iStan

iTrader: (29)

Please? Front and Rear? Benefits of one/both?
Thanks

Jarred@Z1

iTrader: (4)

In a nut shell... it cleans up the air that goes under the car makes it flow smoother and if done correctly like on Ferrari's and Lamborghini's it can create downforce.

terrorist22

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.

fairladyinit

iTrader: (2)

Really? You named yourself terrorist?

terrorist22

It was my BF2 name and current Call of Duty 4 name... it was an alliterative throwback to Counterstrike when I went over to BF2 and I couldn't think of any other name on the forums

fairladyinit

iTrader: (2)

ohh lol nice.

iStan

iTrader: (29)

So what the hell is the point of the diffusers (more so rear) for the 350z?

Stan (Looking for a true functional reason for it, and the Ricer points assigned by Abui)

terrorist22

Yeah, it's a dumb name but whatever. It's easy to pronounce.

There are no downsides to diffusers... unless you get an improperly made one... then it's just ballast at that point. Theoretically you want a body kit made by one manufacturer such that all the parts work in harmony, but more or less kits are designed for style.

terrorist22

For the most part, looks.

Simon McBeath did a study on NASCAR cars with front and rear diffusers in Racecar Engineering; this was probably three or four years ago, and it was purely CFD analysis. No other bodywork modifications. I forgot what the front one did (aside from the obvious), but the rear was an interesting case. Downforce did increase, but the smoother flow invited interaction with all the pipes and crap underneath, increasing drag. And this is a fully-built racecar.

If you want to feel the effects of a diffuser, you would really need to go fast and have an aerodynamically-sensitive car.

iStan

iTrader: (29)

Arg. I'm going to hunt down that book and start reading. Thanks

:Sigh: An f'in Terrorist has been most helpful to me today

buzzardmountain

iTrader: (17)

<----Waits on Cheston's reply...

Chebosto

iTrader: (43)

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.



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.



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.

Jim@Showstoppers

iTrader: (157)

^^ Thee man has spoken


/thread

jaspendlove

iTrader: (3)

^^^lol

Motormouth

iTrader: (44)

I wish a mod could undelete my 2nd post.

groovin_350z

iTrader: (59)

Cheston Is The Man

CEO350Z

iTrader: (21)

cheston......

he's like a little evil genius or something.....

an asian Dr. Evil!!!!!!

groovin_350z

iTrader: (59)

DUN DUN DUN!

terrorist22

The low-pressure area underneath a wing will "drive" the air flowing through the underbody; i.e. help it accelerate faster and hence more downforce. The problem w/ wings of course is that there is a huge drag penalty associated with them, as well as a raised center of gravity.

I still have doubts with aftermarket diffusers for our car. I would like to know if there are real wind tunnel tests/data as well as track testing. I also would like to know if the pieces were tested all together, or if the diffuser was designed/tested solely w/o taking other aerodynamic parts on the car (i.e. if all the parts worked in harmony).

In the mid-nineties, F1 teams would run exhausts such that the gas would be injected into the flow, accelerating the local air underneath. More downforce at higher speeds, but come to a corner, and the engine revs die down. You lose all that downforce, and the balance of the car will be a lot less predictable. Hence the overhead exhausts you see nowadays.

I have read McBeath's book and the Millikens' book and they are quite good for introductory-level kinda stuff. I read about halfway through the latter (it's nearly 1000 pgs); was still trying to understand more of tire/suspension theory.

Borbor

I think one must remember something when it comes to cars kitted with diffusers (whether it's for race application, or straight from the factory)

Almost in all cases, all those cars are flatbottomed, or nearly so. I still can't see how a diffuser would work on a non flat bottomed car.

@ terrorist: Milliken as INTORDUCTORY??? LMAO.

RCVD isn't exactly a bedtime reading material; it's more of a reference when you actually have something at hand to work with and are using that book for reference.