Might be because the exhaust has a bit more pressure driving it out.

 

valves

:smash: :smash: the intake valve is larger,because because the gas is drawn in on the piston down stroke (suction)a larger valve allows more gas into the cylinder. the exhaust valve is smaller because the exhaust gas is pushed out by the piston coming up in the cylinder (pressure) so it does not have to be as large.

 

The intake valve and runner are bigger to allow for the large volume of air/fuel to enter and fill the cylinder during the intake stroke which then is compressed to a much smaller volume on the compression stroke. If the exhaust valve and runner were the same size it would slow down the velocity and scavenging effect that a somewhat smaller runner would afford when the exhaust valve opens. Thats my theory anyway, open for criticism....

 

because that is all it needs to expel the burnt charge?

That is the answer.

 

Alot of cam timing specs also show that the exhaust valve is open longer, maybe it equals out?

There are single pattern cams that have the same intake and exhaust valve duration and lift.

Dual pattern cams generally have more exhaust duration than intake duration. I assume this is to make up for the smaller exhaust valve/port diameter.

 

coming in it is filling and compressing in the cylinder as 327 nut said.going out it is already under preassure so it takes a much smaller area.

 

I guess it'd be like just about any other type of pump. Generally speaking, pumps are almost always designed with the inlet larger than the outlet.