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Discussion Starter · #1 ·
Ran across these photos of an installed Audi "accessory" rear diffuser on an A5 in Germany. This diffuser (along with many other accessories) is currently available in the German A5 accessories catalog discussed in this thread.
 

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damn! that looks like an interesting "accessory". i think it'd look best in black with my car, though. is it like the black plastic on the regular A5? or does it have to be body-painted?
 

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damn! that looks like an interesting "accessory". i think it'd look best in black with my car, though. is it like the black plastic on the regular A5? or does it have to be body-painted?
That piece is made of a metal, at least thats what the dealer was telling me!
 

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Shame it doesn't fit. There's a stupid 'lip' all the way round which ruins the lines of the bumper. If you're gonna do this, it should not be body coloured, otherwise the lip makes it look like it either needs fitting propoerly or is a part for another car!

...carbon perhaps?

Edit: plus I've just noticed a ridge along the bottom edge too, meaning it can't actuall 'diffuse'... adiffuser is supposed to diffuse the air coming from under the car, reducing turbulence to increase the speed of the air under the car and increasing the down-force through the 'ground effect' (sorry that's prof there!) ...so in conclusion it looks like a stick on bit of plastic you'd see on a japanese hot hatch and ruins the lines of the car.
 

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Doesn't look nice at all to me.
 

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Discussion Starter · #8 ·
Shame it doesn't fit. There's a stupid 'lip' all the way round which ruins the lines of the bumper. If you're gonna do this, it should not be body coloured, otherwise the lip makes it look like it either needs fitting propoerly or is a part for another car!
Actually, I think that "lip" you're referring to is supposed to serve as an accent. The pic from the brochure (below) shows this to be an (aluminum?) accent trim piece. But, it doesn't seem to show up well in the pics above...maybe it's the lighting or maybe the owner had it painted the same as the body color. Regardless, that design is not my cup of tea.
 

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oh god, there's more to it than just that. now that i've seen what the whole thing's supposed to look like, i don't think i can consider it anymore, lol.

i like the contrast with the black piece as well, for the record (which is why I asked if it was a black piece). I just wish I could get something a little more aggressive than the regular rear piece. that back plastic piece is, to me, the only blemish (a small one at that) on an otherwise gorgeous car. The only exterior part i'm thinking of changing is that, and this is the first one i see that isn't completely outrageous (i take that back after seeing the rest of it *shivers*)
 

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Discussion Starter · #10 ·
crespo, check out these pics of the stock A5 diffuser painted body color. The first two pics are from an actual A5 painted that way, but the third pic is a Photoshop version of another car (after & before).
 

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IMO if you're gonna change the diffuser, you might as well really change the diffuser, otherwise just leave it alone:



-Ray
 

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IMO if you're gonna change the diffuser, you might as well really change the diffuser, otherwise just leave it alone:



-Ray
That Rieger diffuser looks pretty good..but it needs the larger exhaust tips like in the pic.
 

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Wow, I like this Riegger one.
 

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Discussion Starter · #15 · (Edited)
Wow, I like this Riegger one.
I like the diffuser part of it, but just not the painted add-on pieces that are on the sides and rear bottom part of the bumper cover. It kinda ruins that seamless look, IMO.
 

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I've just noticed a ridge along the bottom edge too, meaning it can't actuall 'diffuse'... adiffuser is supposed to diffuse the air coming from under the car, reducing turbulence to increase the speed of the air under the car and increasing the down-force through the 'ground effect' (sorry that's prof there!) ...so in conclusion it looks like a stick on bit of plastic you'd see on a japanese hot hatch and ruins the lines of the car.
Ian,
One other thing a proper diffuser relies on is what's called a laminar airflow - ie one with no turbulence. It uses a divergent duct at the rear of the car which, as you say, has the effect of increasing the speed of the airflow under the car, thereby decreasing the pressure underneath and therefore 'sucking' the car onto the road. F1, Le Mans and other racing cars use this to amazing effect.

For road cars, there are few that actually use a proper diffuser - most modern Ferraris (from the 355 onwards) have a properly flat bottom and a rear diffuser, so these would have worked. Same goes for McLaren F1, Pagani Zonda and others. Noble are probably the bottom of this sort of range in terms of price - I don't think there's anything cheaper that has a properly flat bottom and a diffuser that actually does anything (at least I can't think of anything at the moment).

Most of the hot hatches (I'm thinking RenaultSport Clio and some fast Seats here) that have them are just for show - as is the case here. It's purely cosmetic, and the fact that it's called a diffuser is because there really isn't an alternative name for that part of the rear bumper. Same goes for the Riegger one I'm afraid. It isn't and never could be a proper diffuser as the flow of air under the car isn't laminar, so unless you shrouded all of the diffs and exhaust to make the bottom of the car flat (and preferably fit a proper front splitter) then the airflow is far too turbulent for the diffuser to have any effect.
 

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...Indeed Scratch... although any device that reduces the air pressure behind the car will have a good aero effect... like the little corner wings you used to get on people carries and estate cars to stop the rear winow getting mucky... they would increase the MPG by 1 or two miles!

...even so if it's for show, it should 'look' like it should function!

FYI I reckon the cheapest road car with a decent diffuser and ground effect design is probably the lotus elise?
 

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FYI I reckon the cheapest road car with a decent diffuser and ground effect design is probably the lotus elise?
Ian,
Yep, fair one - I got stuck at the Noble and then couldn't get lower on price - think your probably right on the Elise though.
 

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