Quote:
Originally Posted by winkosmosis
It's a rearward facing surface, so it's not higher pressure just because the airflow is slowing down. Like the top surface of a fastback isn't seeing high pressure.
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Exactly. The reason that performance cars want a diffuser is to increase their down-force. If it did the opposite and increased pressure (lifting the car off the ground) they would be avoided at all costs!
Diffusers can reduce the drag experienced by the vehicle. Drag is at it's lowest at ~2.66 (long, delta Cd 0.025) or 3.66 (short, delta Cd 0.02) degrees, and is still better drag-wise compared to having a 0 degree diffuser until at least after 11 degrees (short diffuser, delta Cd 0.007) or 6 degrees (long diffuser, delta Cd 0.013). In both cases, down-force (negative lift) is being increased all the way to the 6 and 11 degree numbers above, and in both cases they max out at delta Clr (coefficient of rear lift) at 0.26 (THIS IS HUGE!). I suppose I should quote Hucho on this one, figure 4.48 in his 4th edition.
What you can see is that a longer diffuser allows more downforce and lower drag than short diffusers.