[aerohead]

NASCAR teams would like to have the exhaust exit behind the racecar,however,are forbidden to do so by the rulebook.Of course,in a race situation,they're looking for any edge they can get.Drafting would place the following driver in a position to breathe concentrated exhaust,so perhaps its better the tailpipes remain where they are.In a commuting environment,and at the RPMs that engines typically develop,the volume of gases being expelled out the tailpipe,and the exit velocity at which they're being discharged,I believe would render a "thrust" of dubious value.As to the issue of tailpipe size,my opinion is,that for normally-aspirated Otto-cycle gasoline engines ( and no Nitrous oxide either!),that the inside diameter of the tailpipe should be about the same size as that of the throttle-body,for MFI/EFI,or the combined area of carburetor throttles.