Exhaust tuning is the last task of the year - except for putting everything together, going to the salt, racing, and squeaking past the 140 mph barrier. This build diary addresses basic stuff. This theme is followed for exhaust tuning. "Performance Automotive Engine Math" by John Baechtel is used as a reference.
Two of many things to worry about in exhaust tuning are flow and sonics. A big river is an analogy. We watch if flow by us from the shore. The water always goes in the same direction. Its speed and flow are governed by channel characteristics such as size, gradient, and smoothness. There are waves on the river. Wind waves going one way, boat wake waves another, and waves from a tossed rock following a different path than any. The waves travel quickly in any and all directions regardless of flow direction. This is like the sonic waves in the exhaust system.
First, is the header I am using is the right size? It worked good on the 790 cc engine. Will it be OK for the 865 and 994 cc motors? The procedure is to analyze it using three methods that were developed independently from each other: Baechetl's formula, A. Graham Bell's method, and Meaux Racing's PipeMax program. (Baechtel Chapter 8.)
Baechtel's relationship says that primary pipe diameter is related to the amount of exhaust that must pass through it, and this depends on cylinder volume and rpm. This makes sense. The attached pages show the calcs for the three motors. The pipes I have are 1.40 inches diameter, the 790cc engine needs 1.50 inch pipes, the 865 cc one requires 1.57 inch ones, and the 994 cc needs 1.68 diameter tubes. Clearly, the headers I am using are too small.
How much too small? Looking at diameter and comparing the standard Triumph pipes to the ones needed for an 865 cc engine:
[(1.57 - 1.40) / 1.40] x 100 = 12%, not much smaller. This is misleading. Flow capacity is related to cross-sectional area. Looking at area and comparing the standard pipes to the ones needed for an 865 cc engine: [(1.95 - 1.58) / 1.58] x 100 = 23%. The pipes I am using are 23% too small when looking at area. Sometimes it is a good idea to take a good look at how things are compared to each other. The next post will be about Bell's method.