Just for clarity, I'm not asking in relativistic terms. Assuming the engine's pistons aren't horizontally opposed, would the vertical force of the pistons move the car downward? Or would it not matter because the explosion is contained, causing no net force outside of the engine? Or maybe it would make the car lighter, because the downward energy (I realize energy is not usually used as a vector, bear with me) eventually thrusting the wheels be used up, leaving only energy moving in the upward direction? Much thanks for feeding my curiousity.
no, it doesn't. car engines are balanced in the sense that for each piston moving upwards at any moment, there's another moving downwards, so the reaction forces cancel. even if all the pistons were moving up and down together, their motion would not make the car heavier or lighter, they would just make the car vibrate up and down.