The cat is really just for when the engine isn't running perfectly. If it's running exactly right and the tune is just right, you can pass the sniffer test without a cat with reasonably modern systems, the cat is to make up for the systems not working 100%, to deal with those issues. At least that has been my experience.

 

I thought it was worse than that. Say you took a 96 Camaro and stuck a LS1 in it. Don't they define that has illegal too because it's not the original engine for the car?

 

By federal law you are allowed to use a newer model year engine in an older car, but it has to have all the smog equipment as used in the car that the engine was initially built for. You can also use an engine package which was certified to meet current smog regulations, and can be used in any car, such as the GM E-ROD LS3.

 

Actually when I was checking I was told that Federal law makes no allowance for engine swaps. But because Ca law DOES, and Ca law is allowed to be followed in any state because is it more stringent than federal law, that is what allows engine swaps.

Ca has two classes of engine swaps, one is a swap, that is an engine that is for the same general class of car, same year or newer, and the other is an engine that is available in the same model car in the same model year or later, and they are handled a little differently. The first requires a lot more paperwork, the second requires nothing more than a visit to a state run referee station to get a new under hood sticker with the new emissions standards for the car, so putting an LS1 from a later model Camaro in ANY earlier model/year Camaro is an engine UPGRADE and is much simpler, putting an engine from some other model, or putting an LS1 in a ford, is an engine SWAP and is more complex and involved and takes more paperwork and the inspections and a little more involved, but the basics are the same in call cases, for the most part the emissions equipment must come along, except for cases of vehicles that didn't have cats in the first place which don't get the cats installed, but everything else must be moved with the engine.

 

Each staete has its own little quirks, so it is good to check with DMV first.