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.
I could do some more photoshops of this idea when i have time.
