Intake valves are larger because barometric pressure, (engine vacuum), is all that makes the air go in. Exhaust valves don't have to be so large because pressure from the piston pushes the exhaust out. Intake valves are cooled by the air and fuel coming in. Exhaust valves have hot exhaust gases passing over them so they have to cool while they are seated by transferring the heat across the seat. With a smaller diameter, that heat has less distance to travel. That prevents the valve from melting and burning away.
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Sunday, October 28th, 2012 AT 8:28 PM