# Anyone out there have a custom rear diffuser?



## chrisc351 (Feb 17, 2011)

I was looking at the OSIR carbon fiber rear diffuser because of its motorsport look, but I'm curious, has anyone done a custom more purpose built rear diffuser?


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## Marcus_Aurelius (Mar 1, 2012)

I started a project a long time ago to make a front splitter and a rear diffuser. I never got to finish it (mainly the diffuser) because I wasn't sure what final route I was going with my exhaust. Hopefully I can find time to finish it before next racing season and post something here. 

Most of the ones I've seen for the TT aren't functional and mainly for looks. Are you after function or form? A rear diffuser will also need to be paired with a front splitter to keep the dynamic balance somewhat intact.


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## chrisc351 (Feb 17, 2011)

I was originally going with a form/function approach but I recently drove a 911 through the snow and thought there is no way I want to give up performance. So my plans have changed to a still good looking fitment but with a beefier track approach, I plan on running 18x9.5 VMR's with 235/40's all around or 245/40's. As for the front splitter, I saw a carbon fiber cupra front lip but I'm not sure if that's really gonna do anything either. 

My initial thought when it comes to the rear diffuser would be a much wider rear diffuser, possibly cutting the bumper to fit something much wider, but I also want the car as my daily driver for the summer. So I really don't want to butcher (as sometimes people do, thinking their doing the car some good). I was possibly thinking something like this?


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## Neb (Jan 25, 2005)

Following this for a fun thread idea.

Most custom one's I've seen have just been for looks. Shaved cutouts or moulded diffuser into the rear bumper.


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## warranty225cpe (Dec 3, 2008)

This would be great for someone looking to do exhaust deletes or side exit. This could take the place of the rear valance, and follow the contour underneath. Cool idea


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## chrisc351 (Feb 17, 2011)

It wouldn't necessarily have to have the exhaust delete, it could be possibly be more like this, with the exhaust just pushed further out to the sides, kinda like how this RX7's APR diffuser is with dual cutouts (the reason for the dual cutouts is because I'm planning on doing a custom exhaust with a y-pipe and a switch, one with a muffler, and the other with a straight pipe).










But I'd be more than happy if I could mold this onto the back and hide the exhaust


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## Forty-six and 2 (Jan 21, 2007)

Cutting the bumper to fit one may be a little extreme, but a well fitted functional one may be pretty cool. A carbon one would end up being pricy, but a fiberglass one painted black could be fairly reasonable. Hidden exhaust tips would be a must. To cut cost and ease of construction, I would make the sloped portion as one piece, and the fins separate, then attach the pieces together. 

It could be pretty cool if fitted to the exhaust valance cutout.


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## VMRWheels (Aug 11, 2009)

This should be pretty interested. Can't wait to see the finished product. *Subscribed


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## Tempes_TT (Oct 24, 2010)

Ive thought something like this would be cool, but also having a front splitter to to match as well. 

Even pulling something like this off










Dont know how much of that is actually functional, but as far as design, I think it could be made to work.

Wish we were able to get a hold of ABT tuned TT's


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## chrisc351 (Feb 17, 2011)

Well, I was thinking to save on costs, and because my design would probably never be as good as big companies taking some measurements and talking with a few diffuser companies about the measurements and existing diffusers and see if those can be molded somehow onto the car


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## Forty-six and 2 (Jan 21, 2007)

Sure it could be... Research the function of a diffuser, and what it needs from a design standpoint. Having a company modify an existing product for a custom application is highly unlikely, and possibly the most expensive route. The rx7 one you posted, to me, looks poorly designed. The only good is that it starts at the axle...

It would be pretty easy to make the mold for one of these. The difficult part would be pulling it off in carbon at a pro level. A functional design could figured out simple by reading and getting under the back of the car.


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## sTT eV6 (Oct 26, 2010)

chrisc351 said:


> It wouldn't necessarily have to have the exhaust delete, it could be possibly be more like this, with the exhaust just pushed further out to the sides, kinda like how this RX7's APR diffuser is with dual cutouts (the reason for the dual cutouts is because I'm planning on doing a custom exhaust with a y-pipe and a switch, one with a muffler, and the other with a straight pipe).
> 
> 
> 
> ...


Ive had issues with the electric cutout bypass systems in the past.
I would recommend a vacuum type with a switch operation.








Steve


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## tedgram (Jul 2, 2005)

opcorn:


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## Krissrock (Sep 10, 2005)

I was actually bout to have one made, met up wit the fabricator over a beer to go over some designs and ideas......then I lost my job. 

project scrapped. Perfect for my car since I don't have exhaust back there. 

I will have one eventually


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## chrisc351 (Feb 17, 2011)

sTT eV6 said:


> Ive had issues with the electric cutout bypass systems in the past.
> I would recommend a vacuum type with a switch operation.


