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Originally Posted by stownsen914
I put a fairly large diffuser on my 914 racecar. I had a bit more freedom to do so than you will on a 911 since the engine is farther forward. I did have to move some things around to get it to fit - mostly the exhaust and shift linkage. The car handles really well, but without data acquisition it's hard to quantify. On our cars without serious wind tunnel or CFD time to optimize, we're not going to get thousands of lbs of downforce like the big boys get. Hundreds could be a more realistic goal.
There are a few things to keep in mind with underbody aero:
- It won't do much to have a diffuser if you haven't flat bottomed the entire belly of the car. A diffuser is intended to control fast flowing air under the car and gently guide its passage out the back of the car and return it to "atmospheric." The underside of older Porsches has all kinds of undulations and protrusions that slow down air flow under the car. Putting a diffuser behind that will greatly reduce its effectiveness.
- One of the great things about underbody aero is that it can actually improve your aero drag while providing some downforce. If you flat bottom your car, you're reducing drag under the car which can offset the drag that downforce from a diffuser would add.
- This part isn't so easy on a 911, but having a low wing can help "pump" the air out of a diffuser and have some benefit to downforce. This may sound unintuitive since usually a high wing is better, but the gains in improving the downforce of the diffuser can offset the loss of extra downforce that you'd normally have with a higher mounted wing. Back in the old days of IMSA GTP, they used to run two wings to get the best of both worlds!
For my 914 diffuser, I mostly copied existing designs as best I could. I did put a low wing and flat bottomed the entire belly of the car with a sheet of aluminum. I need to do some data acquisition to see what it's really doing. Would love to do some CFD to optimize it. I'm sure I've left a lot on the table.
Scott
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Quote:
Originally Posted by faapgar
I have a full ground effects 914-6 built on a chrome-moly tube frame chassis.Chassis was built in the 90,s in Texas with a stock floopan to make it appear like a 914.Ground effects were set up by an engineer.It made 1100 lbs.of downforce at 100 mph.It was brand new with 1.25 inches of ground clearance.Full side skirts and slick tunnels under the drivetrain.I put a tie wrap on the shocks and went testing until the tie wrap moved 1 inch. I changed springs 6 times until I got it right.1100 lb.rears and 950 lb. for the front.It hit bottom at around 100 until I got the springs right.It was a trip to drive but became tenuous if you bumped a curb and lost some air.VERY quick to 100 then slow to accelerate because of the drag.Scott Townsend knows this car.I will get a picture of the car and rear diffusor.The big rear wing was designed for the air to pass underneath it.Motor was 2.8 on 3.0 turbo case.Ti everything.45mm slidevalves,Motec.373 hp at 9300 rpm but it needed that Rawknees Turbo Powaaa.Fred
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Aptly illustrates that extensive testing, data acquisition and valid replication is part of the equation, especially when dealing with "black art" topics like vehicle aerodynamics. Without such testing, many vehicles may appear to be "state of the street art" despite being only best guesses as to whether those efforts bear any reality of real-world effectiveness.
I can only shake my head and wince at the "aero" bits tacked onto sporty cars by the marketing departments of car manufacturers. These include rear wings, strakes, rear end "diffusers", spoilers, winglets, ornamental scoops and NACA ducts (many non-functional), etc. A substitute for good car design perhaps.
Sherwood