Rectangular port intake vs oval port intake

[autozoned290]

I am trying to get an engineering explanation to the why the rectangular port intakes are not preferred until you have a higher horsepower engine. A quick search doesn't give good answers

 

[gruntguru]

A cylindrical hole works better because of lower surface area/volume (lower skin friction for a give Vavg). Rectangular ports will do if there isn't enough room for round ones of the correct size.

Engineering is the art of creating things you need, from things you can get.

 

[Tmoose]

The intake ports on stock Chevy small block V8 heads have been rectangular starting in the manifold and extending several inches into the head forever. That probably started as a "packaging" convenience, not for flow characteristics.

What really matters for engine performance is the port shape around the valve guide and thru and slightly beyond the valve seat, and that is closer to circular, but not very circular at all.

http://i956.photobucket.com/albums/ae45/mrbo_photo/DSC00400.jpg

Chevy introduced the "Vortec" head for trucks around 1996 they provided a big improvement in low valve lift flow and combustion chamber shape that worked great in trucks, but made them superior to the factory "Corvette" heads for higher performance street use too.

Can you see the difference in the rectangular ports?
truck Vortec

http://www.2quicknovas.com/vortec.jpg

Corvette "Fuelie" heads

 

[tbuelna]

The simple explanation is that rectangular port shapes often allow more efficient use of the space available to fit them into.

 

[8adf85]

I'm assuming the question is referring to Big Block Chevrolet's based on the choice between oval or rectangular ports.

The answer to that example would be the that the size of the runner is suited towards achieving an airspeed that dictates the torque and horsepower peaks, based on the displacement vs. rpm range.
The oval port heads when ported are suitable for the 650-850 hp range, while maintaining better response below the power peaks compared to the rectangular port heads at the same power level.