they got them.

I thought about using just holes in the bumper cover and have seen them on some cars around here and they look ok but "unfinished" to me. The covers just add a more finished look to me.

Brushed aluminum like in the first photo may be one more option.

05-14-2013, 02:06 AM
Well you got me off my butt and thinking about this. Today I bought the cf wrapping. It's a piece about 2 feet square and should be plenty for two "hole covers". Cost me all of $4.00. I think the last chunk of plexiglass I bought was about 3 feet square and it was even cheaper that will be my next purchase. :p

Got to "figuring" things out and found that back surface has a bit of a curve to it so the plexiglass should work better than aluminum as it is more flexible and should conform to the curve better.

Was going to use a license plate as a template but found they will be too big to look proportional so looked around and found an individual box of kleenex will FIT quite nicely and in proportion.

Not sure yet if I will be using any screen or not that decision will come after drilling the holes and seeing how it will look.

05-14-2013, 03:34 AM
Ha got off my butt and checked my "good stuff" (junk pile) and found I have enough plexiglass left over from my last project to make my two "hole covers. :)

05-14-2013, 03:40 PM
It's interesting how one's choice of material(s) and design is often dictated by what is available in one's "Junk Pile!"
In designing the "Strakes," I had to make them a bit smaller than envisioned. The Plexiglass in my parts bin was smaller than the template I first made. I had originally considered ABS plastic, but went with pieces of scrap Plexiglass instead. I just couldn't get myself to cut up a brand new sheet for this project,

07-26-2013, 12:46 PM
After driving around with this "Diffuser" mod, I noticed an interesting side effect. At first, I didn't want to make any early claims, however after burning through at least 10 tanks of gas, I have to note that i am seeing a bump in mpg.
It's been much easier to hit tank averages of 36-39 mpg lately, than compared to previous struggles to reach 34-38 mpg tank averages.
I am guessing the lower rear lip created more aerodynamic drag than I imagined (or perhaps there has been a change in fuel formulas since mid July).
For the record, I am not going to reinstall that lip to test out my theory. BTW, this may also support why I hear that base Fits get better mileage.

07-26-2013, 10:49 PM
Good report guess that mean now you need some HOLES. :rolleyes:

08-02-2013, 10:11 AM
I looked around today and with my stock sport lip it fastens to the base rear bumper and if one was to cut holes in the sport lip you would see the base bumper underneath it and no air would escape.

So I'm thinking I could cut smaller round holes in the lip then larger round or long square holes in the bumper so you wouldn't see it and let air out.

Still in the planning stages but this stuff cuts like butter so it's just a matter of marking the holes you want and a few minutes with a dremel tool it's done.

Maybe when I get a minute I could cut some "hole" replicas from black duct tape I happen to have and try them out to see what looks the best.

Mike you got me thinking once again. :rolleyes:

08-03-2013, 12:38 AM
Don't waste time cutting holes! Use your machine gun!