heres the simple way...
follow the flow of air from the when it first enters the induction system, through the air filter, throttle body, plenum chamber, inlet ports into the combustion chamber. then follow the burnt gases out of the exhaust ports, exhaust manifold, exhaust system tailpipe and away. If you look to see where the most prominent constrictions are you'll find its probably the valve to valve seat area, the inlet and exhaust ports around the area of the valve guid bosses, and poor alignment of said ports to manifolds causing turbulance in the gas flows

. get these right and your away! just a change of filter will give you not a lot! :'(