Thanks! I appreciate the feedback, I'm actually working with a motorsport company on this so I'll definitely bring it up when I talk with them about it (once I get my car back on the road, damn winters :banghead: )


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## Forty-six and 2 (Jan 21, 2007)

After thinking about it, I may give it a shot pretty soon. I don't have a muffler, so there is plenty of room to work with. I crawled under there this morning, and it really won't work out, space wise, with a full sized muffler taking up all of the room it would need to be functional. Making one would be easy enough, but producing these would take a bit of effort. There would be cutting required. The metal edge that the exhaust valance bolts to would need to be trimmed away, and a new exhaust solution would also be necessary. 

If these modifications don't scare you away from the idea, I can come up with something. :thumbup:


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## chrisc351 (Feb 17, 2011)

I'd be happy to hear/ see what kind of idea's you have! I just can't have a massive diffuser in back because it does have to be a DD


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## Forty-six and 2 (Jan 21, 2007)

Who else would actually be interested in one of these? I'm just trying to figure out if I should consider making a mold or not. Making two or three can be done without a mold, but any more than that, I would start off making the mold.


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## 1.8TurboDaily (Feb 26, 2013)

watching and interested! :thumbup:


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## chrisc351 (Feb 17, 2011)

I'm thinking of buying a really ****ty rear bumper lip, or a used one if I could get one cheap enough, and then I wouldn't be afraid to cut it. My best friend is an autobody guy so he'd be helping me work on this, I would cut the rear diffuser space really wide and put a wide diffuser in place with a hidden or side exit exhaust

I would be happy to cut away at this piece of junk!









As for idea's of what I'm thinking, I'm thinking a generally pretty simple design like this with fins going all the way across with no exhaust showing 










They all seem to have the general shape of the one on this corvette, curved on the edges


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## Krissrock (Sep 10, 2005)

Forty-six and 2 said:


> The metal edge that the exhaust valance bolts to would need to be trimmed away, and a new exhaust solution would also be necessary.
> 
> If these modifications don't scare you away from the idea, I can come up with something. :thumbup:


already done...i'm interested 

I've been running no valence for years now


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## chrisc351 (Feb 17, 2011)

Here is another idea of what I'm thinking of, possibly make it into two pieces that can be screwed together or attached somehow, I'd obviously have no exhaust tips.


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## Forty-six and 2 (Jan 21, 2007)

That one is a little too much for my taste. I'm going to be shooting for something more fitting for the style of the TT. Functional and simple is the way to go. :thumbup:

It looks like the weather around here is warming up, so I will be taking care of a couple things this weekend. After that, I will get the templates cut for this thing. I have a good, quick plan for the mold.


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## chrisc351 (Feb 17, 2011)

Well if your doing your molds real soon, maybe I'll hold off on mine and see where yours goes, I still gotta get wheels and coils first!


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## Marcus_Aurelius (Mar 1, 2012)

I think at this point, it is safe to say that you guys are after look and fit more than functionality. That's the reason I asked what the purpose was in the beginning of the thread; and judging from the comments and pictures posted, you guys seems to be after a rear valence that looks like a diffuser (although it might not necessarily work like one). A functional diffuser is not going to be subtle or pretty (high angle of attack and very protruding fins are a necessity to get something remotely effective).

Below, is a general idea of what functional splitters end up looking like - and like I said once you make it functional in the rear, something needs to be done in the front or it's understeer city. Overall balance screwed due to added increase in rear grip, and no increased in front aero grip to match the addition to the rear (we all know that fromt mechanical grip isn't the platform's best attribute).


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## Krissrock (Sep 10, 2005)

of course. Anything really functional would be destroyed from street driving pretty quickly (speed bump) 
Almost all of the diffusers on the market are hardly functional.


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## Forty-six and 2 (Jan 21, 2007)

Marcus_Aurelius said:


> I think at this point, it is safe to say that you guys are after look and fit more than functionality. That's the reason I asked what the purpose was in the beginning of the thread; and judging from the comments and pictures posted, you guys seems to be after a rear valence that looks like a diffuser (although it might not necessarily work like one). A functional diffuser is not going to be subtle or pretty (high angle of attack and very protruding fins are a necessity to get something remotely effective).
> 
> Below, is a general idea of what functional splitters end up looking like


 I have to disagree. Not all diffusers have to be ugly and huge to be functional. The example you posted is a pretty extreme one. I can agree that the wider and "taller" they can be the better they perform. There are plenty of street cars that came with a functional factory diffuser, that is still aesthetically pleasing. 

F1 has restrictions on the height and widths of their diffusers yet they still account for 30% of the overall downforce of the car. Obviously an F1 car's aero design is extremely efficient, and make any full body car somewhat laughable. Regardless the diffuser can still create downforce without being massive on any car. Like you said some sort of front splitter would be necessary keep the front in check. The front end is said to be the most important, while the diffuser is the second. 

