ok, here goes... ive been confused for a long time about how much HP cats steal...my mechanic says i can get about 20% more HP with no cat on my 1997 Civic EX(or any NA 4cylinder,). The mechanic has a friend with a WRX with no cat(mechanic also said the faster the car, the less of a difference in HP gains when cat is taken out)... and he said that the person could most definitely tell the gain in power(about 20 more HP)....but my confusion lies in what other people write in civic forums. people say its a waste of time because its only a small gain in HP (3-5) and when emissions come, its a bitch (ya know)...heres my Q. HOW MUCH OF A HP GAIN DOES A (stock) CIVIC GET WHEN THE CAT IS TAKEN OFF and DOES GUTTING THE CAT DO THE SAME AS JUST PUTTING IN A STRIAGHT PIPE ? (the only mods done to my car is an intake and 2.25 inch catback with a magaflow muffler) and with the o2 sensor simulators...do they still keep all the shit working in your car! im thinking that the rear 02 simulator(OBD 2 is GAY) only checks if the catylst is fuctioning properly..and that the front 02 sensor is the one that checks on rich or lean exhausts, correct backpressure, and output voltage...and now my second Q.. do the 02 sensor sims keep everything cool in the car, or does it just always tell the computer that everything is good when something might be going bad? i hope i didnt confuse ne one with my questions, i was never good at grammar. ---ill get a good signature later.