First law of thermodynamics. A few points: 1) Internal Combustion engines work by harnessing explosions. Each explosion pushes a piston, which is then harnes

First law of thermodynamics. A few points: 1) Internal Combustion engines work by harnessing explosions. Each explosion pushes a piston, which is then harnessed to do useful work rotating the wheels. The Exhaust note is caused by agressive expansion of gasses rushing into the exhaust. That expansion _could_ have been used to push the piston more (in practise you'd need a secondary expansion unit, which isn't all that practical .) 2) The exhaust system is there to reduce (and modulate) noise levels (and emissions). It weighs a not insignificant sum, causes back pressure, which affects the efficiency of the overall system. (although now that heavily mitigated, even more so with a turbo.) Granted, most of the energy is wasted through heat, but thats a different matter.