i have had one bo1 and i'm on my third bo1a. very pleased with grp engines so far! holds a tune very well, easy to tune. i can get 10:50 on a tank running under 260*f with a nice stream of smoke! i run on a small track, but thats the best mileage i have had out of a .21, by far. i consistantly break 10:30 with a "safe" tune. these times are also from a bone stock engine!