Well, here is a well-engineered example from Deiter Weiß, you may have seen his systems (DW-Technik) on YouTube...

http://www.dw-technik.de/produkte1/09_dw-xf470.php

Above is the quad-tip example. They have a few other tip configurations, but either way, the exhaust is super expensive... $1,200 rings a bell.

There's another cat on YouTube who made his own... and to me, it sounds far too raspy but to each his own.

[media]http://www.youtube.com/watch?v=PuUei-11ZH0[/media]