We sealed the copper gaskets...
We sealed the copper gaskets with a thin film of silicone around the deck openings at both sides. The pistons were fly-cut at the intake notches to provide the radial clearance for the 2.250-inch intake valves.
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.
To keep things as equal as...
To keep things as equal as possible, the same rockers would be used on both sets of heads. The large-port Chapman heads required .850-inch offset rockers, so we used these 1.7:1 ratio offset Harland Sharpe rockers on the iron heads, even though standard rockers would be fine.
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.
We couldn't, however, use...
We couldn't, however, use the same intake on both sets of heads due to the difference in port size. For use with the iron heads, we selected a ported Mopar Performance M-1 intake with a Dominator-pattern carb adapter.
Topping the combination was...
Topping the combination was a King Demon carb fitted with the blue venturi sleeves for a flow rating of 1250 cfm. This carb works very well.
Putting a lid on the valvetrain,...
Putting a lid on the valvetrain, we use a set of sheetmetal aluminum valve covers from Granatelli Motorsports. These valve covers have an internal sealing rail, providing increased clearance for the valvetrain, a necessity with the Chapman heads. Some hammer-work was required on the valve covers to clear the MSD distributor.
The substantial pipes for...
The substantial pipes for our brute were provided by tti: 2-2 1/8-inch step headers, with 3 1/2-inch collectors. All tests were run with open exhaust.
With our baseline top-end,...
With our baseline top-end, the 470 belted out some solid numbers, making 642 hp and 611 lb-ft of torque.
Next came the swap to the...
Next came the swap to the MP/Chapman heads. Because of the raised large ports, the stock valley pan no longer works. Dedicated Stage VI intake manifolds are available from Mopar Performance, which seal the lifter valley. We were using a low-deck Max Wedge intake manifold from Indy and a pair of MP adapter plates, which are designed to run with a cut back valley tray. Just snip off the port section of a stock valley pan gasket.
The adapter plates take up...
The adapter plates take up the slack when using a low deck intake with the raised Stage VI ports. They also have a recess to accept the cut back valley pan. It seals with a bead of silicone sealant.
Slight modification of the...
Slight modification of the inner bolt holes of the adapters, meaning they had to be slotted to get the bolts into place, was required.
With the new top-end installed,...
With the new top-end installed, the 470 looked impressive, but would it deliver the goods?
Westech's John Baechtel took...
Westech's John Baechtel took the dyno controls, and we zeroed in on the tuning for our new combination. The Chapman's proved their worth, twisting the big block to the tune of 774 hp and 667 lb-ft of torque. Our iron heads were good, but the CNC Chapman's kicked them in the gut with an increase in output of 134 hp with no other changes. Wow!