To me, the entire question of whether keeping the exhaust hot is beneficial remains unanswered. > Expanding gases such as those in the initial phase of an exhaust event, contained in a fixed size tube like the exhaust, open at one end, will go faster when they are hotter because they are under more pressure. "Simple physics" (Newtonian) tells us that the pressure of this body of gas will bear equally in all directions; against the sides of the pipe, against the lower speed gases the pulse encounters, AND against the gases still trying to leave the exhaust port. Anything that lowers the pressure of the gas body will lower that resistance. > Nothing so far offered has answered this question: If it is so important to keep the exhaust velocity high, why do modern exhaust systems feature so many increases in section over their length, each of which not only slows the exhaust, but also cools it? (Abrupt drops in pressure cause a corresponding drop in temperature. That's how refrigeration works.) I know why they are there, but I want to know how something that so obviously works against high gas velocity could be a good thing, given the assertion that higher velocity is so important. > There is simply no support for the idea that even fractionally more heat can be drawn off of a single exhaust pulse during the less than 20 milliseconds it spends in the header and midpipe by exposure to an uninsulated exhaust system than results from the loss of pressure that occurs from just letting it out of the cylinder. There is also no hard evidence that I have seen that shows that increases of velocity actually result from insulating the exhaust.