Oversquare vs undersquare referrs to the Bore/Stroke ratio of an engine. If the bore is larger than the stroke, the engine is considered to be oversquare. I saw a recent magazine article discussing the advantages of oversquare vs undersquare engine designs. I can’t exactly recall what they said, but it was pretty off base.Here’s what you need to know in a nutshell:
- How much torque an engine can make is based on its cubic inches. Torque comes from how much air the engine can ingest every intake stroke. The more the cubic inches, the more air it can take in every stroke.
- How much HP an engine can make is based on the total valve area. HP comes from the total air the engine can ingest per unit time, like every second. The intake valve it typically the largest restriction for air flow in an engine. Therefore, an engine with more intake valve area (total valves in the engine x valve area) can ingest more air than one with less valve area. Or, it can rev higher before it runs out of breath. Higher revs mean more HP.
If we look at an oversquare engine, its larger bore allows for larger valves to be used. Therefore it can make more HP than the same cubic inch engine with a smaller bore. That’s it. It’s just that simple.The old axiom that long stroke engines make more torque is related to this. It’s not that they make more torque, its just that they can’t make HP because the valves are just that much smaller to fit inside the smaller bore.It also explains why 4 valve engines make more HP, lots of valve area for the displacement. It also explains why the engine with more cylinders will make more HP for the same displacement. The total valve area (adding up all valves for all cylinders) is greater.Now there is a downside to big bores. It takes longer for the flame front to travel across the entire bore, meaning a slower burn rate and being more prone to detonation. These are typically not a problem until higher RPM. Also, the larger valves are harder to control at higher RPM.Now I’m sure you are thinking that superchargers, cam timing, runner lengths, etc all have an effect. They clearly do. But I was trying to keep it simple.