Final Design:

The dual-stage twin plenum manifold was selected for the team’s final design. In order to tune the manifold for the boost points at 7000 and 5500 RPM, a simple optimization procedure was undertaken. Since it was desirous to have small plenums, V was fixed at 109.1 cubic inches as mentioned earlier as the minimum volume.  Z1 and Z2 were 1 and 2, respectively for this configuration. L2 then remained as the parameter whose value would determine the system natural frequency. Based on the design parameters mentioned earlier, a plot of L2 versus system natural frequency was produced. Note the isocline at a plenum volume of 109.1 cubic inches (0.001788 m^3).  The desired resonant frequency is reached at a secondary pipe length of 6.5 inches (16.5 cm). All dimensions for the system were known at this point.

Optimization graph for system dimensions



The final design

The image above shows the engineering drawing of the final design.  Note that for the desired resonance conditions, only the interior dimensions of the system need be specified. Outer pipe diameters and plenum wall thickness would be influenced by the material chosen for construction. Here, pipe walls are assumed to be 1/16 inch thick, and the plenum walls are 1/8 inches thick. Sections would be joined by welds or glue, and the manifold remains in two pieces until installation. Also shown is the mounting flange, which has been designed to bolt onto the frame where the original airbox was. Thus it will provide support for both the manifold and carburetors.

To install the system, it will first be necessary to cut a hole in each side-cover to admit the primary pipes.  The side-covers are mainly cosmetic in nature and serve only to isolate electrical components from the elements. This function will not be harmed if the holes are sufficiently toleranced. Next, the mounting flange will be bolted to the existing attachment points. Then, the system halves will be inserted from the left and right sides of the motorcycle. The primary pipes are connected to the carburetors by flexible rubber connectors, the type of which was found to be sufficiently effective during the experiment. Ordinary hose clamps secure the rubber connectors.

If the increase in VE due to the new design is high enough, it is conceivable that some adjustment to the carburetors may be necessary to maintain an adequate air/fuel ratio for combustion. An increase in air mass due to the pulse effect may not draw enough fuel through the jets, so measures may need to be taken to richen the mixture. Appendix C contains a simple diagram of the Mikuni side-draft carburetor. If the mixture is found to be too lean at idle, an adjustment to the pilot air screw will probably suffice. If the charge is found to be too lean at high speeds, however, an adjustment to the throttle needle may be necessary. The carburetors are provided with needles which can be raised or lowered along a series of graduations at their point of attachment to the throttle slide. This adjustment will require disassembly of the carburetors, but no new parts will be required. It is foreseeable, however that no adjustments will be necessary, since the boost will likely be of subtle proportions. In addition, small, high-output engines like those on motorcycles (largely unregulated by pollution standards) are generally tuned significantly richer than stoichiometric. Thus, the new mixture may be sufficiently rich without carburetor adjustment.

To ensure the engine remains free of debris, it is necessary to include filters for the intake air. The team desired to incorporate the most efficient air filters on the market to ensure a minimum of flow restriction. Toward this end, K&N Filtercharger Corporation was contacted. Their technical support department provided suggestions on filter selection. All filters are of high-flow type, reinforced cotton-fiber construction and are designed to help straighten airflow to the manifold. Appendix D contains excerpts from the K&N Motorcycle catalogue, in which a filter selection method is suggested. The manufacturer claims that if the simple selection formulas are followed, one can expect a less than 1% reduction in VE over the no-filter case. In addition, the filters are reusable and carry a ten-year/million-mile warranty. Universal styles are available, and the design team selected a round-straight variety (as shown) with a bore of 3 inches (#RU2420). Two filters were necessary, and are listed at $43.72 each.