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Experimental Analysis:
A physical experiment was undertaken to determine the optimum primary pipe length corresponding to a VE boost at 7000 RPM. A detailed description of the experimental procedure is found in Appendix A. In short, a section of straight pipe was attached to the carburetor feeding cylinder number four by a flexible rubber coupling and hose clamps.
Straight pipe attached to the carburetor
The fuel system was disconnected from that carburetor and the throttle slide was held in wide-open position to simulate hard acceleration. The engine continued to be driven by the three cylinders still supporting combustion. Using an electronic tachometer in contact with the crankshaft, the engine was held at the test speed of 7000 RPM.
Measuring RPM with a hand held tachometer
A solid-state pressure transducer then took dynamic cylinder pressure readings through the spark plug hole.
The Pressure transducer
Installed transducer
Different lengths of pipe were tested, from 24 inches to 8 inches (roughly the distance from the valve face to the bare carburetor mouth). The resulting plots of peak cylinder pressure, generated using data acquisition equipment, were compared to determine the pipe length corresponding to the highest trapped air pressure (proportional to VE). The image below shows the plots of cylinder pressure versus pipe length at 7000 RPM. It was apparent that a length of 12 inches from the carburetor mouth, or 20 inches from the valve face, was optimum. This represented a significant increase in VE over other pipe lengths, as well as over the bare carburetor. This value of L1 will be used in future calculations.
In addition, pressure readings were taken using pipe elbows of different angles to determine the magnitude of any detrimental effect pipe bends might have on the wave or flow in general. It was found that smooth bend angles of up to 45° had no significant effect on VE. For sharper angles, a slight reduction in performance was shown.
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