Exhaust cut outs are the common term. There are several variations from cheap to expensive and trick. If you have headers with 3 bolt collector flanges, the cheap way is to remove the bolts at the track and let the exhaust pipe rotate down and put one bolt in to hold them in-place. They make a weld in T/Y with a removable end cap to un-cork the headers (sometimes incorrectly called “Lakes Pipes”) and bypass the exhaust. They also make T/Ys with a diversion flapper that is controlled by a cable or electric solenoid (trick and most expensive) so no crawling under the car is required. Take note that most states where e-check is an issue will almost always flunk a vehicle equipped with cut-outs. Contrary to popular beliefs, opening your headers will probably decrease performance unless you have a totally restricted exhaust system or an outrageous race mill. Exhaust gas scavenging accomplished with a properly designed exhaust system helps facilitate the flow of exhaust gasses out of each combustion chamber through pressure inversions created by exhaust pulses from the other cylinders. Plus from a sound stand point, unless the engine in question has a radical cam profile, open headers can sound kind of crappy.