Why are intake valves always bigger than exhaust valves?
The laws of physics and common sense suggest that it is easier to compress than to take in a gaseous substance. In actual practice, during the exhaust stroke of a four-stroke engine, combustion gases are ejected as a result of the upward travel of the piston. This operation will happen “no matter what”, in other words, notwithstanding the dimensions of the exhaust ducts or diameters of the valves. Better exhaust conditions (larger valves and ducts) will only have a limited influence on the work carried out by the engine (effort). By contrast, during the intake stroke, air must be called into the cylinder: in other words, you must “attract” into a “hole” a gas otherwise free to stay in the airbox or the environment. As you can well imagine, it is hard work to “draw in” an air column which will very easily “break”, therefore, it is vital to create the best possible conditions to facilitate air intake. Clearly, the intake volume is the same at each cycle: what changes is the density, or t
