mikegarrison

At high temperatures and pressures, like inside your car's combustion chamber, N2 disassociates into molecular N. That reacts with oxygen. Thus some of that nitrogen does become NO and NO2 (which together are known as NOx).

Sideways

Larger exhaust valves also have problems with heat.

Elistan

Oooh, good one. I hadn't thought about that - a major component of exhaust valve cooling is having it seat against the cylinder head. So a larger valve will have a smaller seating area to mass ratio, resulting in less cooling.

alexf20c

It's easier to get exhaust out than it is to get air/fuel mixture in, isn't it?

More to the point, why would you have a smaller intake valve? If it can't get in, why would it need to get out?

APEXSEAL

RACER

A 4 cyl has one or more cylinders firing at any given time, meaning that as one cylinder is on the exhaust stroke, another one is firing. The force from combustable ignition is far greater than the force generated from air and fuel rushing into a cylinder because it is forced to do so. All it is doing is filling a void (reducing vacume)

AusS2000

After a series of rather silly posts no_really has got it right.

There is a lot more volume after combustion than before, but it is being pumped out. The only thing driving air in is atmospheric pressure (talking NA here) and as there are plenty of restrictions between the air filter and the port you want to make it as easy as possible for air/fuel to get in to the combustion chamber.

ttah

It's the same reason your mouth is bigger than your butthole.

Prolene

That's partly the case. Really, the exhaust valve opens before bottom dead center, before the piston is pushing anything out, and the pressure built by combusting fuel begins the flow of exhaust gasses. That flow can continue to scavenge the cylinder even as the piston in past top dead center. That scavenging effect is helpful to bring in the intake gasses as well. "Valve overlap" is what allows this to happen. This overlap is not needed as much in forced induction, as the flow of intake gasses might go out the exhaust to varying degrees before the exhaust valve closes.

Prolene

Good explanation of cylinder flow dynamics <here>.