Rear Diffuser

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Postby CMS-GT4 » Wed Dec 31, 2008 7:36 pm

Well here is the room made once the gas tank is removed. It might give those who want to get a fuel cell and a rear diffuser an idea of what they are working with.

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A quick PS of a dual exhaust 3-piece diffuser. I would never do the dual exhaust myself, but w/o the tank in the way you would have a lot more options for the muffler style and location.

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Postby NissanSkyline901 » Wed Dec 31, 2008 8:47 pm

Im still thinking about making one out of aluminum come spring. I have a side exit exhaust so I dont have to worry about that and I can form the diffuser around the gas tank.
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Postby CMS-GT4 » Sun Jan 18, 2009 8:14 pm

So I had seen some interesting info on how these work. Might be ideal for you guys to read up on them before making one. From what I have read, the oem tank cover might even benefit from having the holes in the bottom covered so that air flows over it and does not get trapped in it. The muffler will still cause some issues though. I am sure will do it mostly for looks though and may not see much benefit unless they drive high speeds. If I do one I will likely get a smaller exhaust that is closer to the size of the gas tank so that I can make it closer to the body of the car and more uniform.

There is lots of good info on the my350z forums.
http://www.mulsannescorner.com/diffuser.htm

I also suggest reading Competition Car Downforce by Simon McBeath to see how all those other aero mods make downforce.

You will see a crazy amount, and design, in the vanes in sports cars and open-wheel racers, all to keep the flow one-dimentional and somewhat predictable at the limit. The diffuser is pretty much defined as "free downforce" (along with a properly designed inlet and undertray) as there is almost no drag penalty. But you'd have to spend a crap load of money to see that happen.


EDITED: for addition of pictures.

For the Z, we already have a factory diffuser-- it's actually the OEM exhaust, the underside has been shaped to allow for air flow to exit the underside of our car in an upward motion.

Image

The effect is called Venturi. you might remember it from high school physics or college physics. Venturi Effect states (from wiki)The Venturi effect is an example of Bernoulli's principle, in the case of incompressible fluid flow (in our case, let's just use air flow under the car) through a tube or pipe with a constriction in it. (think the road to the underside of the diffuser) The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is supplied by a drop in pressure or a pressure gradient force. (read: same amount of air flows faster in a smaller channel than through a bigger channel.)

As the diffuser sweeps upward, it creates a lower pressure. think like the top side of a wing. it'll suck upward. well the diffuser is 'upside down' when its installed on the underside of our car, so this sucking motion will in a sense suck the car down toward the ground, thus theoretically giving the car higher downforce. starting the diffuser closest to the rear axle and having it gradually sweep upward to the end of the car is most beneficial. but with all the junk underneath our car, its a little bit harder, so we have to work from rear subframe back...

a diffuser works better if the car were lower to the ground (think smaller venturi opening), and if you can channel the airflow in straighter lines, or 'clean up' the flow from the underside. hence you always see the high end race cars with smooth underbodies with strakes that 'guide' the air all the way from the front of the car toward the back.

Image

the faster the airflow underneath the car when it enters (thanks to a front diffuser or similar device) and you can maintain that air flow speed underneath, then you're in business. (thats why some F1 cars, they say produce enough downforce that if you turn the track upside down, they would still suck to the track)

so now let's talk why some of us might want a diffsuer.. a majority of the after market exhaust systems for the Z, kinda screw up the flow behind the axles. because it leaves a big gap where the stock exhaust used to be (remember? our stock diffuser...), and now at higher speeds, you might have noticed if you were following another Z on the freeway, the rear bumper buffets slightly, depending on which aftermarket exhaust that car had. (think the single or dual canister designs, happened to me when i had my RSR) all though its not that bad at like free way speeds, but do some triple digit runs, and you'll probably notice more drag on the car or it might not feel as stable as it did when you had the stock exhaust. so companies are now producing rear diffusers to 'seal' up the gap left by these canisters..

you could negate some of these weird pressures by installing 'speed holes' (haha i like to call them that) by venting the rear bumper with mesh. (example: look at all the JGTC race car rear ends. they're all open or have openings to allow air to exit from behind the wheel and under the car) but then you still won't have the benefits of having a diffuser.

to negate this turbulent airflow underneath the car by not having the stock exhaust, you can overcome the lack of undercar downforce by installing a high-pedistal GT wing that will push the rear end down.

there are alot of companies that call their rear end addons 'diffusers' which i think is a little misleading.. like the nismo piece for the G35coupe. not really. the charge speed rear piece. well. it looks nice and has a speed hole, but its not really a 'diffuser'. the varis piece, top secret/mastergrade, first molding, yea. those are diffusers.

hope that helps. if you really want more info. check out these books. I have them and read thru all of them. very good info.. the last one. R146 is a text book so if you dont have the want or desire, you can skip it.:

Race Car Aerodynamics: Designing for Speed (Engineering and Performance) by Joseph Katz (Paperback - Aug 1995)

Competition Car Aerodynamics: A practical handbook by Simon McBeath (Hardcover - May 31, 2006)

Race Car Vehicle Dynamics (R146) by William F. Milliken and Douglas L. Milliken (Hardcover - Aug 1995)



^^
rofl.


