Small Port Size Better?


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I was looking for one thing and found another. I had been trying to decide on the early head with 1.5 ID intake and small valves or to go with the later large valve head on the 200. If the pro's are reduceing the valve size it makes you think. Check the quote below.

Has anyone worked with an outfit that ran your new head on a flow bench? What are your thoughts?

Thanks in advance, Ric.

Hot Rod
Tips & Tricks
Why Small Ports and Valves Work Better

Forget colossal port sizes. They don’t work. When working on cylinder heads, the idea is to gain flow by making the port more efficient. A large port is usually lazy. The flow has no energy. Need proof? At one time it was likely possible to drop tennis balls down the ports of a Pro Stock engine. Today, they’re rather small (so small that the exhaust valves typically measure 1.80-inches in diameter, and will no doubt shrink even more over time). Certainly the port size required for a 500 cubic inch Pro Stock engine is larger than that for something like a 468-inch street machine combination, but the idea is to pack as much velocity into the port as possible. For example, Reher-Morrison has come up with a "smallish" oval port configuration for Big Chief-style 14-degree heads used in any number of Super Category bracket-race big blocks. This port is considerably smaller than what is currently in vogue. But there’s a catch: While the port looks small, it actually flows well in excess of 500 cfm. Those are numbers once reserved for the most elite Pro Stock cars, yet today, the configuration is available to anyone.
8) while port shape is more important than port size, you still need a minimum port volume to feed any internal combustion engine. the biggest thing is the velocity of gas flow, to a certain extent the faster the better. there is a point of diminishing returns re: high velocities, you have to be careful not to go too fast. sometimes higher velocity is achieved by a smoother port shape and sometimes by a larger or smaller volume. remember that pro stock engines turn 9500-10,000 rpm, and that can create excess velocity that is reduced by a lower port volume oddly enough. they also try to make the engine work in a fairly wide(for a drag engine) rpm range so they need a different port volume. you can really only determine what is best for your engine with a flow bench and a dyno. with engines where the port size is small and the gas path tortured as our heads are, and considering the fact that unless we can get smoother transitions from carb outlet to intake valve, large ports are an advantage because it smoothes out the airflow.

I agree. It is more common now to use small ports nad big valves because the small ports have good velocity.

On the small port I6 head I would open up the carb seat because that is where the carb sits and you need the inlet area there.

I would port the valve pockets and get at least a 3 angle valve job. I would also go with a 1.73" intake and 1.55" exhaust valve. The Ford heads are notorious for poor exhaust flow. Another way to help this without extensive porting is a split duration cam. Crane has a split duration for our Inline six's thats pretty nice.