Diffusor bottom plate plans for 911

[petri]

We have done computer aided simulations of the diffusor: the width and height of the canals is continuously widening parabel towards the rear: it should be quite near of perfect.

Cooling beneath the engine will happen two ways as you can see in the 3d model pics: when standing= to 4 directions (this is the most critical part and we will test this carefully). There is an option we will need some extra fans especially for the turbo area when standing.
When running= through the rear panels and over the rear bumper beneath the engine. This is not that much of a problem due to my oversize oil cooling capacity: now my engine hardly reaches the low line of the temperature, to say now my engine is about to run almost too cold.
Last summer all other development parts of the car are basically finished, so now we are capable of doing complete testing and racing program this summer.

[Reaper930]

Lookin' good man, best of luck to ya!

[stownsen914]

Very nice work ... one thing I will be curious to see how you deal with is the hot air from the cylinders and heads (I'm assuming this is an air cooled motor). I see you have addressed this in your analysis, but I have to wonder if the design will be able to handle the volume of air produced by the cooling fan -- several thousand cfm -- if you drive the car hard for sustained periods. You mention the oil coolers, but that may not effectively replace proper cooling on the cylinders and heads, especially on a turbocharged car. I put a flat bottom and diffuser on my track 914, and was impressed at how hot the engine bay and diffuser got, even though I left the back of the car essentially open to allow hot air to exit that way. Closing over the bottom of the engine indeed restricted the air coming off the cylinders/heads. I would up building some special shrouding to guide some of the air out the sides, and louvering my rear fenders to allow the air to escape.

Please continue giving updates, and good luck in testing your car!

Scott

[petri]

Yes, the cooling fan runs through about 1500 liters/second when running full. Even if running how fast, the capacity of the fan hardly can do more than that. In my engine bay (upper part) the air intakes in the deck and rear windows altogether creates the situation that up to 100km/h the engine space is in under pressure situation and over 100km/h it will be overpressure situation.

When running engine and standing still, I'm quite sure there will be problems if doing it long enough, to say too long. This momentum might need some additional fans beneath to suck hot air off.

Anyway, when running below 100km/h the amount of air through the fan is slightly less than 1500L/min. Air beneath, around the cylinders and exhaust can go both sideways (toward the tyre) and to the rear the space in between engine and diffuser must also be widening toward the rear. Our simulation tells, that because the rear diffuser is lower than the center bottom plate of the car, the front edge of the rear diffuser will take major part of air running near the center bottom surface. The simulation shows that at least between 100-160km/h the cooling should be better than without, because of the speed of the changing air between the engine and diffuser. Toward the rear growing space and fast flow of the air will direct and in some momentums even increase ( suck) the cooling air exit. I will add some kind of sketch, what are we thinking of doing it.
This means the diffuser will not ever be a bolt on solution: all the changes and redesigns we have done to this car really tells it. Everything behind the engine (turbo-exhaust-silencer etc.) must be aerodynamically relocated and redesigned according to the air exit purposes.
Do you have pics or sketch of your car and solution? would be interesting to see them.
I agree there might be a need for more openings in the rear for the air as you have thought.

[petri]

Additional sketches, dimensions may vary, not drafted on a top of any section or photo, but just the idea with free hand and imagination during the coffee break.
The problem would be easier without silencers down to 95dB which is quite normal noise limit in many European tracks for street car events (and those events are most fun with this 'street car').

[petri]

After some rough quick'n'dirty test modules, here begins the story for the final product. This cardboard mock-up is more of measuring the absolut maximum size (this is for sure oversize because of too much exhaust lines etc. coming to the air stream) of the diffuser.

Anyway, 4 tricks for cooling the engine:
-scraper-like front edge,
-two nacaducts in the front part of the diffuser, both sides of the gearbox,
-plenty of air exit options in sides
-huge air opening in the rear (no bumper or middle lamp unit).

Story will continue from here.

[Flieger]

And Brawn GP thought they had a trick diffuser

http://www.racecar-engineering.com/news/cars/313897/diffusers-they-are-legal.html

[petri]

Thanks for an excellent link .

[Miguel Antonett]

What are you going to do with the headers sticking out/down into the air flow path?

An effective diffuser must have laminar flow all the way out, those pipes will generate lots of turbulance distroying - partially - the ground effect.

[mph911]

Originally Posted by Flieger

And Brawn GP thought they had a trick diffuser

http://www.racecar-engineering.com/news/cars/313897/diffusers-they-are-legal.html

Thats what I was thinking, with all thats going on in F1 with the diffuser debate!

Great work Petri, good to see some innovative thinking, good luck!

[petri]

"An effective diffuser must have laminar flow all the way out, those pipes will generate lots of turbulance distroying - partially - the ground effect."
Yes Miquel, you are very correct. Each canal has 3 surfaces for laminar flow, the major surface is of course horizontal.

What you see here is a maximum size mock-up, still with one level top plate. Canal heights will differ in future:
- altogether the amount of 5 canals will change to 7 canals.
- 2+2 side canals will be with full height (according to mock-up)
- 3 canals in the middle will be lower ones and 2 of them (all others except centre canal) will be equipped with nacaducts for engine bay cooling. These nacaducts will also decrease part of the air following laminar flow of the top surface . These middle canals will go beneath the exhaust lines.

Next test proto will be of aluminum, possibly too heavy, but not so easily burning .

[Miguel Antonett]

You are touching all the bases here. Go on and keep us posted. Great effort!!!!

[petri]

Thanks for the support Miquel. I forgot to say one of the new Brawn innovations is Gurney flap-like addition around the exit edge (both horizontal and vertical). Of course: why it wouldn't work there if it works in the edges of rear spoilers as well as rear edges of racing sails? So we will do some study with it as well

[Flieger]

Brawn and the other Double-Deck Diffusers declared legal!

http://www.f1technical.net/news/12129

http://www.racecar-engineering.com/news/cars/316234/diffuser-gang-legal.html

Good news for you, Petri. Are you going to bring in some air from the sides to feed the top diffuser?http://www.formula1.com/news/technical/2009/0/634.html That air could replace the disrupted flow from the engine cooling air.

[petri]

Thanks for a set of great links... Here is a sketch of the next step. I have to study through-the-diffuser-holes, anyway this can't go too much worse than air flow now under the standard 911 with umbrella-style rear bumper like most of the bumpers.