I got this one. Get the 28 in the Rossi, Top, or Rex. Go with the nonpull version, I think roto and pull are B.S. Use a starter box, they are almost foolproof. Nothing worse than being stuck cuz of a dead battery or having to fix a broken string (don't tampons have strings?) or have the one way bearing go south. I hear people say that it frees up some h.p. too, which makes sense cuz there isnt the additinal load of a one way bearing for the crank to contend with. Plus the advantage of a lower center of gravity, the motor is closer to the chassis in the nonpull version. O.K. getting back to your question. Running a .28 vs. a .21, hmmm. Rick from R.B. mods is porting mine as we speak (Novarossi n528xr $412.00 Delivered to my door hotrodded in just 1 week) from what I hear these motors are truly unruly. U.M.T. and Truggy classes afford us the use of bigger motors than the sanctioned buggy classes, up to a .28. If the big dogs are running modded 28's, why would you consider running anything less? Never such a thing as too much power, thats what the trigger finger is for, learn to use the power, much as "Luke" learned to use the "Force." Call Rick from R.B. mods, 1-479-200-0286. Tell him Ekber sent you. May the Force be with you.