[hyi1976]

All, what does rear diffusers actually do?
Is it only for looks?
Thanks all

[Ccatznip]

I believe a rear diffuser "smooths and straightens" air flow from under the car when exiting the back, so turbulance is not created to lift the rear tires at high speeds. I think this is why BMW are very stable at high speeds...they are almost completely flat on the bottom. I could be wrong. i am sure someone might have a better idea.????

[Year's_End]

Originally Posted by Ccatznip

I believe a rear diffuser "smooths and straightens" air flow from under the car when exiting the back, so turbulance is not created to lift the rear tires at high speeds. I think this is why BMW are very stable at high speeds...they are almost completely flat on the bottom. I could be wrong. i am sure someone might have a better idea.????

I think this is more or less correct. I think it has to do with Bernoulli's Principle (however you spell it). By straightening and smoothening the airflow beneath the car, there is negative lift because the air on the top has to move faster to reach the same destination within the same time.
...okay...so that wasn't the clearest explanation but it has to do with negative lift. It's like a functional spoiler that goes underneath the car.