Things like cat-back exhaust systems, K&N filters, STB's, etc. will not void your warranty. Some dealers will hassle you if it's not OEM dirt on the tires, but most are very reasonable and will not give you a hard time. If you've gone into the motor with a cam or heads, swapped gears, put on a blower or nitrous, many dealers will not honor any powertrain or drivetrain warranty claims, but will honor things like power windows, radios, etc.

Like was posted earlier, the burden of proof is on the dealer to prove that your mod caused the failure, not on you to prove it didn't. If you've got one of the few bonehead dealers that will hassle you over the slightest little thing, it would probably be better to find another dealer or reputable repair shop to do your work. There's no shortage of knowledge here on this board to help you find someone you can trust.

I bought my car at Betley Chevrolet in Derry NH. They've treated me very well. I recently had to have my M6 repaired due to a busted 5th gear synchro and bent selector fork. I've got a few minor bolt-on mods (check my sig) and they know I've been to the strip a couple of times. They could have said, "Oh well, you drag raced it, so that's probably why it broke. Sorry, we won't honor your warranty." But they didn't. Whether or not they would cover it was never an issue. They fixed it and covered the whole repair. Since a 5th gear synchro failure is somewhat unusual, they took the time to thoroughly go through the entire transmission to find the root cause rather than just throw parts at it. They even kept it stored inside in a special bay while they were waiting for parts so it wouldn't have to be outside in the slop. They did an excellent job and took great care of the car for the 10 days they had it. If it had been on my dime, the repair would have cost $2,100.00.

I know NH is a long way to go and not practical for you, but there's probably a good dealer in your area that someone on the board can recommend.

I can appreciate you being concerned about it. It's bad enough to have to bring your car to the shop in the first place for warranty service, let alone having to deal with a service department's ignorance and indifference to their customers. The good dealers recognize that the service department often makes or breaks a decision to do repeat business and they're smart enough not be jerks over minor bolt-on mods.

Not only does he sell 'em, he races 'em. Last several years it's been the F-Body, this year a new Z06. He still runs his 3rd Gen in the American Sedan class (SCCA).