Welcome to the board, you'll find this to be a valuable resource into this mental illness! And while you're searching for a cure, be sure to include dosages from www.busnut.com
(which has a tremendous archive!) and the GMC busnuts group on Yahoo: http://groups.yahoo.com/group/gmc-busnuts
Please, as Arthur suggested above, take a few minutes to edit your profile to include a signature line that includes at least your first name and home/base city & state. Once you actually get a coach, adding it's make/model/powertrain is also useful. As mentioned, it will help us help you a whole lot more with suggestions for parts, service and even some local busnuts you might not be aware of!
What's the VIN on this 4106 you're contemplating? PD4106-XXXX?
The original OEM powertrain for the 4106 from the factory was an 8V71 mated to a 4-spd manual transmission. GM never installed any automatics as options, so if the coach has one, it's a retrofit.
OEM 8V71 bus/coach hp was 275, with about 770 ft/lbs of torque. A stock 6V71 is rated at 238 hp with 600 ft/lbs of torque. So if, in fact, the coach does have a 6V71 in it, then you've lost 37 hp and 170 ft/lbs of torque, pushing 25,000 lbs thru a sloppy slushbox.
OTOH, this might be a 6V92T engine coupled to a V-730 automatic, another popular powertrain swap. If that's the case, it's a whole different ballgame, because the 92 can easily be pumped up to 350 hp with different injectors and different turbo housings. Easiest way to tell is to look on the exposed camshaft pulley on the radiator end of the engine. Embossed in the center of the pulley it will say which engine it is attached to.
Once you go over 375 hp in either of these engines in a coach (8V71 or 6V92), cooling becomes an issue, not to mention fuel consumption!
Keep us posted!
FWIW & HTH. . .