A muffler (4) for motor vehicles comprising an intake pipe (8) of the exhaust gases that separates, at a fork (10), into a main pipe (12) and a secondary pipe (16), a muffler body (24) which delimits an expansion volume (28) and houses at least partly the main pipe (12) and the secondary pipe (16), wherein the intake pipe (8) of the exhaust gas is in fluid continuous connection with the main pipe (12) through the secondary pipe (16), and wherein the main pipe (12) comprises, downstream of the fork (10), a throttle valve (32) which allows or prevents a further fluidic connection of the intake pipe (8) of exhaust gas with the main pipe (12), and an output of the exhaust gases (40), for the expulsion of the gases from the muffler pipe (4).