i see the terms "no air" and "single air" a lot when shopping for hi-flows...what do they mean?

i also see 2-way and 3-way and stuff like that...can someone break it all down for me?

also does the replacement cat have to be similar in size to the stock one? will the hangers still work?

This ought to help..

The air injection system is designed to introduce outside air into the exhaust stream to assist in burning the gases produced by the engine. Air is injected into the exhaust system at one of several locations including the cylinder head, exhaust manifold, or directly into the catalytic converter. Outside air is drawn by an air pump (sometimes referred to as a smog pump

and
http://auto.howstuffworks.com/catalytic-converter.htm

Lots of info there.

As long as the inlet/outlet pipes, and physical area provided allow, they will work.

Bigger isn't better, as there are only so much pollutants in an exhaust charge.
Changing one of their levels, will cause a change in the other two.

When they begin to clog (Think of a toll plaza at rush hour) they clog at the front.
the road after the toll is useless if you can't get there....

Many people think that they can simply replace the catalytic converter and pass the emissions inspection. If the problem is the catalytic converter, of course this would be the right solution. But, in many cases, a new catalytic converter will simply "mask" an underlying problem with the air/fuel ratio, ignition system, or some other problem with the engine. A catalytic converter should only be replaced if it is operating inefficiently or not at all.

A tune up to specs, including (if needed) timing, PCV, EGR, Air pump, Spark plugs, plug wires, idle speed, MAF test, clean intake system, and air filters .............
(what have I forgotten?) will afford you better gas mileage, ( Leadfoot excluded) power, economy.
Don't waste money on a "Bigger-n-better " converter.

Hope that helps ya!

You need a non-air 3-way cat in answer to your question.