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Old 07-31-2007, 08:58 PM
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Intake Vs exhaust port sizes

Maybe you folks can help me understand something. Why are the exhaust ports and valves smaller than the intake ports and valves on cylinder heads? I understand about velocity and cam durations, but I would think that they would be equal in size and volume to expel out what it brought in and that a smaller exhaust port and valve would hurt performance by being restrictive! Is the intake side that large just to supply more than enough air/ fuel charge so the engine can have all it wants at any given point and the exhaust small because that is all it needs to expel the burnt charge?

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Old 07-31-2007, 09:31 PM
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Might be because the exhaust has a bit more pressure driving it out.
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Old 07-31-2007, 09:34 PM
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valves

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.
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Old 07-31-2007, 09:39 PM
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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....
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Old 07-31-2007, 09:52 PM
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Quote:
Originally Posted by camaro freak
because that is all it needs to expel the burnt charge?
That is the answer.
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Old 07-31-2007, 10:33 PM
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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.
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Old 07-31-2007, 10:40 PM
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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.
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Old 07-31-2007, 10:46 PM
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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.
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