Hmmm, I think I see where you're coming from. Because petrol engines are a long way from 100% efficient to start with then if all you are doing is improving the amount of chemical energy that is converted to kinetic energy and reducing the amount that is wasted as heat and noise then you have a more efficient engine without affecting the net energy in or out of the system. But it's probably best not to say things like "you are going to have more power for free" - that will always wind physicists up! Probably better to phrase it as "you are going to waste less of the power you have already bought and paid for".
Given the billions that the motor industry has been investing in trying to make cars more energy efficient I would guess that there are good reasons why they haven't gone down this route. But I don't know what those reasons are. Could be that it doesn't work, could be that it does work but significantly reduces engine life, could be something else.
Dave P.