Everett Ogilvie

I posted this in the prior diffuser thread - it is the bottom of my car. It is a bit hard to tell from this pic but the entire bottom front to rear and full width is as flat as a 4 X 8 of plywood. The rear undertray is under the lower A-arms, and it extends out to within an inch of the wheels/tires.

No matter which diffuser - if there are openings, projections, parts, gaps etc. for "many" feet in front of the diffuser (that really means ALL lower surface area forward of the diffuser), it in reality means the diffuser is doing very little.

John Shiels

Which is near impossible on a stock type car and really hard on a street car. One street bump and there goes your work. Throwing on a rear fascia does nothing in my opinion. I am very willing to hear how a fascia on the rear of a car would work?

VGLNTE1

I'm no expert but I think if just the back half of the car was flat it would help...of course its not going to be 100% because I'm not going to add coolers to the tranny and diff. The bpp one looks to be a stuck on the back one to look Like a C6. Is there a pic of it installed on the bottom underside of the car.

John Shiels

What you see from the rear is what you get I think. BPP can chime in I am sure if there is more. There are a lot of obstacles even in the back half of a stock car.

TheSkunkWorks

If I might add, a 7* max slope is the typical figure I've seen, but the further from the road surface the more likely as much as a 10* angle can be gotten away with in a single deck diffuser. Keep in mind a more rapidly increasing volume will tend to "stall" at lower speeds, with the resulting turbulence from air stream de-lamination causing drag without benefit.

IMCO it would be good to include a hand full of well thought out straighteners, but in any event don't overlook the importance of the side fences. If no pan is ahead of it, you'll also need a generous leading edge radius to promote good air stream attachment. FWIW, the flat floor ahead of the diffuser on the Reynard in my avatar was made of nature's composite (read, "marine plywood").

Of course, there's much more to this, but hope that little bit is worth $.02 (PayPal accepted ).


TSW

Solofast

Good points, it's also very very important to have a generous radius in the transition from the bottom to the diffuser (which is something that I don't see in the shots above and could be a reason that there are limits to the divergence angle in that design). In some of the other diffusers that are commercially available you are seeing very high divergnce angles and obviously those things would separate and won't work very well. If you are willing to do a multi level design you can get higher, and that's what would allow you to generate some really big downforce.

TLGunman

Even better than a "generous radius", would be an elliptical curve, ie conic. Air tends to stay attached better, than just a pure radius, but in this case, the length of radius is fairly small with a 7° slope. 7° is the common number, because that is the separation angle of air, I had to deal with that number quite a bit, when I was doing instrumentation design work for air craft jet engines, and low and behold it shows up again with race cars.

That said, I have tried an 11° diffuser, due to lack of length, and it didn't work for me. We ended up changing the chassis, so we could do 7°. Panoz had luck with 11° on their LMP car though, but I'm not sure why...my guess would be the length of the diffuser and the transition was allowing the air to reattach, or at least not detach, as bad as it did in my case.

dfinke23

How do you change the wheels and tires on this? It looks like the lower CA's would make contact at full droop.

Everett Ogilvie

Diffuser behavior is extremely complex and sweeping statements usually can't properly characterize it - there are too many interactions of too many variables.

As a refresher I just finished re-reading the two best papers I have found to date and one always has to ask questions such as;

- at what ride height are you trying to determine optimal diffuser angle (because at extremely low heights drag effects override any pressure recovery effects of the diffuser and the required area ratio becomes too great)?

- what is the length of the diffuser relative to the flat underbody length (N/L)? Diffuser length that is half the body length, or less, is always best.

- what is the pitch angle of the flat underbody? For greater pitch angles, less diffuser divergence is needed. Body pitch angle drives the all-important area ratio (inlet area vs. outlet area).

- optimum diffuser length decreases as area ratio decreases (meaning that at low angles shorter diffusers work best) AND optimum area ratio increases as ride height decreases.

The last bullet indicates that short low angle diffusers work very well, and at extremely low ride heights a greater area ratio is required (meaning a larger diffuser angle), which can then lead to the drag penalty that eventually surpasses any diffuser pumping effects (i.e. reduced downforce than was had at a reduced diffuser angle - the literature to date still indicates optimal diffuser angle is between 7 and 9.62 degrees).

Dave, I believe that you found 7 degrees to work better than 11 degrees because of the last bullet, and as you surmised it was directly related to the length you had to work with.

TLGunman

Here are a couple links that may help:

http://www.mhest.com/spotlight/autom...rodynamics.pdf

http://strangeholiday.com/oops/stuff...304.092016.pdf

Everett Ogilvie

Now HERE is a diffuser. Would like to get these parts for my car - bye bye street car then I suppose.....

John Shiels

just need an air suspension to raise it for the street. What car is that?

Everett Ogilvie

Mosler. Super GT car. Theoretically I can get those for my car....

Once I put on that rear end I might as well put in the sequential trans, right?

John Shiels

times ah wast'in!

vms4evr

This is why most things you see for sale are for looks and make very little real use of aero. Some of the ones that are hyped up are built based on theory with limited testing. There is a very small percent that are actually wind tunnel tested and truly track tested. Those parts require very deep pockets to buy. They give you maybe a second or two off your lap time. For the average driver doing DE and such. You could do a top end engine build up for far less and enjoy that power all the time.

I love the look of some of the aero designs but they aren't practical for street cars. Many are difficult even with a track car. Most of our trailers won't even allow cars with those front spoilers to load up. You need to have removable parts just to get them loaded/unloaded from their tow vehicle.

The more I look at aero pakages the more I'm starting to think of just doing the head/cam upgrade and calling it a day. Or just doing home made engineering like the OP and see how it goes.

TLGunman

That would strike fear in the hearts of those on the street. I'd do it!

Everett Ogilvie

I remove my diffuser for street driving (and probably need it removed to get the car in the trailer) and if I get the longer front splitter that too will probably need to be removed to get the car in the trailer.

(not that removing parts is a trainwreck, just thought it was a cool emoticon)

John Shiels

Go off track with a splitter glue to the ground and see what it cost you!

Aero makes your car stick and stop faster something HP won't do. It also doesn't blow up. It gives you more confidence driving as the car feels nothing like stock. I'd give a bunch of HP up to have the aero that works. When I did my car I had that choice. I chose a cheaper H&C with all the coolers and aero vs. big dollar engine. Big dollar engine could go pop and all is gone. 450 RWHP and aero has put down some big HP cars. If I had the money to waste I put on drop spindles to lower my car but that would lead to tire clearance issue and the need for two sets of rims. It never ends $$$

Good news is I worked on the car today for the first time in a long long time to get it back together.

Everett post up some pictures of your car please!

Everett Ogilvie

John, here is a pic with the diffuser in place but before the final fitment so it was not quite right yet but getting close;



Here are the new BBS track wheels and Hoosiers. You can just make out the front dive planes (canards);



Like you mentioned, I would like to consider some drop spindles like the LG units to lower it for the track without having to put my coilovers down close to the shock bump stops.

P.S. Glad to hear you are working on the car!

VGLNTE1

Omg. That is amazing..