It's no secret that unlocking the power potential of an engine requires unlocking the airflow through the cylinder heads. A set of stock Mopar big-block heads can move somewhere in the neighborhood of 220 cfm of air and produces a nice torquey engine. Common aftermarket replacement-style heads can push flow upward into the high 200s, which can provide sweet street/strip horsepower. Now, let's forget about nice and sweet, and just get plain mean. When Mopar Performance decided to put the gloves on and step into the ring with a killer cylinder head, they also decided not to pull any punches. To handle the development effort, Mopar Performance teamed up with Chapman Racing Heads. Real world-class ability in port-form development is a very specialized expertise, and there are only a handful of companies with the credentials of Chapman. At the professional racing level, Chapman has earned the kind of respect that can only be gained by putting teams in the winner's circle. They've done just that in highly-competitive venues, such as NASCAR, sprint cars, and pro drag racing. These guys back-up the big boys with high-end cylinder head development and flow research.

The Stage Vi
Mopar's Stage VI castings have been around for years, and this was the basic casting Chrysler brought to Chapman for development. The Stage VI was readily compatible with the existing big-block Mopar engine configuration and had some innovative and sound engineering behind it. The most unique aspect of the Stage VI is a raised intake runner, which essentially moves the intake port to the RB height on a low-deck block, allowing the engine's top-end combination to bolt together using a standard RB-width intake manifold on a low-deck block. In tall-deck applications, the wider spread between the raised intake ports is accommodated by either a set of Mopar Performance adapter plates (PN P5249189) or by using a dedicated Stage VI manifold. Ok, the runners are raised, and the engineers at Chrysler figured out a way to make it all bolt together. but why all the fuss? Raising the intake runner reduces the transition, or turn-in, to the cylinder and provides better flow potential. All else being equal, it's an edge in getting the airflow increased, but that isn't the whole story.

Chapman looked at what Chrysler brought to the table in the basic machined casting and saw, that while it had excellent potential, there was room for improvement. First, the Stage VI stubbornly retained Chrysler's original valve length of 4.86-inches. Again, so what? It's stock, right? The stock-length valve leaves a limited space between the top of the valvespring retainer and the seat in the combustion chamber, and there's quite a bit of stuff that needs to fit within that length. Think about it, that 4.86 inches has to accommodate about 2 inches needed for the spring, space for the valve tip, retainer, spring seat, and the roof of the casting. As with raising the runner, a taller short turn in the port floor offers better flow potential. The floor and short turn will need a good 1 inch or more of height from the valve seat to the port floor to get good flow. Now we've used up a big chunk of the space under the valve, and we haven't even considered room for the port. With the stock valve, space is at a premium, and the port height under the valve is crunched down to about 1.6 inches. That isn't much.