Side valve engines are fine for smaller mowers and will offer good performance but when it comes to large scale machines like ride-on mowers and lawn tractors, OHV engines are almost always fitted.
Side valve engines, as the name suggests, have intake and exhaust valves fitted to the side of the combustion chamber, whereas the OHV configuration dictates that the intake and exhaust valves are fitted in the cylinder head. This makes OHV engines slightly larger as well as more powerful.
In fact, OHV engines offer considerably more power and run more smoothly and quietly than side valve engines. They are also more fuel economic and can therefore save money. They are generally considered to be more environmentally sound.
In very general terms, according to a number of sources, a great deal of this difference is down to the shape and configuration of the cylinders.
With side-valve engines, the positioning of the valves means the shape of the cylinder itself can interfere with airflow and exhaust flow, reducing the efficiency of the engine.
The design of the OHV engine means more air is drawn in to the cylinder for a longer stroke, helping to make it more energy and fuel efficiency.
Side-Valve Cutaway OHV Engine Cutaway