alexjc4

The standard engine pan has a bit of a kick up at the end, what's the betting that its around 7 degrees?

I have skimmed the chapter in Milliken. so, with the proviso that I am rank dilettante and really don't know anything; I think the 7 deg rule of thumb comes from the ground effect era, and skirts are needed to work really well, without skirts you can run much higher angles, 14 deg say, somehow the extra vortesesese that get generated as air rolls under the side of the car stop the air flow separating and "stalling", you can even promote this with vortex generators, I guess these are the vertical baffles you see in diffusers on everything from road cars to f1 cars.

It's been suggested (with some good provenance) the stock 964 engine undertray reduces lift, perhaps smoothing flow under the car and then the short flick up at the end is where this effect is gained. IIRC the reduction in lift isn't dramatic only 5-10kg at 100mph (the rear wing gives 60-70kg at 100mph) but I is enough to restore the remaining imbalance caused by the 911 teardrop profile at high speed, that the wing almost completely addresses on its own.

Here's a crazy idea; I wonder if you could make a blown diffuser by diverting the cooling air exiting the engine, so it exits in the short "venturi" section at the rear rather than in the middle of the undertray!?

Captain Ahab Jr.

Don't forget to add diffuser strakes, to help maintain the airflow direction and to reduce the turbulent air deflected sideways from tyre squish disturbing the diffuser airflow.

If you want to get really trick, start it as far forward as possible to create as much volume as possible, don't go too steep with ramp angle as it will cause seperation. Tilt the engine up slightly from the gearbox end will also help to create more volume as it will allow a higher ramp.

For the cooling slots/holes below the engine, these could be ducted internally so the engine air flow exits through an external slot above the diffuser to keep the diffuser aero surface unbroken.

A flat floor from the front the the diffuser will also help get the most from out of it.

The McLaren F1 road car took it a stage further than all this and used fans to suck the boundary layer away making the diffuser even more effective at high speeds.

Alex, blown diffuser is a difficult beast to control, doing as you suggest would make it RPM dependent, this is not good idea during the lift off throttle stage of a corner as it will shift the centre of aero balance all over the place and exactly when you don't this happening. Its possible to use the engine as an air pump leaning off the mixture but keeping the rev's high to keep the the airflow consistent but would require complicated engine software and hours of track time with aero measuring to get it working.

alexjc4

Something subtle like this maybe

alexjc4

Lol good point; even changing gear would effect downforce, not really ideal!

kos11-12

I new we shoudn't say the word d_______r , ....
Anyway headlights on !

I think less is better, covering this area and enough cooling would do the trick, ok maybe when we are at it make 2 side fins to tied up the flow, that would do it for our little sports cars ....
Diffusers needed to be made in conjunction with the rear spoiler downforce, the way to start would be to do a simulations, then test it on track or wind tunnel , there is so many shape, slanted, concave, the length, where it starts and finish the fins shape , and so on.....maybe worth it if someone is intending to sale a few for race cars...

kos11-12

I just had an aspirin and read that...

http://www.f1technical.net/forum/viewtopic.php?t=11477


http://www.f1technical.net/forum/vie...hp?f=6&t=10943

alexjc4

Adding dive planes and a meaningful (ie not cosmetic) front splitter seems like a good idea so as to try to keep the centre of pressure from moving backwards and adding to the c4 understeer.

Also if you fit a cup pipe you could cut out the bottom of the rear bumper to allow you to run the optimum ramp angle, it may only be a couple of inches that you'd cut.

Ps didn't max3.2 say he was a fluid dynamics guy? maybe he could point you in the dirrection of some quick and dirty cfd.

Cheeksyboy

I can see why an aspirin would be necessary, my brain hurt just after the page loaded

Front dive planes are what the GT3RS 4.0 had I think....and I've seen them on a number of Westy and Caterhams, definitely one thing to consider to go along with my front splitter (that I don't think does as much for adding high speed grip or stability as I'd like)....