Being an engineer I know I take things a little extreme. Does anyone offer rear diffusers and if not, why? I know they work in conjunction with rear wings, but can be beneficial as a stand alone item.
ANY THOUGHTS? I should have also mentioned - this inquiry is strictly toward the C5.

 

Most rules for GT and GTS cars prevent the wing from overhanging the car body, so it's hard to use them to drive a diffuser, unlike some prototype and open wheel cars.

 

Front and rear defusers YES plus all kinds of C5 body parts. C6 and C6 Z
parts too.

Corvette Race Bodies AKA ACP

http://www.corvetteracebodies.com/corvetteracebod.htm

Ask for Kerry

 

If you get a rear diffuser from ACP, please post pics.

 

I don't know of any rear diffusers for Corvettes. They are illegal in the series' that ACP feeds. They Viper guys tell me the Comp Coupe rear Diffusers are junk and I imagine anything develped at a reasonable budget would be more of a parachute.

If you look at what ferrari and others have done, I think you'll find the development needed to make the system work efficiently is well out of the budget of those of us intersted. Conversely, you can make a ton of downforce by just bolting on an off-the-shelf wing!

Dave

 

Yes, you can get tons (literally) of downforce from an off the shelf wing. The problem is you get hundreds of pounds of drag too. The idea of the diffuser is to improve the downforce while not significantly increasing the drag.

What most folks don't realize is that in order for a diffuser to work properly they have to be designed to work with the entire underside of the car. Unlike a wing, just bolting a diffuser under the back of the car isn't going to help things much. Also note that with the latest Ferrari, (there is a picture of the underside pressure contours in last months R&T of C&D) they are stabilizing the flow under the car with an underchin wing and then the bottom is flat and smooth all the way to the diffuser. With a front engine car the flow is a mess as a result of the engine cooling air being discharged under the car (this is why the racing vettes are using a top discharge radiator). Even if you have an extensive Computational Fluid Dynamics (CFD) program (which I do) you aren't likely to get it right without a bunch of wind tunnel time.

 

C5west(.com) has rear trays that are very inexpensive.
They probably don't do much good at regular road course speeds though

 

I believe the C5West kit is just a rear ground effects kit. They're website is scary anyway. I wouldn't ever buy anything that looked like the crap on their website.

If I understand it correctly, a rear diffuser is the last part of a venturi tunnel. The venturi tunnel should pull air from the front and even side of the car to accelerate to a single small point beneath the car, then suddenly expand...the expansion produces the suction (downforce). The diffuser helps pull air thru the venturi tunnel. -- I'm probably way off, so if someone really knows how it's supposed to work, please chime in. There are threads in www.ten-tenths.com (www.mulsannescorner.com) that explain it very well, but I haven't read it in a while.

Eugene

 

Generally speaking, the diffuser gives the underbody air a place to expand, because it is expanding, the air under the car can accelerate to fill this expanding area. As air accelerates, the pressure drops, and this lower pressure under the car pulls it down.

Most rules don't allow the diffuser to start until the rear axle centerline, with the area between the axles having to be flat. Diffuser angle (rule of thumb is 7deg), strake location, and the number of strakes can all help or hinder the diffuser too.

 

I used the C5West kit as a starting point for C4R ground effects.
We plan to eventually add down strakes to the rear tray to aid the air flow

http://community.webshots.com/album/122909680iDIHyq

 

Evan,
Some parts of your aero kit makes sense to me, like the splitter, rear windors strakes, rear spoiler, removed rear signals and the hole behind the license plate. Rocker panels don't make sense to me unless it helps a venturi tunnel work. The rear tray just looks like a ground effects kit that doesn't do anything, which is what I think the C5West kit is, but it sounds like you're actually trying to make an effective undercar and rear aero package, and that's pretty cool.

I also agree with Solofast. Experiments will undercar-to-rear aero packages will add negligle drag and may add significant downforce. Experimenting will be expensive at worst, and has the potential to be highly effective. Wings are the easy sure bet, and I'll run one eventually, but I'd rather take care of downforce under the car if I could do it and still retain mufflers.

