Quote Originally Posted by dr. d View Post
why?
exhaust gas is very fluid, and is not easily prone to the effects of gravity.

it does not have to be perfectly vertical.
to effect gravity drainage, a mild slant is fine, in my view.
When hot vaporized water (ie. steam) from the exhaust gas gets into contact with the relatively cooler surfaces of the exhaust system, it changes state (condenses) to water. It then collects on the lowest parts of the exhaust system due to gravity. One such area is the muffler.