"The first three rules to designing a port are air speed, air speed, and air speed. The forth rule is port shape. When I say port shape, this encompasses the geometric shapes throughout the port as well as port wall angles. The floor height and roof height are very difficult to define, and it changes depending on the engine and its airflow requirements. You basically choose roof and floor heights that will give you a slight converging angle to the short-side radius. The roof and floor will converge anywhere from 4 to 8 degrees on high-port heads. This gives area for port bias and a wider bowl without decreasing velocity too rapidly. As the roof and floor converge, the side wall expands and gets wider at the same rate the roof and floor converge. This is the way we change shape without decreasing velocity. Certain shapes will also let us change shape as the port gets deeper with a smooth decrease in air speed as it approaches the short-side radius. The air speed must decrease to turn the short-side radius but not too much. With high ports, we bias the ports less, but with low ports like the conventional 24-degree BBC heads, you always see a drastic bias in the port over the short-side radius."
Tony Mamo: "The bulk of the reason why a raised-runner head usually shows better results than a non-raised-runner head is that its straighter intake ports make it easier for air to negotiate the short-turn radius. What you might find interesting is our new 235cc SBC head is a non-raised-runner design, and has already made more power on the dyno than a larger-but-older AFR raised-runner head, even in an 8,000-plus rpm race engine. Our new 235cc design has an explosive curve through a moderate cross section compared to the raised-runner design with peak flow numbers of just 10 cfm less. It was a very interesting dyno test and proves that you need to use caution when making blanket statements that raised-runner heads will always make more power. Nonetheless, there is no question that a raised runner improves your ability to get more airflow from a particular head design, and it also straightens out the angle of the intake manifold exits as it directs the air into the cylinder head."
Al Noe: "Raised intake runners make a "line of sight" port more easily obtained, which helps a high-rpm head make power. A head designed to make peak power at 8,000 rpm or less is less sensitive to how high the runner is raised. Everyone assumes higher is better, but if a port with a lower entry has better mid-lift airflow, it will usually make more power. A very interesting comparison of port entry location and power potential is a 363ci drag race engine we built that made over 800 hp. It did this with a standard intake port location 20-degree head, and also with raised-runner 15-degree head. Interestingly, these two heads were less than 5 hp apart in power production, despite the fact that most people would expect a much larger difference. As the valve angle gets flatter, it is usually necessary to raise the runner to keep high-lift port stability in check."