QC For Me
One look around Chapman's facility, complete with the latest CNC machining centers, and you get the idea that these guys are serious about quality. The place has more an aura of a research lab than a machining operation. Early on, the MP Stage VI heads had developed a questionable reputation due to outsourced casting and machining. At the casting level, problems with porosity and core shift were reported. At the machining level, tolerance control sometimes required reworking of critical clearances by the end user. Chapman realized that casting consistency is an absolute requirement in having a sound foundation for a quality cylinder head. To accommodate the CNC port forms developed, revisions in the casting cores were required in order to put enough meat where it was needed, and a high degree of accuracy was demanded of the castings themselves. Along with revisions to the tooling, the casting operation was moved and contracted to Zeus foundry. This is the company that pours the metal for Chapman's NASCAR and Sprintcar P-7 and W-9 heads. We had an opportunity to view cutaways of final machined heads at Chapman's shop, and the casting quality was beautiful. The critical machining is handled in-house at Chapman's, and their reputation speaks for itself.

Power Please
Flow numbers, port volumes, dimensions, and the like, make us feel like scientists, but when all is said and done we're not specifications believers-we're horsepower junkies. The bottom line is just how much power is up for grabs with a smokin' set of heads? We thought that was a fair question, and set out to do a little experiment to find out. We obtained a set of the large-port MP/Chapman heads for our little exercise, and had on hand a healthy 470-cube low deck stroker short-block. The bottom end featured flattop pistons to deliver 12.6:1 compression, with a trick Total Seal ring set, and a custom Comp MM-series flat tappet solid camshaft with 260 degrees duration at .050. The bottom-end was custom-built for minimal rotating friction, and featured extensive internal oil control mods, mostly with hand-made parts.

As a basis of comparison, we had an extraordinary set of ported production 915 iron heads to run against. These iron wonders had been fitted with 2.25 and 1.81 valves, with extensively modified closed chambers. The ports were developed after an intensive R&D program, cumulating with over 45 man-hours of porting. Intake port flow topped 300 cfm with exceptional low and mid-lift flow-seldom seen territory for production based heads. For the traditional iron-head Mopar racer, we'll go out on a limb and say these heads are as good as it can get with the old-school ported and modified stock stuff-and we are willing to back it up.

We had the Westech crew bolt our 470 to the SuperFlow pump and arrived with the pair of top-ends for the challenge. First up were our 915's. These production-based heads retained the stock port window, so by necessity, the intake manifold would have to match. Linking the heads to the fuel mixer, we utilized Mopar Performance's low-deck M-1 intake, topped and blended to a Dominator adapter, to get intimate with a King Demon RS carb bearing a set of blue 1,250- cfm venturi sleeves. The combo worked really well. after a few jetting pulls, we found a set of 92 jets, coupled with 20 high-speed air bleeds, had the mixture at its optimal ratio. Output was a muscular 642 hp at 6,100 rpm, with a wrenching 611 lb-ft of torque, twisting the water wheel at 4,200 rpm. This combination was no pushover.