Runner Length

The original plan was to shorten the runners (via shortending the air horns). However, flow numbers tended to indicate that more harm than good may be done without some empirical data correlating runner length, flow bench numbers and real world HP numbers…or some knowledge of the tradeoffs between measured flow and tuned length. Although the belled end of each stock air horn doesn't look like much, the flow test tends to indicate that they provide a significant, and positive, flow affect. For example, a sampleair horn was cut 1" shorter than stock and showed about 12% LESS airflow. The stock air horn flowed about 365 CFM @ 28 in H20 and the 1" shorter air horn flowed about 334 CFM at the same vacuum. The cut air horn (i.e. a straight cut) was considerable 'louder' during the flow test. Beveling the edge of the cut end of the air horn showed about a 2-3% increase in flow. An angled cut with beveled edge showed about a 4-5% increase over a straight cut without a beveled edge.

Shortened Runners

Dynoing the modified without shortened runners yielded miminal gains, relative to all other variables. So, after some more thought and research, the basis for the comments above regarding loss of flow were found to have one key flaw. The level of flow evaluated for a single runner was about 50% that of the entire engine at 7000 rpm. At such a high level of flow, the conclusion was made that the runner was stalling - essentially too much air flow for the capacity of the runner. At 7000 rpm, the whole engine is flowing about 570 CFM. So each runner would be flowing about 71 CFM. Based on this more practical flow level any loss in flow associated with the loss of the belled ends was justified as being negligible. With this in mind, the total runner length was shortened by about 10%. Total runner length is defined herein to be the distance from the back of the intake valve to the inlet of the runner (air horn), including the thickness of the IMRC plate. This 10% reduction was achieved by shortening each air horn by about 2". Some of the air horns are different shape than others. So, the 2" shortened length is an approximation. A volume check showed that the volume of each air horn was reduced by about 21%.

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