Interestingly, the Stage VI port is cast with plenty of material in the spring seat area, but it is also deeply plunge-cut during machining to make room for a spring with the short stock-length valves. The machining limits the port height in this critical area. A primary revision in the Chapman version was to upgrade to a longer 5.400-inch valve, adding .540 inch to this critical dimension. This frees some precious space for the port. To make use of the longer valve, the spring seat machining was altered, while still allowing room for a 2.00-inch installed height. Since the valves are inclined at 15 degrees, a longer valve moves the centerline of the valve tips inward towards the lifter valley. To accommodate this change, the rocker location is revised to match. Besides making the valve longer, Chapman saw no need to retain the stock 3/8-inch valve stem, preferring to reduce the stem size to 1 1/32 inch, which offered a considerable reduction in valve weight-a critical element in rpm potential.

The changes made to the valve specifications and machining represent the most obvious alterations to the familiar Stage VI, providing more flexibility in the port design. Port development is the hallmark of Chapman's operation, and an intensive effort was undertaken to produce a highly effective port shape to be replicated in the production versions with full CNC porting. Two versions of the Chapman CNC heads were developed, one with an intake port featuring a production-sized opening, and a large-port version with the Max Wedge style port window. Both of these heads would share a common exhaust-port design and 1.81-inch exhaust valves, while on the intake end, the large port version would receive 2.25-inch diameter valves, with the smaller head having 2.200-inch valves. With its larger intake ports, the Max Wedge sized head is much wider at the area adjacent to the pushrods, necessitating the use of .850-inch offset intake rockers for clearance. In tandem with the port development effort, the casting walls and water jackets were subtly revised to provide additional material as required to accommodate the final port configuration.

As might be expected of Chapman, the resulting flow is staggering, particularly considering the port sizes involved. The Chapman intake ports are remarkably efficient, drawing over 350 cfm through the standard intake port window of the smaller sized head (See Flow; Chart 2). This is a head compatible with commonly available, conventionally sized intake manifolds, and utilizing the stock valvetrain layout. Achieving that kind of flow level with a moderate 260cc runner, points out that the port efficiency is exceptional, and an efficient port is one that strikes the most advantageous balance of flow and velocity. The Max Wedge window head is equally impressive, raising the bar on intake flow to over 380 cfm, while adding a modest 25cc in volume. Again, the flow versus port size shows this to be an exceptionally efficient port configuration. Both cylinder head versions share the same exhaust port shape, with peak flow topping 250 cfm, without the aid of a flow pipe at the port exit. We had the opportunity to test the larger version of the Chapman heads on Westech's SuperFlow 600 bench, and found the advertised numbers to be valid, something that is not always the case with manufacture's numbers. On the exhaust side, the addition of a flow tube brought peak flow to just over 300 cfm.