Sorry guys....new EPA regulations....it's part of the new system that Re-Burns the exhaust gases, and use UREA mixture that is injected into the motors....James, if your buses don't have those engines yet, they soon will be, although International Harvestor has a different approach, more like the old system....but, that is what it is all about....I'm still running the pre-emmisions motor in mine....thank god, and that is also why CAT quit making truck engines. They refused to use this set up in their motors...it isn't required in bulldozers, excavators, and farm tractors.......yet....lol