Does modifying a car void the warranty?
Yes and no. Under the Magnuson-Moss act, if the failed part was not caused by a modified/aftermarket part, then the warranty must be honored. For example, if your engine blew but you only changed/installed an aftermarket stereo system (you did not install or modify any other component; your car is stock) which did not cause the engine to fail, then the manufacturer (Mazda) must honor the warranty. However, if you change something like the pistons with aftermarket high compression/performance ones and the engine blows, then you are most likely out of luck.