I was thinking the same as Jeroen, some holes or slots at the very back of the aluminum...

With the back of the aluminum totally covering the exhaust as well, not only might air from the fan be prevented from flowing properly but the exhaust heat can not radiate away and may be somewhat trapped adding to the problem.

Hmmmmmmmmm

How about you reverse the fan? Instead of pulling air from above and pushing through the engine to below, you pull from below and exhaust out the top? Would that work? Do you need a 917 style vertical fan? If you geared the fan to spin it even faster or enlarged it could you not get the cooling AND get some suction effect thus to a degree turning the diffuser into a mini-venturi?

Final fitting and pictures:

http://forums.pelicanparts.com/uploa...1112290827.jpg

http://forums.pelicanparts.com/uploa...1112290844.jpg

http://forums.pelicanparts.com/uploa...1112290794.jpg

That is so cool looking! If there are any cooling issues.... you MUST resolve them in a manner that permits the diffuser to stay. Even if it results in no net gain in lap times or anything, its just too cool looking not to use it!

That WILL be functional. Nice job.
One experiment you might try is this:

Bring the engine up to 5k+ with someone's help.
Have your car on the ground
Feel under the engine to where the air is coming out from - get a general idea of what (engine fan) air flow is ocurring wihout/with the diffuser in place.
Be careful doing this -

I need something like that! Not for aero reasons, but to collect the oil drips!

Nice job!

Continuing with that theme, there is a useful test that you can do which involves putting some dots of oil along the leading edge of the defuser prior to going out for a session. When you come back the streaks that are generated will be able to tell you quite a bit about the air flow at the surface. The results might look something like the following examples that I've pulled from Simon McBeath's excellent "Competition Car Downforce".

http://forums.pelicanparts.com/uploa...1112294449.jpg

My concern with your design (in addition to the cooling issues described earlier) is that the angle of the defuser it too abrupt thus causing the defuser to stall. If this is the case you might be picking up more drag then you gain in downforce. The oil-drop test will help to clarify if in fact the defuser is stalling or not.

Jack - Maybe my original question was too general. Did you happen to make a template of the aluminum cuts? I'm very interested in your project, I'm just not the best at "fabing" however I think I can figure out the assembly.

Bigrubberjeep: Unfortunately, I made it up as I went along. But my particular configuration -- early car license panel/bumper, Flowmaster muffler, engine moved 1.5 inches forward, cut-and-rewelded B&B headers -- probably wouldn't match any other car.

If I can show meaningful gains (which I'm still a little doubtful of), then maybe we can put Widebody911 to work on it with his CF skills.

John, the oil drop test sounds like a good idea. If it seems to help (based on lap time, 'seat of the pants' and DL1 data) the first time out, I'll definitely do some more specific tests like that.

You could solve the potential flow separation problem by extending the diffuser into the rear bumper. Unfortunately, that would detract from the looks of the car, and you run the rist of adding a RiceFactor

I see an added benefit here - there is a lot of air off of the engine; this would join your smoothed air underneath and provide even more air volume for the diffuser to 'lift'

I'll never forget the wind storm of air behind my 911 as it ran on the dyno...

HUGE!

You could run holes in the upper rear portion for hot air exit with no loss of 'suction' -

Jack - I think you really are on to something here.

From pictures that I have seen, here is my 2 cents.

Maybe a streight line out like the ferrari works better vs. the curved.

Also, maybe a gap in the middle section of the diffuser where it meets the bumper, like an inch or two just to allow "hot" air from the motor to exit.

I'll try to draw my idea and post later.

Ideas....????http://forums.pelicanparts.com/uploa...1112304092.jpg http://forums.pelicanparts.com/uploa...1112304102.jpg http://forums.pelicanparts.com/uploa...1112304109.jpg

Jack - what top speeds are you going at on the track?

Also, a small camera can help with the oil drops - and use a colored oil or red ATF on a white or silver painted surface.

From my "bling" POV, I would have to say some of these diffusers are more for looks than effect.

Here's some interesting pics.

Sherwood

Depth, arch and proximity to the ground are the keys to making these work.
Jack's looks pretty damn good!

How about if you run your engine fan backwards and create a sucker car :)

Just kidding of course, but you have to figure Porsche tried it at some point.

"Depth, arch and proximity to the ground are the keys to making these work."

I'm also thinking a large volume of air would make this more effective for ground effects, but from where? Is there that much air sneaking in from under the perimeter?

Or maybe I'm thinking something else. Is the diffuser for ground effects or to allow under-vehicle air to escape more easily?

Sherwood

I'm really concerned about the engine cooling. The engine fans push about 1000 to 1500 liters per second at higher engine speeds. That's a LOT of air. Where is all of that air going to go with that current configuration? Even adding some slots isn't going to provide enough capacity. More importantly, when you're tracking the car at high rev's the engine is generating an awful lot of heat. If the air has no place to go, the fan will stall and your engine temperatures will go up really fast.