1 As a general rule, the longer the exhaust and the larger the diameter, the more power it will make at lower revs. On the flip side, smaller bore and shorter systems will promote a gruntier motor. 2 Back pressure being beneficial on a four-stroke system is a fallacy. In reality, you don’t want an exhaust to create any restriction to gas flow, as this will compromise performance. The only caveat is the addition of exhaust butterfly valves, as these are designed to fill-in torque deficits on systems that are tuned for high end power. 3 Unlike race cans, which are essentially perforated tubes wrapped in sound deadening material, road legal equivalents utilise specifically calculated chamber sizes to eliminate specific sound frequencies. 4 Catalytic converters contain precious metals that promote the conversion of toxic pollutants gases into harmless gases, such as switching carbon monoxide into carbon dioxide (and water). 5 While titanium, stainless and mild steel are commonly used in exhaust production, lightweight aluminium isn’t capable of withstanding the high temperatures present in exhaust gases (up to 700ºC). Some very high performance race engines exceed this temperature and require systems made from nickel based alloys.