Ok I got used hotshot headers and I am curious what is the best thing to use to clean the insides? Carb cleaner? or would seaform be good too? I just want to get all the black stuff(assuming it is carbon build up) out. Please let me know thanks.

edit: Oh yea I heard since I have the hotshot headers I would have to get rid of the cat or get a high flow cat cause the it will just kill the cat... is that true? I want be able to pass smog with it and not have to worry about a dead cat.

Can't say much on that one, but try brake parts cleaner, I use it to remove anything, and it works. But don't go using it on painted surfaces. It might involve some scrubbing if you are up to the task.