a high flow cat..well its a cat pipe which u have to have to pass emissions, and well high flow..it helps flow more exhaust fumes throught it, and the flex pipe, im not sure really..cept that it flexes lol
catalytic converter, it is an enzyme piece that converts your exhaust into a less harmful emission. a higher flowing one may have less of the enzyme to clog the system, and a larger diameter piping. a flex pipe is what it states. It's a metal tubing that has a flexing element on it, it allows for flexing of the pipe. The engine vibrates a lot, enough to crack the exhaust piping and exhaust manifold, this allows the whole exhaust to flex, along side the rubber hangers.
If you want one, get BOTH, they will allow more air to flow through the exhaust system than the stock ones ever could. I noticed such a dramatic change in my car when I installed it the other day (thanx ray, I got everything and the car is amazin) the cat will still clean the polution out of it just allow more air to flow threw, then the larger flex pipe (2.25 inches vs 1.75 stock) carries it on to your cat back which is also much larger than stock. Result, spinnin tires, burnin people, and turnin heads!!!!!!