Without a full flat bottom of the car I could definitely see the diffusers function be limited. The best I can do is make it extend up to the control arms. Even without being as extreme as others, there is air flowing under the car that will benefit smoother exit out of the rear. It naturally will be a lower pressure zone, creating downforce. May be minimal, but will still function. 

The main reason for personally wanting one is the fact that without a muffler, I have a massive amount of open space that just creates a big dragging air pocket. I could simply make a flat panel to enclose the open space, but a diffuser would just make more sense. I'm not interested in chopping up my bumper, so fitting it into the existing valance opening is my outlet restrictions. The vertical strakes can be made to extend even further downward, past the lower edge of the bumper. The sections on the sides of the main diffuser section in the center can also have it's own strakes. Much shorter of course, but will still help with channeling air out from under the car.


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## G'D60 (Mar 11, 2002)

Correct me if I'm wrong but didn't the stock exhaust system (rear muffler) act like a bit of a diffuser/splitter? The way it was shaped and designed it acted as a big panel to fill the hole (or open space) 42 is talking about. I thought I read about that somewhere... 

Anywho I have this 'valance' sitting at home... Not sure if I want to go with this or find an OEM 3.2 honeycomb rear... I'll get it painted and see how it turns out. 

To be honest I'm not even sure who makes it... anyone know or seen it before??


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## warranty225cpe (Dec 3, 2008)

G'D60 said:


> Correct me if I'm wrong but didn't the stock exhaust system (rear muffler) act like a bit of a diffuser/splitter? The way it was shaped and designed it acted as a big panel to fill the hole (or open space) 42 is talking about. I thought I read about that somewhere...
> 
> Anywho I have this diffuser sitting at home... Not sure if I want to go with this or find an OEM 3.2 honeycomb rear... I'll get it painted and see how it turns out.
> 
> To be honest I'm not even sure who makes it... anyone know or seen it before??


 Not sure who makes that, but it is NOT a diffuser. Thats a valance.


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## G'D60 (Mar 11, 2002)

Valance. thats the word I was looking for. thx


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## Tempes_TT (Oct 24, 2010)

Ive always liked the idea, but in terms of functionality, doesnt one have to be going at a fairly high rate of speed, generally associated with some sort of racing, to make good use, if use at all, of the splitter, under-tray and diffuser? And wouldnt you need all three to really claim full functionality as youll most likely also need 'fences' running the skirts and such to keep outside air out and keep flow smooth and directional to produce the downforce?  

I want to say I have basic understanding, but correct me and enlighten me where Im wrong! 

either way, Ive thought about having a 'semi-functional' splitter, undertray and diffuser for a long time. I may take some time to make some 3d renderings...but dont hold your breath on it. 

Putting a car on the road course has been a long time goal of mine and am interested in where this is going, whether its for function or aesthetics. :thumbup:


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## Marcus_Aurelius (Mar 1, 2012)

Forty-six and 2 said:


> I have to disagree. Not all diffusers have to be ugly and huge to be functional. The example you posted is a pretty extreme one. I can agree that the wider and "taller" they can be the better they perform. There are plenty of street cars that came with a functional factory diffuser, that is still aesthetically pleasing.
> 
> F1 has restrictions on the height and widths of their diffusers yet they still account for 30% of the overall downforce of the car. Obviously an F1 car's aero design is extremely efficient, and make any full body car somewhat laughable. Regardless the diffuser can still create downforce without being massive on any car. Like you said some sort of front splitter would be necessary keep the front in check. The front end is said to be the most important, while the diffuser is the second.
> 
> ...


 Matt, the reason I said what I said is because there are all kind things that can be proven to work on paper but in practice doesn't work as well as they did theoretically. The race community have a tendency to find the "smoke and mirrors", simply because if something really works it also shows on the clock. There are well known and respected companies that came up with "functional and pretty" carbon fiber diffusers, well they didn't perform as intended at the track. They were more of a cosmetic piece for the guy filling your rear view mirror to look at in front straight. The ugly and "extreme" one posted was an actual functional upgrade over one of those pretty carbon one that did absolutely nothing (at least at the speed reached in typical tracks across America). Same goes for rear wings, there are a well known company that made a big name for themselves with carbon rear wings for all applications, however anyone serious about getting some aero help for their rear end know to stay away from those (even if the real functional solutions meant ugly and "extreme"). 

