CFM and Power
Darin Morgan: "If you use a 235cc port flowing 340 cfm and they hurt power over a set of 210cc ports flowing 300 cfm, then the air velocity was to low in the 235cc heads. It's not always that simple, but that's the general idea. The 210cc port heads may have flowed less, but their air speed matched the engine combination more closely than the higher-flowing cylinder heads. This scenario shows the end result of installing a cylinder head that flows too much air without enough air speed, but what if we reverse the situation? If you install a 220cc head flowing 300 cfm on an engine that needs a 235cc port flowing 340 cfm, you will decrease the engine's rpm range and peak power.
"Air speed is the most important tuning factor when designing a port, intake manifold, or any induction system component. It's not the only one, just the most important. People are infatuated with cfm numbers because that's all they have to judge a cylinder head's worth. It's extremely frustrating for people in the cylinder head business. Blindly sacrificing air speed for airflow is a fool's errand. When a customer demands the most airflow possible they are not always correct in doing so! The problem is when customers fail to ask for the most airflow possible within the limitations of the air velocity and sizing constraints for his particular engine. This is and will continue to be the root of the problem."
"It's not often that a head with lower advertised cfm figures makes more power than a head with higher cfm figures, but it does happen. The reason for this is velocity just about every time. When you sacrifice air speed for airflow, you may be getting yourself into trouble. This of course depends on the engine combination you're dealing with and the airflow requirements of that engine. Today, professional port designers have been able to develop ports that do both. I have ports that flow a lot of air and have an exceptional velocity profile. I always try to get the most air possible, but never sacrifice air speed in order to achieve it. That is unless I can get away with such a trick, which is very rarely."
Kevin Feeney: "The public's fixation with flow numbers is a constant battle in the cylinder head market. At RHS, we stress to our customers that the quality of the flow is more important than the quantity of the flow, and this is mainly based on the balancing act between flow, velocity, and runner cross section. To properly design a port, you need to know what size the engine is, the target rpm band, and the application. At this point the taper in the port is determined to accommodate the criteria outlined. Ultimately, the taper will dictate what the runner volume will be. This is why the port volume on an engine will differ on another engine of the same displacement that's designed to operate in a different rpm band. It's not uncommon to have a properly designed port that does not yield the highest flow numbers on the bench, but makes more power and performs better on the track."
Darin Morgan: "Shaping a port for maximum airflow and velocity is a painstaking process. In addition to using a grinder, I often use tiny pieces of epoxy to add material where needed, and sand it down with sandpaper until my fingers start bleeding. It's a process that's impossible to repeat by hand, and considering all the hours that go into designing a head, you couldn't charge someone enough money to make it worth your time. That's where the CNC machine comes into play. Without a CNC, the port you just spent months designing couldn't be replicated. This repeatability is a big part of the reason why modern cylinder heads offer much better performance per dollar than the heads of just 10 years ago."