We wanted to look at peak numbers, but were just as interested in how these varied header designs affected the entire curve. To summarize the results, we calculated the average outputs over the rpm range tested, from 3,000-6,600 rpm. While the overall averages give an indication of the area under the entire power curve, we also broke down the results to include average torque production above and below 5,000 rpm. Header changes will sometimes take power away at below-peak-torque rpm, but show added power at above-peak-torque rpm. We chose 5,000 rpm because it was roughly the rpm point at which this engine peaked in torque output. In a pure race application, output at peak-torque rpm and above is the most relevant range, since this is basically the operating range the engine will see on the track. In a dual-purpose application, torque output lower down becomes much more of a consideration. Here's what we found.
The Mill:Our Iron Headed Air Hammer
The mule for our test wasn't your ordinary street mill, but rather a stout 440 combo. The short-block features a reground stock-stroke forged-crank, swinging Eagle H-beam rods, and 12.5:1 Arias piston with Total Seal Slant Gap Gapless rings. With a stock 3.75-inch stroke and a .060-inch overbore, the final displacement is 452 ci. The piston domes were massaged to smooth the sharp edges, and the spark plug reliefs were radiused, which helps flame travel with high-domed pistons and the big wedge's distant plug position. The final compression ratio calculated to 12.66:1 with a -.005-inch piston deck clearance, and Milodon's copper .040-inch gasket. Milodon studs were used to secure the crank and heads, along with their deep sump pan, tray, oil pump, 1/2-inch pickup tube, and gear drive. The block's oil feed passages to the oil pump were massaged with a carbide bit.
The heads are production 915 castings, selected for their closed quench chamber configuration. The ports have been seriously reworked with a max-effort (no welding or epoxy) porting job, and fitted with custom Manley 2.25-inch intake valves and 1.81-inch exhaust valves. To minimize shrouding with the large valves, the chamber was plunge cut adjacent to the valve, well beyond the sealing surface of a stock gasket. Milodon copper gaskets were modified to match the chamber, and the bores were notched for a smooth transition from the chamber to the cylinder. Flow was improved to the tune of 25 percent more under the entire flow curve (as opposed to just an increase in peak flow) on both the intakes and exhaust, with peak flow figures up by 34 percent. Working the valves is a full Competition Cams cam-and-valvetrain package, including a custom Comp roller camshaft, with their NC-Series 4149 intake lobe, and High Tech .420-inch exhaust lobe. It works out to 260/258 at .050, and over .650-inch lift after lash with Comp's 1.6:1 aluminum roller rockers.
To handle the induction duties we used an 1150 annular discharge-booster Holley Dominator 4500 Series carburetor, and it performed flawlessly mounted to a ported Weiand Team G intake manifold. Getting over 630 hp from a single four-barrel, all iron 440 without any juice, spray, or squeeze just doesn't happen by accident. What it takes is a working balance of flow, valve action, and compression.