From the web:

"Big-block enthusiasts have always looked down their noses at oval-ports, but we’re here to tell you that round is the new square. As a general rule, round (or oval) ports will outflow their rectangular counterpart given equal dimensional area. The reason is that the corners of a rectangle are actually wasted space in terms of flow. The flow rate in the corners is very limited, so the wasted space does little more than increase port volume. This is obviously not a desirable situation, as power production is a product of both airflow and volume. The ideal situation (for most street/strip motors) is to maximize flow while minimizing port volume. There’s an argument to be made for increased port volume on high-rpm (dedicated) race motors, but for most, small port and big flow is the best combination."