"In high-rpm race applications, we aren't so concerned with velocity as much as we are with having a lot of airflow to feed a very hungry engine with a big appetite at very high engine speeds. This assumes that you have the right cam profile and valvetrain hardware to match, but the bottom line is that a head that can provide similar airflow through a more efficient design, potentially with a smaller port, would still make more average power and torque and propel the car faster down the racetrack, most notably improving 60-foot times. However, even in my high-rpm, take-no-prisoners race example just mentioned, the basic elements of total available airflow to be processed is still a balancing act that needs to be properly managed. The highest flowing head may not perform the best if it only flows 10 more peak cfm of air, but has worse low- and mid-lift flow and is 30ccs larger than a different design, which is smaller and more efficient with a stronger curve through the low- and mid-lift range."
Al Noe: "One very important characteristic of airflow is not only peak cfm, or relative velocity, but where in the lift curve those events happen. Are you looking at flow numbers at 0.100- to 0.200-, 0.300- to 0.500-, or 0.600-inch lift and beyond? Are you building a street car, or an 8,000-rpm race engine? The head has to be tailored to what it's being used on, and if this is done properly, the higher-flowing head will almost always make more power. However, we need to clarify what we mean by higher flowing. At Trick Flow, our goal is to maximize the airflow from 0.300- to 0.500-inch lift, and for typical street cars we're not concerned with flow beyond 0.700-inch lift. That's because the majority of hot rodders use cams that peak in the 0.500- to 0.650-inch range. We also develop airflow with a different flow bench from most, and we feel this makes a difference as well.
"The next thing you must consider is the valve diameter. A larger valve diameter will almost always produce higher 0.100- to 0.200-inch flow figures, which can be counterproductive to making power. Larger valves also tend to be more shrouded and have trouble with the all-important mid-lift airflow from 0.300- to 0.500-inch, but then these larger valves tend to shine at the highest lift points simply due to shear volume. For race engines we use larger valves, but are able to reduce the 0.100- to 0.200-inch flow even further with steeper seat angles. We then try to maximize airflow from 0.400- to 0.600-inch. As rpm increase to 10,000 the intake and exhaust port shapes become very critical for power production. Lower-rpm engines don't seem to be as sensitive to the port shape as higher-rpm engines. Our customers are generally looking to make peak power by 8,000 rpm or less, so the importance of flow numbers are still very relevant in their search for power.
"The velocity around the circumference of the valve is actually more important than the velocity in the port. We feel having equal localized velocities around the circumference of the valve is far more important than having equal velocities in the port, and as these velocities are equalized at the important lift points, the port will flow more air at these same lift points as well. We can measure the velocity of the air around the circumference of the valve every 45 degrees, and having these velocities equalized during the all-important mid-lift flow area yields max airflow and best power.