BTW, you mentioned "creating downforce" with a rear diffuser, correct me if I'm wrong but isn't the purpose of those to reduce drag-inducing turbulence and lift when the low pressure air from under the car gets to rejoin the high pressure air moving above the car? I may be off, but I haven't heard of diffusers on production cars actually "generating downforce", and if one did I'd think it would look more like what I posted. In any case, function is subjective. My idea of functional may have nothing to do with what others may consider functional. :beer::beer:


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## Forty-six and 2 (Jan 21, 2007)

You're right. While the function of a smaller, restriction fitted diffuser will most likely be minimal. Only being effective at high speeds, but there is still function. It is completely obvious one built as large as possible, with a full under pan will be close to the top of the function list. 

Your description of the function of the diffuser is spot on. Downforce is created by the accelerated speed of the low pressure air re-entering the high pressure zone at the rear of the car. This creates a "vacuum" under the car, inturn pulling the car down aka downforce. It is essentially a zero drag spoiler. 

While tunnel tested race cars will benefit from a diffusers in major ways, since they can be tuned close to the track speeds the car will be racing at. What I am getting from different designs, small to large, is the smaller will really only have an affect at high speeds. The larger will start functioning at a lower speed. I'm sure the speeds will still be relatively high, due to the need for air speed. 

I recently read a story in a diffuser debate thread. The guy has/had an Elise that he would run track days at the Nurburgring. He said he was having trouble with a particular high speed crest in the middle of a turn. As he came over the crest, the rear end would lift, and roll the side walls of the tires as he regained control. He just thought he would fight off the loose rear, until he lost it and tapped the barrier. He then fitted a reasonable diffuser he can feel the car stay planted as he comes over the exact same, previously problematic crest. 

I know, it's one case of a specific scenario. We may not see high speeds regularly in our street driven TT's, but I'm sure at highway speeds, the may be a little more noticeable stability. I'm not thinking I will make a diffuser that will make night and day difference. Building one with the knowledge of what makes them functional, and doing the best to fit those attributes into the limited space available, is in every way better than creating a diffuser looking valance, like nearly every one posted before the Evo.


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## Marcus_Aurelius (Mar 1, 2012)

My problem moving forward with the discussion is the idea of creating downforce by the diffuser. For that to happen, in my untrained knowledge, you would need speed that this car doesn't make in real life use, and a diffuser that is far more extreme than what I posted. A diffuser, in production cars with aero bits adapted to help, is a helper to other pieces at best. Not capable of creating downforce but reducing lift (opposite of downforce) and drag. 

Due to turbulent underbelly design, relatively high ride height (distance from ground to belly), production cars generate tons of lift at speed. Added aero parts have to first counteract that lift (call it negative downforce) before they can generate any actual downforce. Doing a semi "functional" semi-visually pleasing diffuser is a sure way of achieving nothing in terms of real function. 

Here is a basic and simplistic extract that explains what a diffuser is to a production vehicle: 

"*The diffuser has two (2) purposes: 
1. To reduce drag. 
2. To reduce lift. 
What goes on at the rear-end of a car: 
The area immediately behind the car tends to have turbulent, slow-moving air. The fast underbody air that exits the car ends up meeting with this slow-moving outside air. The greater the air speed difference is, the more turbulence is created. This turbulence causes unwanted drag and disrupts the airflow that exits from the underbody, which in turn decreases the underbody's ability to reduce lift. 
How the diffuser works: 
The diffuser itself does not actually create downforce - it works in conjunction with other aerodynamic components to reduce drag and lift. The upward front-to-back angle of the rear diffuser causes the fast-moving underbody air to expand and slow down. This slower air is then better able to meet up with the slow outside air, thus reducing the amount of turbulence behind the car. The vertical fins are there to make sure that the air at the rear left and right sides do not disturb the function of the diffuser. 

When is a diffuser effective: 
As a general guideline, the rear diffuser needs to be angled upward at least 10 degrees. In order for a rear diffuser to be effective, the air that travels under the car from the front needs to be as fast and smooth as possible. Slow-moving, turbulent air is not going to help a diffuser do its job. 
*" 

I am glad you brought the Lotus example because there is plenty of real gear heads and racers in that community. One of them actually did grassroot wind tunnel testing on the effect that the said diffuser was having on the Elise. The result was quite deceiving (as most racers were already pointing out) because the diffuser didn't do nearly enough to the gobs of turbulence that was happening when the high and low pressure had to meet again. Judge for yourself: 

http://m.youtube.com/#/watch?v=bXMg...ktop_uri=/watch?v=bXMgwRlNWLk&feature=related 

Here, we kind of got into it in general, but I got booted out of the forum in that period and the thread went into the deep end. However you can have a general idea of what we're working with in the TT (including the fake ABT numbers). The TT has a somewhat developed underbelly for what it is, but I don't see a splitter, without the help of functional wing, overcoming the inherent lift and then generate downforce (at least not a visually pleasing one). :beer: 

http://forums.vwvortex.com/showthread.php?5266619-Lets-talk-Aerodynamics&highlight=Diffuser+splitter


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