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  1. #1
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    Who has made their own rear diffuser out of aluminum?

    Out of frustration that there appears to be ZERO companies who sell a pre-made lower rear air diffuser for a 1969 Camaro/Firebird, I've decided to embark on making my own out of aluminum. I have no metal fabrication experience and no welding experience.

    I plan to use CAD for the templates (cardboard aided design) and tack everything together using a torch and Alumaloy rods. Then I'll take it to a local shop for proper TIG welding of all the seams.

    It will be designed to attach fully to the bottom of the rear valence and to the bottoms of the rears of the fenders (think a giant rectangle). I plan on using dzus fasteners. I *think* there is just enough of a lip on the fenders and rear valence for drilling to make this happen so the flat sheet will be close flush with the actual body panels.

    So, I have a few questions lol.


    1) What gauge aluminum sheets and what grade aluminum should I buy for this?

    2) What size Alumaloy rod should I use for the tacks?

    3) What should the front look like in terms of where I end it? Like do I wrap the front up around the rear of the fuel tank and fasten the front sealed to the bottom of the trunk between the rear frames above the axle?

    4) For the verticals, how many should there be? I'm obviously not an aerodynamicist so I need to depend on what others that found to work. Knowing how many will then tell me how far apart to evenly space them.

    5) How far down into the air stream do the verticals need to protrude?

    6) Should the verticals just be perfectly straight, or slightly contoured out towards each side just a tiny bit?


    To clarify, the car will be participating in top speed events, however, I plan on staying in the sub-199mph class. I am more concerned with the stability of the vehicle from a safety standpoint than I anything else. I also plan to run the car on road courses which can be 145mph+ in some areas (think Big Willow).

    Pictures are worth 1,000 words if anyone would care to share pictures of their functional rear diffusers (not just made for looks).

    Thanks!!!! All comments/suggestions are welcome!

    Alex
    http://www.TheFOAT.com/92GTA
    1969 Pontiac Firebird
    w/BP 461ci stroker kit, 670 heads & XE274H cam. Primer black w/black interior.
    1992 Pontiac Trans Am GTA w/SLP Performance Package. Dark Jade Grey Metallic, grey leather, T-Tops.

  2. #2
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    I can't offer you any structural or design advice, but have you tried looking at what the guys have done with the Big Red Camaro? You might be able to pick up a few ideas from then since they've been there, done that.

    There's also a series on Amazon Video, I think it's called "Going Fast and Taking Chances" that's all about the Big Red Camaro and you see every square inch of that car and what they do to make it go.

    Good luck!

  3. #3
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    I've seen the Hoonigan video of Big Red where they get her up on a lift. I don't have Amazon tho. I'm sure theirs is overkill for me, but you're right, I'll give it another look for ideas before I start.

    EDIT: Upon taking another look at Big Red, she doesn't have a rear diffuser as such, partly due to the way the exhaust is designed to blow away the turbulent air at the rear of the car. My exhaust exits in front of the rear tire like an old Trans Am race car so I don't have the benefit of a setup like that.
    http://www.TheFOAT.com/92GTA
    1969 Pontiac Firebird
    w/BP 461ci stroker kit, 670 heads & XE274H cam. Primer black w/black interior.
    1992 Pontiac Trans Am GTA w/SLP Performance Package. Dark Jade Grey Metallic, grey leather, T-Tops.

  4. #4
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    Aug 2008
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    jacksonville,fl
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    I'll give some advice towards the fab parts.
    Cad design is good, sharp sissors & material about like the back of writing tablets is what you want
    If making from aluminum, #3003 h14, .062 or '063 thick is the common stuff to find & use. As far as the alumaloy rods, only people I see using that crap are the ones selling it at shows, attaching soda cans together which are really high tech metal. You could try some, but I'm not even considering those type of products, which I tried 25 to 30 years ago. Maybe they are better now..
    I would either do all welding or have a welder do all welding, tig welding & low temp rods would not mix. Also, just a fact , it's better if the same person making the part welds it, there can be so many fit & warp issues between the 2 fields one person could blame on the other. Tig needs really tight joints to do good & warping to the pint of changing fit can easily happen with aluminum.
    Option 2 with aluminum is to make parts with flanges to be attached with epoxy panel adhesive, such as 3-m 8115. spears can be test fit with small self tapping screws, then screwed & glued, then remove screws. Application gun can cost about $40 these days & Panel adhesives can cost a little over $ 30 to $50 per pack. I've been using Smart 263 I think it is from Finishmasters.
    Make no mistake, aluminum fab work takes practice & skill to get good at. An awful lot to learn to make compound curve parts.



    Another option to present would be fabbing from abs plastic sheets. Easy to cut, can bend & be formed with heat, abs glue will pretty much melt parts together & can be sanded, filled with something like evercoat polyflex & easily painted. Sheets can be like 1/8 1/4, or 1/2" thick without getting very heavy. Could make spears from 1/2" thick plastic, sanding front to a taper.
    Abs or UHMV (slick plastic) can also make a good bottom edge of a front spoiler to take up some road abuse. I'm not an areo engineer, but keep in mind that many cars featuring a rear difussor have a crapload of custom work designing the underbody to be flatter & less turbulent in airflow. I think the most aggressive front spoiler design to keep as much air as possible out of car will be the biggest help.

    https://www.google.com/search?hl=en&...ntYag7LSfY4LM:


  5. #5
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    Thank you VERY much for all of that advice!

