There's a lot of discussion about single-plane intake manifolds versus dual-planes, so we tried one of each, an Edelbrock RPM Air Gap dual-plane (bottom) and an Edelbrock Victor Jr. single-plane.
Dual-plane manifolds (left) essentially divide the engine in two, feeding each bank from one side of the carburetor. Aside from a small 2.65x0.5-inch machined passage in the plenum divider (pen points), each bank of cylinders only has access to one half of the carburetor's flow capacity. The idea is to keep the velocity of the air/fuel mixture that passes through the carburetor and intake runners as high as possible for optimized atomization and cylinder filling at all rpm levels, especially down low. This yields more efficient combustion, which maximizes cylinder pressure, which equates to good torque output. The downside is reached at high engine speeds where the restricted carburetor access can decrease top-end power output by starving the engine. By contrast, notice how the single-plane configuration (right) uses an open plenum that gives all eight cylinders straight-shot access to all four barrels during the intake stroke. Flow velocity may not be ideal at lower rpm so torque may suffer, but once the crank gets spinning, power potential is much greater than with the dual-plane.
We were surprised to see that the dual-plane RPM Air Gap has larger runner entry openings than the Victor Jr. While the widths are very close (Air Gap: 1.162 inches, Vic Jr.: 1.125 inches), the RPM Air Gap's runners are 0.092 inch wider.
With our iron head testing out of the way, we can now get serious and switch to aluminum heads for Tests 3 and 4. As we demonstrated in previous installments, refurbishing the stock iron heads (top) cost us $909 in parts and labor (see Part Two). Why settle for puny 1.94/1.50 valves, big 76cc chambers, and unnecessary mass when for $1,295-a mere $386 more-you can step up to 2.02/1.74 valves, 64cc chambers and superior ports and shed 38 pounds with a set of Super Street 215 cast-aluminum heads from Patriot Performance? It made sense to us. For a comparison, check out Part Three for a detailed overview.