So i have heard some people talking about off road exhast systems that arnt legal for use on the road. Who makes these and do they give you any more HP's?

 

Most true "off-road" systems are systems with no catalytic converters. In many stats it is illegal to run without cats. You do get some more HP without cats though, but in many cases you also lose some TQ. You can also get Hi-Flow cats that add a little HP.

Roush sells a cat-back exhaust system for the '04 and up F-150s, one is the "legal" system the other is the "off-road" system. The legal system has a larger muffler and sound deadening material in the tips. The off-road system has a much smaller muffler and straight through tips with a more aggressive tone which is why the call it "off-road" even though it is still technically legal from a catalytic converter standpoint as it is a cat-back system

Hope this helps a little.