I've seen some for sale at auto parts stores, and in Jeg's and Summit. These are for carbureted vehicles only. Now why on earth would you use this? AND, would they help something like a 15 second vehicle?

 

Hehehehehehehe

There is alot of science that goes into PROPERLY setting up a fuel system.

When a needle and seat closes in a non return regulated system it's in fact deadheading the pump. The pressure can raise to like 9-10 PSI. Closing against this high of pressure will generate a flucuating float level. With a return sytle system, there is a min pressure raise, so the float level stays just about constant.

With a return sstem, if you plumb it correctly you can use the return line to purge the system of any air bubbles, so that the needle and seat don't chatter, and lose their adjustment. SOme cars are so bad that the acclerator pump setting have to be played with because of this trapping air

It eliminates vapor locking. And can be used to help keep cool fuel available for the engine. Cool fuel helps HP as well as cool air.

Most of the above also applies to EFI.