Many people here will STRONGLY recommend against high speed gears and anything more powerful than a Systema M100. It's possible to go higher than that, but you're putting a much larger amount of stress on the mechbox shell itself, and you're going to have to invest in the electrical system... low resistance switch assembly, Systema high torque or Eagle Force motor, thick (14 guage) wire and a very powerful battery. Even then, you're probably going to toast your motor quite a bit quicker than you would normally.
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