It seems all kinds of people are slapping diffusers, canards, slats, etc on their car. If you're doing it for looks, that's one thing. Here's the scoop on what it is, what it does, and whether or not you need it.
The Venturi Principle
This principle states that as a fluid (which air is) flows through a constricted area, its velocity increases. Think of putting your finger over a water hose.
The Bernoulli Principle
As a fluid's speed increases, pressure decreases.
Application in Race Cars
Venturi Tunnel
The car has a shaped underbody. The first third of the car underside slopes towards the ground, the middle third is flat, and the last third slopes away from the ground. The front compression section accelerates the air by constricting it (nozzle effect). The middle part contains the air at a rapid speed and low pressure, creating suction. The final part (diffuser) slows the air back down to ambient.
Flat Underbody Plus Air Dam
The car has a flat underbody. The car also has a low front air dam. The air dam forces incoming air under the car, which then flows at a high speed/low pressure over the flat underbody. The diffuser section at the end slows the air back to ambient.
Factors essential to venturi principle operation
1. Measurement B must be A/3 or less. This translates to about 1-2 inches for a low-sprung race car.
2. The tunnel must be adequately shielded from spilling out on the sides. This means, fairly close to the center of the car.
3. The compression and diffusion section must be at pitched 17 degrees or less, otherwise the flow will detach and become turbulent.
4. For angle T to be 17 degrees or less, and C to be B*3, F must be at least 1/3 the length of the car.
What purpose does the diffuser serve?
The diffuser decelerates the air under the car and returns it to ambient speed and pressure. If the air isn't decelerated, it continues at a negative pressure past the end of the car. This causes drag.
Is this practical in my street car?
Probably not. In order to achieve a low enough center section ("B"), the car would need incredibly harsh suspension to control the ride height. This would be most unpleasant, and the amount of normal body roll may ground out parts of the car around corners.
The center section, if it generates enough suction to make a difference, will probably be ripped from the bottom of the car, unless it's built as an integral component and/or welded on. Then, you'll have a real fun time working on your car - with the bottom welded shut! If you hit anything or went over a speedbump, your painstakingly-attached carbon or aluminum underbody work would probably shatter or gash.
The diffuser's job is to slow down and increase the pressure of the air. If the air is already moving at ambient, slowing it down below ambient will immediately cause drag.
Does the diffuser I saw for sale help me?
Unless you have bodywork which accelerates air to faster-than-ambient, probably not. Unless the diffuser is a real 10-17 deg diffuser, probably not. Unless the lowest point on your car is 2 inches off the ground, probably not.
Here is an example of a real diffuser:
Here is an example of a gimmick diffuser:
