The new D21T MK II Red head has a turbo button and standard button as well as both 7mm and 8mm carb venture. The Red Head has much better bottom end then the old green head. I have two D21B one stock and one RBmods, which are very good, engines as well as I have the STS 28 for my Savage. My D21B keep up with engines that cost twice as much! Right now it has put me in 7th place in my class in the Georgia Championship Series, which I have only been racing seriously for about 4 months. My next engine will be the new T red head!!!
One other thing the T models have a different piston then the B as well as it T is factory modified like musclecar64 said.
Here read more about the new red head and Rick Brakes for RBmods impression of it.
http://www.stsmicroengines.com/