Good points. That is why it is important to compare heads within the same family of port volumes. Within a given port volume the higher flow rates will have higher port velocities. This combination must be sized to the desired engine. For example the AFR 225 would be a dog on a 302 CI engine, but the AFR 165's would severely limit power on my 427 CI stroker. AFR has a nice article where they substituted a set of AFR 185's on an otherwise stock 392 Ford Crate motor (
http://www.airflowresearch.com/artic...le04/A4-P1.htm ). The result was a 75 HP gain with torque gained accross the board. One might assume that you would loose low in torque (due to velocity issues) to get high end power, but in this case it was a gain everywhere. AFR has put a lot of time into efficient port design to get high flow for a given port size.