claykos

The factory piece from a 997 RSR? Or have one fabbed? You do not want to exceed ~10-14 degrees of angle on it anyway or you will get separation which will increase drag.

J richard

^^^like Clay said the rsr uses a lower engine tray/diffuser in combination with venting in the rear bumper cover, likewise another place to look would be a gt2/turbo rear, I think champion and ruf both have narrower undertray/diffuser options that could be adapted to a cup.

mark kibort

when there are no rules, (or basically no rules), its amazing what is bolted on or the shape of the cars that compete for the best lap time possible.

I think Sofro can attest to how fast these cars are ,whe his best a year earlier was about 4 seconds off the mark of some pumped up monster EVO on DOTs at the buttonwillow time attack a few years back.

HJK

Only 500 out of a 951? Come on 600 over can do from a 3.0 16v I'd venture and a 1000kg will come a long way. At least you'll have fun.
H

stownsen914

I've seen RSRs that had diffusers. Maybe they were running in a European or Japanese racing series? A little research might lead you to something.

Scott

TrackDays247.com

There is a lot of crap BS diffuser idears out deah.

wanna911

A diffuser should still reduce drag on street cars that have rear bumper covers that catch air and produce a parachute effect. So maybe used in that context they should in fact reduce drag, but that may get overlooked by the actual intended and most beneficial use of the diffuser which is to produce downforce without the increase in drag of a top mounted wing.

95ONE

LOL Pat.. This turned south pretty quick. TIME ATTACK is fantastic. I have yet to compete and plan to ... SOMEDAY. Anyway. I would work on the WHOLE underbody to expect any real benefit from a rear diffuser. That underside pic of the Ferrari is pretty cool. As you know, I'm spending a ridiculous amount of time on Aero as well as weight. Right now, I've completed the rear air Block outs, and now I'm trying to figure how to get the high pressure areas out of the front fenders. Then on to as flat bottom as i can get. Trying to keep the exhaust as high and large as possible. The two hate each other. if you look at the front of the Ferrari, you can see a low pressure area that doubles as a radiator relief pocket. Also going to try and make one of those, but one closer to what the NSX Type R has. Larger Canards are also going on.
Did you know that the Formula 1 cars have almost -1G of braking force from their aero alone? No engine braking or brakes. Where horsepower per pound is HUGE, you can do things like that. Yet another thing to balance in the quest for road course speed.

M758

Aero in general is well beyond the abilites of home racers. There is alot of complex calculations that go into basic aero theory. You start with that to create some basic (at least basic for aerodynamicists) models and concepts. Then to prove they work is series of emperical testing both on the car and in wind tunnel combined with CFD (computational fluid dynamics). 99% of of us don't have any access to wind tunnels nor have the tools or skill to do complex aero calculations. Even some of the aero engineering I work will can only go so far as their assumptions do not apply cars (just in the inside of jet engines).

Point is for most of us Aero down to trial and error along with enough technical knowledge to have some direction. That also means any thing we may see at our local track could be way off the mark. Even some of these super Time attack guys with all these wings and flip ups etc have no idea what they are doing. Some elements work, but may work rather poorly.

So when staring the path on aero realize that it is highly complex and highly interactive. It is also highly effective, but when compare to chassis tuning and mechanical grip well. It is probably 10x harder to get right. It is also very restrictive in terms of what can be done on street car based race car. Formula cars or proper sports racers by contrast have much more options to do aero properly.

mark kibort

without the flat bottome, the diffuser is more of a drag reduction bolt on. with it, it still is more about drag until the ride height gets really low to take advantage of the venturi effects. So, im not convinced that there is a lot of time to be found for us road racers to go nuts with aero. (unless you are always at VIR ) Plus, the cost of some of his aero might be pretty pricey as well and certaninly at worst a lot of custom fabricating.

As far as 1 g deceleration forces for F1, while this is true, its the same amount force to decelerate at 1g, as it does to accelerate at 1g. in otherwords, the force acting against the F1 car at only very high rates of speeds (like 200mph), causes this kind of deceleration. (and that force goes down inveresly to the square of speed too) I dont think you would want that kind of drag, unless you had the power to drive it in the other direction for acceleration! Everything about the F1 stats is pretty amazing.

What you can do to relieve some of the high pressure in the fender well, is create the top of fender slat vents, or vents behind the front fender panel.

