The intricacies of airflow dynamics through a cylinder head are some of the most difficult concepts to grasp in all of hot rodding. That made it the perfect topic to discuss in the inaugural installment of CHP's new "How it Works" department last month. As it turns out, we underestimated the sheer volume of meaty tech info our expert cylinder head designers not-so-willingly coughed up, so now we're making a call to the bullpen and heading to extra innings.
This month, we'll cover the aspects of head design we didn't get to address the last go-round, such as the effects of valve angles on airflow, combustion chamber theory, the relationship between cfm and power output, CNC machining, and how to scientifically determine an engine's airflow needs. Returning for battle is our razor-sharp cast of experts that includes Tony Mamo of Air Flow Research, Kevin Feeney of Racing Head Service (RHS), Al Noe of Trickflow Specialties, Tony McAfee of Dart Machinery, Darin Morgan of Pro-Filer Performance, Jason Neugent of Brodix, and Bryce Mulvey of Dr. J's Performance. At the risk out tooting our own horns too loudly, it's the next best thing to eavesdropping on a Pro Stock team's R&D department.
Valve angle
Darin Morgan: "There is no substitute for a low valve angle head design just as long as you raise the port high enough to make it work properly. Low valve angles let us increase valve size, decrease chamber burn time, reduce pumping losses, reduce detonation or pre-ignition sensitivity, reduce chamber shrouding, and achieve discharge coefficients higher than any other cylinder head designs. Low valve angle heads in the 7- to 12-degree range with port heights of 1.250 inches or more are the most efficient designs possible today. Pro Stock heads are 9- to 10-degrees for a reason. If you look at the most powerful normally aspirated engines, you will find an overwhelming percentage of low valve angle heads. When comparing the flow numbers of a 22-degree BBC head versus a spread-port head, we see that we need to look at the overall induction system design and package. A low valve angle head with low ports that flows high cfm numbers will not outpace a cylinder head with a low valve angle and high ports flowing the same cfm. With the early LS-series engines, there were underhood packaging considerations that the factory engineers had to consider. The short-side radius on the LS1 and LS6 heads is what we call hypercritical, and is not conducive to high flow above a certain lift point. They are great heads, but GM saw the light when they came out with the LS3 and LS7 cylinder heads. These heads have a low valve angle and high ports, not to mention a really nice chamber designs. These heads are the most efficient heads ever to come from GM, and unlike other OEM heads they have room for the high-performance enthusiast to expand upon."
Tony Mamo: "Flatter valve angles allow the valve to be unshrouded from the chamber walls faster than with steeper valve angles. That's certainly a perk, but it's not the end-all-be-all of cylinder head geometry. It's just another important piece of the puzzle. Also, there is a point of diminishing returns. There's a big improvement going from a 23-degree head design to a 15-degree head, but there isn't such a big difference when going from a 15-degree valve angle to a 12- or 13-degree angle. A better-designed 15-degree head could make more power than a 12-degree head. The real one-two punch is when you raise the entrance of the port and move to a flatter valve angle. That's when things really start happening regarding significant improvements in airflow and power production."