Wilson's team used an advanced computerized CFD (computational fluid dynamics) program to help design the tapered spacer. CFD is a science that is concerned with fluids, either at rest or in motion, and deals with pressures, velocities, and acceleration in the fluid. It is also a study of fluid deformation and compression and expansion. Through CFD, the influence of prototype spacer designs can be studied before they're bolted to a test engine. This is referred to as modeling and can show the actual mixing of the fuel and the air in the plenum, and identify any main and minor eddy currents that may exist. The goal would be to reduce or minimize low-velocity eddy currents around the manifold runners as they meet the plenum.
This is the engineering science of how a spacer influences the intake manifold flow. Historically, an open spacer produces two main eddy current vorticies that occur in the spacer itself and the plenum of the manifold. The upper eddy currents are usually generated at the exits of the carburetor throttle plates where the corners are cut square. But an additional eddy current can form at the bottom of the plenum (floor) and near the middle. This eddy current is rather large, may cover a major portion of the plenum area, and have an undesired influence at lower engine speeds. For this reason, an open spacer usually doesn't produce desirable results at low engine speeds and with frequent throttle-plate modulations.
The Wilson tapered spacer was designed to have a positive influence on the common intake manifolds that are used on either a street/strip or dedicated Pontiac race engine. It features a computer-generated variable radius taper design that's intended to maximize airflow through the carburetor and enhance the charge distribution for more horsepower. Part throttle performance and distribution are improved by redirecting the flow pattern created by the partially open throttle blades and gradually reducing the velocity of the charge entering the plenum to create a smoother transition to each runner. At full throttle, the performance is improved by increasing the airflow. In some applications, the Wilson spacer has improved airflow through the carburetor by as much as 110 cfm, though an increase of this magnitude may not be realized with every installation.
Made from aluminum, which is military specification hard-coated black for durability, the spacer's taper design is intricate and partially a function of its height. For 4150-style bolt patterns, the spacer is offered in three dimensions (1.0, 1.5, and 2.0 inches) and for 4500 Dominator flanges in a 1.00- and 2.00-inch height.
The spacer is able to increase airflow through the carburetor because a venturi effect is created at its top, just below the throttle plates. This causes the carburetor to flow more air through the booster. As the air travels through the spacer, the bore is widened to slow down the charge as it prepares to turn toward the manifold runner. Thus, the Wilson tapered spacer works as a venturi, while it also at the proper time slows the charge for the necessary turn into the cylinder head. This is what makes the design so effective when compared to lesser spacers.
Due to the increased airflow through the carburetor, it's customary to have to increase the jet size after the spacer is installed to create the proper mixture strength. A good rule would be six jet numbers referenced from a Holley jet chart.
With our subject 455, the 1.00-inch spacer performed the best due to the low-rpm nature of the engine. It produced 17 hp and a like increase in torque, not only at peak, but also throughout the test rpm range (reference the dyno chart for exact values). The 2.00-inch design proved to limit the gains to 8 hp and 8 lb-ft of torque. It increased the volume of the plenum more than the engine liked. We could've seen a more substantial gain in power from the 1.00-inch Wilson spacer if we'd had more jets available to increase the fuel-flow capability.