    I for sure planned on having someone do the full TIG welds and just use the aluminum sticks to tack the spears. Great point about tho about something changing during the welding process and there being fitment issues possibly. I had only chosen aluminum thinking it would be the easiest for me to fabricate with.

    Upon consideration after your post, I think the best direction for me given me experience, is to go with ABS plastic sheets. I'm also thinking about doing rivets in addition to the glue instead of screws and leaving the rivets.

    I absolutely agree on the front spoiler and I have been struggling with that for a long time. I'm going to have to fabricate my own unless I can find a used Spectre front spoiler. I figured the rear diffuser would be a good place to start since it looks like it will be much easier. Then I'll do the front spoiler out of the ABS as well.

    Working on the bottom side and adding some flat covers to the bottom of the car is something I plan on getting into, just not yet. I need to finish my custom headers and exhaust first. Plus I need to consider placement and how I do the leading edge of the sheets and how air is going to exit over the trailing edge that gets caught between the sheets and the underside of the car. I'm thinking maybe NACA ducts on the inside of the sheets facing up, to draw moving air to the underside as much as possible creating an area of low pressure between the sheets and the bottom side of the car. But then I'm not sure how that would affect the air stream going under the car above the road.

    Thanks a lot, much appreciated!
    http://www.TheFOAT.com/92GTA
    1969 Pontiac Firebird
    w/BP 461ci stroker kit, 670 heads & XE274H cam. Primer black w/black interior.
    1992 Pontiac Trans Am GTA w/SLP Performance Package. Dark Jade Grey Metallic, grey leather, T-Tops.

  6. #6
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    Seems like you have some pretty solid advice, the only thing I have to add is I would caution on fully TIG welding the slats. Unrestrained thin aluminum can warp pretty severely, I'd recommend lightly stitch welding them and jump around on the heat.
    1972 Plymouth 'Cuda - Not LS-swapped, 5.7L Hemi [MS3 Gold Box], T56 Magnum 6-speed - 'Cuda Build Page
    1976 Dodge D100 - Warlock
    2016 Subaru WRX - E30 Tune

  7. #7
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    Quote Originally Posted by GoodysGotaCuda View Post
    Seems like you have some pretty solid advice, the only thing I have to add is I would caution on fully TIG welding the slats. Unrestrained thin aluminum can warp pretty severely, I'd recommend lightly stitch welding them and jump around on the heat.
    Noted, if I decide to attempt aluminum vs. ABS.
    http://www.TheFOAT.com/92GTA
    1969 Pontiac Firebird
    w/BP 461ci stroker kit, 670 heads & XE274H cam. Primer black w/black interior.
    1992 Pontiac Trans Am GTA w/SLP Performance Package. Dark Jade Grey Metallic, grey leather, T-Tops.

  8. #8
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    That is part of the main reason not to fabricate something for someone else to weld. Unless they are a car fab shop, you more than likely would get back a warped pile of crap that no longer fits.

    With an ABS plastic glues can melt it in giving a strong bond & spears can be done from 1/2" thick plastic, which could either be screwed from backside, glued, or both.

  9. #9
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    A rear diffuser will be pretty much just for looks unless you have a flat smooth under tray in front of it preferably stretching forward to a splitter/air dam at the front of the car. There was a long aero thread on this forum but the OP got booted or something and the thread disappeared it seems. Rear diffuser is to allow expansion and slowing down of air from an under tray so without a smooth bottom you don't get much of an advantage if any.

    Couple books that have good general aero info. Both explain the same basic basic concepts similarly. The Katz book has more math in it.
    https://www.bing.com/search?q=race+c...ZD&form=MOZLBR
    https://www.amazon.com/Competition-C...GTGF4KBREJ3DJ3

    Pics below of what I've been working on for my 2nd gen bird. Information about my aero stuff in these threads.
    https://www.pro-touring.com/threads/...nd-more/page10
    http://www.frrax.com/rrforum/index.p...pic=17185&st=0

    Discussion of 2nd gen bird aero here but a lot of the info is general and applies to your 1srt gen. http://transamcountry.com/community/...?topic=71522.0




  10. #10
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    Sep 2016
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    Thanks for the reply! I'm a member on those forums as well.

    Looks like I have a lot of reading to do.

    What's interesting is that I have strongly considered adding those 2nd gen rear fender flares to my 1st gen. Might look off, but it seemed like a good idea. I also would like to pursue the lower body skirts too.

    Thanks!
    http://www.TheFOAT.com/92GTA
    1969 Pontiac Firebird
    w/BP 461ci stroker kit, 670 heads & XE274H cam. Primer black w/black interior.
    1992 Pontiac Trans Am GTA w/SLP Performance Package. Dark Jade Grey Metallic, grey leather, T-Tops.