Final Edit:

oh yea. and it looks freakin bad azz.
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Postby toayoztan » Mon Jan 19, 2009 2:02 am

CMS-GT4 wrote:Image


If you were to do dual exhaust, I believe rectangular exhaust tips/mufflers would look the best. Due to the alltracs flat/pancake bumper and rear end-ish look, I think something like this would accent it nicely with a lip/rear diffuser.

Image

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Postby CMS-GT4 » Mon Jan 19, 2009 2:46 am

Possibly, but I will never do a dual exhaust. If I did consider a dual exhaust, it would be if I had no fuel tank in the way, then i might do something like the 350z/ mr2 exhaust. Its one muffler with twin outlets. But I want to keep weight down and things simplified. If I do switch mufflers I am thinking of the twin tip single design.
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Postby OnAll-FOUR » Tue Jan 20, 2009 2:30 am

I haev one in development and with any luck I will have some pics to show you soon ;)
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Postby EricGT4 » Sun Jan 25, 2009 3:20 pm

Sounds good keep us posted. Can't wait to see how it comes out.
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Postby Chai » Fri Apr 17, 2009 7:29 pm

Image

I have a question wouldn't air get stuck in the area where the diffuser starts right before the suspension parts and around the area where it outlines the muffler?
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Postby Roundy » Fri May 08, 2009 6:02 am

The effect is called Venturi. you might remember it from high school physics or college physics. Venturi Effect states (from wiki)The Venturi effect is an example of Bernoulli's principle, in the case of incompressible fluid flow (in our case, let's just use air flow under the car) through a tube or pipe with a constriction in it. (think the road to the underside of the diffuser) The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is supplied by a drop in pressure or a pressure gradient force. (read: same amount of air flows faster in a smaller channel than through a bigger channel.)

As the diffuser sweeps upward, it creates a lower pressure. think like the top side of a wing. it'll suck upward. well the diffuser is 'upside down' when its installed on the underside of our car, so this sucking motion will in a sense suck the car down toward the ground, thus theoretically giving the car higher downforce. starting the diffuser closest to the rear axle and having it gradually sweep upward to the end of the car is most beneficial. but with all the junk underneath our car, its a little bit harder, so we have to work from rear subframe back...


Oddly enough i think that is backward...

The diffuser in isolation does not generate low pressure, so doesn't generate downforce.

Imagine a very simplistic model, you go from under the car of a certain cross sectional area, then the diffuser is a gradually increasing area.

The air has to LOSE velocity through the diffuser and hence GAIN pressure, creating LIFT

However to gain pressure the air needs to be more dense...so where does that extra air come from?

It comes from IN FRONT of the diffuser, which accelerates the air flow before the diffuser, thus generating a low pressure under the car, and subsequently more downforce.
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Re: Rear Diffuser

Postby CMS-GT4 » Tue Jul 13, 2010 8:17 pm

Anyone done their yet?
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Re: Rear Diffuser

Postby UndaGrwnD » Tue Jul 13, 2010 9:10 pm

i would also be interested.
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Re: Rear Diffuser

Postby CMS-GT4 » Wed Jul 14, 2010 12:52 am

A couple ideas i played with, one based on the savage bumper.

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Re: Rear Diffuser

Postby WarTowels » Wed Jul 14, 2010 4:06 am

And if you did switch exhausts... let me buy yours off yah.

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Re:

Postby Simba » Fri Jul 23, 2010 2:25 pm

Chai wrote:I have a question wouldn't air get stuck in the area where the diffuser starts right before the suspension parts and around the area where it outlines the muffler?


Simply put, yes, which is why that "diffuser" is for show and is functionally useless.

A few things to note about diffuser development on the AT:

- You don't really need it unless the car is extensively modified with higher gearing. It's perfectly stable at ~150 mph, which is about as fast as the majority of cars can go. If you gear it higher, run a higher rev limit or tire setup that allows you to get into the 180 range, it's not a terrible idea to add some downforce on both ends.

- A rear diffuser is entirely useless without a reasonably flat bottom to the car. No diffuser design is going to perform to a reasonable degree unless the area between the front suspension crossmember and diff housing are enclosed to a significant degree.

- Most of the diffuser photos posted have been largely cosmetic. For a functional low-speed diffuser, you need considerably more side spill prevention, which means: Longer strakes.

- Use of a diffuser should be met with a flat front closeout panel, and a fairly aggressive front splitter.

- Both a diffuser and splitter will create more than a few clearance issues on the street if executed in a functional manner (most are not).

Lots of fast, loud things.
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Re: Rear Diffuser

Postby nbx33 » Wed Sep 22, 2010 9:12 pm

Buy an SRT Charger Diffuser. you shouldnt have much issues installing it with the Bumper off. Just get a dremel and take ur time.
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