The last two weeks I have been running the new 13mm rotor from Novak in my 13.5 powered Gen X. The motor feels very different than when it had the origonal, smaller rotor. It's not as punchy and doesn't have the same high strait-away speed, but it is a lot smoother across the throttle range and holds speed better in the infield. I don't think that the motor itself is faster or more powerful, but because of the difference in the way it acts, it makes me faster overall. I did have to gear up significantly to keep up in the strait. All in all I think it is a good tuning aide, worth investing in. I posted a new track record last night.