Mk

mark kibort

From the link that Scott posted,

"The diffuser's aim is to decelerate the air without it separating from the tunnel walls, which would cause a stall, reducing the downforce and inducing a large drag force."

The diffuser also allows for a more gradual interaction of the air coming from on top of the car, and past the wing, to the air coming from the bottom of the car. depending on the design of the diffuser, and there are MANY, some dont really accelerate the air coming from under the car, they just smooth flow. Some actuallly enhance the accelerated air under the car with the use of flat bottom , tunneled designes, and then become a major component for the downforce system at the lowest drag possible.

I dont think we could ever expect to see huge gains at most of the road courses we visit, due to the speed of most of the turns we see. At our tracks out west, Thunderhilll for example, has one turn at 105mph for the faster cars. all others are between 60 and 80mph. I dont think flat bottom, and diffuser aero will allow for much more downforce here if optimially designed. I think primary downforce is the wing, hood vent and splitter configurations.
It would be very interesting to see a wind tunnel test of the effects of all of the above. so far, i have found only one video on youtube, that actually shows a team optimizing wing and splitter configurations and showing the data for the changing forces, in downforce and drag.

edit: Here is the link. it shows downforce before and after some major mods and shows some flow streams. listen carefully!

http://www.youtube.com/watch?v=kqjKx...eature=related

JackOlsen

I built a diffuser (along with an underbody kit) for my car. I'm no aero expert, but here was the problem I ran into: underbody aerodynamics rely on fairly-specific placement of the bottom of the car over the road surface. The problem with diffusers on production cars is that our cars have fairly soft suspensions, so when the car turns there's body roll. This compromises the conditions you've created under the car at the exact point when you actually need the downforce.

My underbody kit produced measurable downforce. But only at high speeds and in a straight line -- which of course is the one time when you don't want it. In corners, it went away as soon as the bottom of the car tilted in relation to the road surface.



I have 600# springs in the back. I'm told that for this kind of stuff to be useful I'd need 2000# or so springs to keep the car level enough for the results to be consistent in corners, when the downforce would help and not hurt.

mark kibort

+1 ^^^

great shot . Is that corner worker running for his life

by the way,did you remove the underbody panel and keep the rear diffuser for drag reduction, or remove it all?

911SLOW

I am running a clio sport cup car for 4 years now.
Renault was so sure that the diffuser works, that they didn't even bother fitting a rear top wing.



"Clio Renault sport is also equipped with an air diffuser, an aerodynamic feature normally only found on high-end sporting cars. Air passing underneath the vehicle is channeled via the flat bottom to the diffuser where it accelerates before being expelled at a higher speed. Combined with the shape of the diffuser, this creates a zone of depression under the car, sucking the chassis to the ground. Compared with a conventional wing, diffusers generate significant downforce without resisting forward movement. On a track, at 80mph, lift is reduced by almost 40kg. The diffuser is designed to function in association with the rear flat bottom which leaves space for the exhaust silencer and lateral-mounted tailpipes."

mark kibort

Now, there is a substantial value. 85lbs of downforce at 80mph. thas as much as my cup car wing at 80mph. it would be great to see where the downforce is measured. Is it over the entire car or just the rear wheels? And how much of it is due to the flat bottom and air entering from the nose? There is where I get intersted in the why and how. I don think that just bolting on that moderately sloped diffuser would give an instant 80lbs of downforce, but the entire system of the front of the car, underbody panel and rear diffuser creates this.


Here is some other verbage from the Clio sport cup:


New Clio Cup adopts the aerodynamic features first seen on New Clio Renaultsport 197. Very closely related to those used in Formula 1, the rear diffuser and the front wing-mounted air extractors contribute to the stability of the car at high speeds. New Clio Cup is the first entry-level sports hatchback to feature such a sophisticated aerodynamics setup.

The rear air diffuser is designed to reduce any lift that is naturally generated by the car’s profile without adversely affecting the finer points of the aerodynamic setup. By channelling airflow beneath the car, the diffuser accelerates and expels the air, creating a zone of low pressure, which "sucks" the car to the ground. The twin exhaust outlets fit perfectly into the diffuser without hindering its performance in any way.


EDIT: Here is some flow analysis of what happens when just a diffuser is added to the porsche chassis.
you can see the flow to the rear of the car is made more efficient. with a flat bottom and inlet tunnel, im sure downforce would be enhanced as well.