Intake & exhaust valve sizes

[Esky]

Been wondering for a while- why are exhaust valves always bigger in diameter than intake valves? I mean, doesn't the hot and expanded exhaust take up a larger volume compared to the cold, unburnt air and vaporised + liquid gasoline, and therefore shd be the side having larger valves?

[curtis73]

You are quite correct that there is more volume of gases exiting the chamber than entering, however not by much since the gasoline is in a gaseous state when it comes in as well.

The main reason is because gases respond better to negative changes in volume (compression) than they do positive changes (vacuum). Since the piston positively displaces the gases on the exhaust stroke, the size of the exhaust valve can be a bit smaller.

The other reason is what is referred to as scavenging. Increasing the velocity of exhaust gases helps "suck in" more intake charge during valve overlap.

[Steel]

uhh... as far as *I* know, exhasut valves are usually quite a bit *smaller* than the intakes, due to teh fact that the pressure of the burned gasses far outweigh the small diameter of the valves, thus allowing designers to use larger *intake* valves to make a more efficient engine.

that or they're the same size.

[jibbastack]

although scavenging is the main tech reason for this it is also easier to push ait through a hole than pulling it through. I remember this statement from school East Coast Aero Tech (now Wyotech}

[2.2 Straight six]

also an old thread. closed.