i've been running a mbx 4rr for two years on a os rg .21 with no problems, but i see that the real fast "race" motors can be touchy or thats what i have been told, i fully understand tuning and reading the manual for the motor, and how the motor works, i just dont want to go over kill with the price. i would think $200 would be good enough to run with, i see the sts, os, and sirios all have a motor in that range that people seem to like.