Rear bumper diffusers?

[Peer]

Besides the looks, what's the purpose of rear bumper diffusers?

-- peer

[Chris_in_NH]

Besides the looks, what's the purpose of rear bumper diffusers?

-- peer

nothing. they're not even real diffusers. ;)

here's a real diffuser...

The purpose of the diffuser is to allow the air that has been accelerated (so that its higher speed can produce a lower pressure) under the car to decelerate back to close to the same speed and pressure it was before the car ran into it. The better the diffuser works, the less drag the underwing produces.

[RascalRuss]

The diffuser work by using a phenomena called the Bernoulli effect, this is increase by taking advantage of ground effect also. Basically, think of the old SU style carburettors that had a venturi. This venturi uses Bernoulli effect to increase the air speed through the Carb's, thus lowering the pressure and drawing the fuel trough the fuel jet.

This is the same principle as a under body diffuser. The sudden increase in cross sectional area at the diffuser causes the airspeed to increase the so lowering pressure.

it all comes to the simplified form of the Continuity Equation which states,

(P1.V1)/A1 = (P2.V2)/A2

This is the basics of the diffuser and what most people know of it. However, as an aerodynamic engineer, you would also know that a diffuser does a whole lot more. The flow pattern over the car and rear wake changes completely, as well as increasing the efficiency of a rear wing application.

hope this helps

many thanks

Russ

[RascalRuss]

Also, just wanted to add, the picture posted by Chris_in_HN, shows a great example of a diffuser, but they is alot more complexity to that design than you think. For instance, the exhausts are positioned there for a diffuser related aerodynamic reason.

[Peer]

The Rascal Russ wrote:

> The diffuser work by using a phenomena called the Bernoulli effect [...]

Sorry, but is this a joke..?

(remember I'm a foreigner, so I wouldn't know if someone's pulling my leg)

If it's not in jest -- can you, or someone, please explain how diffusers like those would do anything besides their looks?

-- peer

[Loren]

A discussion of the use of Bernoulli principal in race cars be found here.

[Chris_in_NH]

The Rascal Russ wrote:

> The diffuser work by using a phenomena called the Bernoulli effect [...]

Sorry, but is this a joke..?

(remember I'm a foreigner, so I wouldn't know if someone's pulling my leg)

If it's not in jest -- can you, or someone, please explain how diffusers like those would do anything besides their looks?

-- peer

hi Peer. this is not a joke. i was saying that the Porsche 'diffusers' are not *really* diffusers and i posted a picture of what *actual* diffusers look like. RascalRuss pointed out the technical reasons why diffusers work and Loren linked to why they are beneficial to race cars.

in other words, the Boxster diffusers are not going to win you any races! ;) they are for looks only. hope that helps!

[RascalRuss]

I did not say that the Boxster diffuser are actually diffusers, I agree with you that these are for looks only, was simply stating how diffusers work.

So before you start slating people on forums, ask politely first

Russ

[RascalRuss]

Hi Loren,

Nice link, this gives a nice outlay of the Bernoulli effect.

If anyone wants to get even more in-depth in this subject, try buying some fluid dynamic or aeronautical books off ebay or Amazon, the Bernoulli effect is a very very deep subject and the mathematics behind the highly complex applications an become quite serious.

I am a CFD/Aerodynamics engineer and used work in the race world so if you want some CFD images posted on this thread of a simple Bernoulli application (simple venturi or something) let me know.

Russ

[986Jim]

The bernoulli effect is real. However that lip on the bottom of a stock Boxster S bumper is only for looks. Real difusers like shows on the Ferrari above do work. Look at the C5R race cars, those are difusers...

[RascalRuss]

Ok, Im am just running a CFD model of Bernoulli to help this thread.

Just to add a note, the addition of the boxster diffuser do not work the way proper diffusers work, however, they should have an effect on the aerodynamics.

With out seeing data, I would say that they "may" induce a Vortex pair, that will be further energised by the high energy exhaust gas flow. What benefits would this have.........it will improve the efficiency over the rear lift wing and lessen the wake......resulting in less drag.

Porsche would not just add this for the sake of looks, for the simple reason that is an extra complexity in the mold tool of the bumper, thus increasing manufacturing costs. If that diffuser was not there....would you not buy a boxster......Of course you would so its not just a styling tool to make you buy it.

Im not saying the diffuser has a dramatic effect, and with out seeing data o one can answer its effect, we can only assume, but it will have some effect to the rear wake

Russ

Edited January 1, 2007 by RascalRuss

[Chris_in_NH]

I did not say that the Boxster diffuser are actually diffusers, I agree with you that these are for looks only, was simply stating how diffusers work.

So before you start slating people on forums, ask politely first

Russ

hi Russ,

looks like this post is directed at me. i have no idea what you're talking about. i didn't think i 'slated' anyone. ;)

i pointed out to Peer that i said the Porsche diffusers weren't really diffusers, that you pointed out why real diffusers work, and that Loren linked to a racing application for diffusers. ???

Happy New Year!! :)

[RascalRuss]

hi Chris,

No mate, was not directed at you at all, is was more directed at these comments

"Sorry, but is this a joke..?

(remember I'm a foreigner, so I wouldn't know if someone's pulling my leg)"

happy new year to you too, and all on RennTech

Edited January 1, 2007 by RascalRuss

[Chris_in_NH]

hi Chris,

No mate, was not directed at you at all, is was more directed at these comments

"Sorry, but is this a joke..?

(remember I'm a foreigner, so I wouldn't know if someone's pulling my leg)"

happy new year to you too, and all on RennTech

ahhh, i see. ;)

i think Peer's question was sincere, not a dig on anyone, for what it's worth.

[RascalRuss]

ok, My bad

Will post a bit more detail on diffusers shortly

[vfxbox]

This is from kokeln's rear diffuers and so far... this is the best I ever seen.

[Peer]

Chris wrote:

> i think Peer's question was sincere, not a dig on anyone,

You are correct.

-- peer

[Chris_in_NH]

Chris wrote:

> i think Peer's question was sincere, not a dig on anyone,

You are correct.

-- peer

;)

Peer, i just read in the Bentley Boxster repair manual that the Boxster 'diffusers' are to help in cooling the muffler, as the newer (bigger, more powerful) engines produce more heat than the previous engines. makes sense!

hope that answers your question! i am also glad to hear that these 'diffusers' are NOT JUST FOR LOOKS and actually have a purpose other than styling!! sometimes it's good to find that you were wrong! ;)

Edited January 2, 2007 by Chris_in_NH

[djomlas]

Porsche would not just add this for the sake of looks, for the simple reason that is an extra complexity in the mold tool of the bumper, thus increasing manufacturing costs. If that diffuser was not there....would you not buy a boxster......Of course you would so its not just a styling tool to make you buy it.

Russ

i think that they actually may add it for the looks to hide the exhaust and even things out a little in the back, i dunno....even if it doesnt do anything it looks nice

Edited January 3, 2007 by djomlas

[Peer]

Chris wrote:

> i am also glad to hear that these 'diffusers' are NOT JUST

> FOR LOOKS and actually have a purpose other than styling!!

I concur -- it's always nice to see that some snazzy lookin' add-ons actually have a practical purpose, (although my wallet will suffer).

-- peer

[Rickw1313]

I liked the look of the diffuser so much when my car was rear ended I changed from the 1999 bumper to the 2003 just to get that look. It did require the spoiler update as well to fit the bumper but it had been damaged as well. You also have different side and center heat sheilds on the newer models but they also needed replacing.

This was the only bright spot of an accident, getting to do a little updating.