(23-12-15, 12:32 PM)pilninggas link Wrote: When I was at Uni, a mate of mine did some research around this for his dissertation. The aim is keep the walls of the exhaust hot so that when the exhaust gases contact the boundary layer they don't cool rapidly and lose kinetic energy/drag (and that change in pressure due to contraction doesn't adversely affect gasflow). I know he did his research primarily around a single cylinder engine, but measurable gains (on an old school dynamometer) were made by reducing the heat losses in the walls. There may have been other pressure wave effects also, but fluid mechanics is not my strength.....
Sort of my understanding as well. By reducing resistance and keeping the density down it allows the exhaust gasses to move faster. I believe there is a risk of it leaning out the mixture though so sometimes re-jetting is necessary.
That said I know a lot of people do it purely for cosmetic reasons. Not a fan of it myself though