[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