Eugene

 

The rocker panels aren't low enough to make much difference at low speed < 120 but it's cheaper to put them on Before the car gets painted.
The rockers and and rear tray may come into play at higher speeds and since this car was planned to race at Fontana it made sense to include them since I had the kit .

But now the next kick application has started to emerge...

Class entry Land Speed record!!!

The car is very few mods away from a possible class record in Landspeed at El Mirage for about 215mph

The subject is up for discussion amonst the boys right now.

Anyone in Norcal have a spare D44 w 3.07s to loan for a few weeks?
I'll be real nice to it, Promise...

 

Evan,

I agree that you are on the right track with the mod package. Look carefully at what the leading edge of the footwells look like because it is there that the flow gets pinched and acts much like a second spolier, which garbages the flow under the footwells. You want to encourage the flow to go under the footwells and not thru the tunnel, although you need enough flow around it to cool the tranny if you aren't running a cooler.

Assuming that you have the stock footwells, You shoud look a putting a radius on the footwell leading edges, suggest that it be at least a two inch radius (four inch diameter at least, bigger if you can manage it). This can be made of sheet metal and even glued under the leading edge of the footwell and stick forward from there. This will allow the flow to accelerate out the engine compartment and attach itself to the bottom of the footwell. This will reduce front end lift by not allowing pressure buildup under the hood and will reduce the under the car lift by increasing the velocity under the footwells. This is a really easy mod that will likely have a high effort/downforce payoff, it is pretty much invisible so you aren't giving anything away to the competion....

 

I'd run the side skirts before a diffuser, cooling becomes an issue. A diffuser and belly pan, would close the whole area up, and not enough air would circulate to keep it cool, especially if the exhaust is enclosed.

But any kind of ground effect, except for the front splitter, is killed off by all the air coming out underneath the engine compartment. That's why the C5R's radiator exhaust is out the top.

 

Since you don't have an unlimited budget and really expensive tools the best thing is to do as you have done with the spoiler. Make an engineering judgement as to what you think will work and apply it. Road testing with a car as large and heavy as a Corvette is difficult and the data, unless you are on a really smooth road, with no wind will be iffy.

The key to aerodynamics is that there aren't any "silver bullets", that is there isn't any one thing that will make a world of difference. What you have to do is a number of small improvements and they will add up to a total package that will have big gains.

I did a drag improvement package on a light plane that I had and did a large number of small changes. They were things like adding a radius at the end of the wheel wells, sealing gaps and getting rid of the antennas. When all was said and done I had improved the top speed by almost 20 knots, from 155 kts to 175. Drag and downforce on a car has to be approached in a similar manner. You aren't going to get a huge gain from just the diffuser. You need to add the afforementioned footwell leading edge radius pieces, add the skirts, and do as much as you can to smooth the air under the car.

You already have the right idea on the front spoiler and engine inlet. You want a small inlet that expands into the radiator to diffuse the air and do your engine cooling with that air. It would be great to reaccelerate the air and eject it smoothly under the engine, but that is difficult to do with the radiator so close to the engine.

Your spoiler is pushing most of the air over and around the car and that is the right thing to do too.

Ground effects don't do much unless the bottom of the car is closer than two or three inches (around one inch is optimum) to the ground. Lower as much as you can and then work with what you have.

Also, the leading edge of the diffuser is important. Since your diffuser is starting fresh after the rear suspension you need to make sure that the leading edge of it has a good radius and that the midpoint of the radius is higher than the footwell or suspension pieces. This will assure that after the flow separates from the bottom of the car it will reattach and flow up the diffuser. I have seen a lot of diffusers with a sharp leading edge, the flow separates right there and they do nothing other than add drag.

Tuft testing is difficult to tell anything other than gross separation, but you can add something to dribble white water based paint under the car and then run it down the road at speed. The paint will streak and you will be able to see if you have separation and where the air is going. Then hose it off and start again after you have made changes. Make sure that the diffuser is flowing fully and that the airflow doesn't get messed up at the start of the footwells and you probably have done as much as you can with the underside.