Just chiming in, I did my original MrX3 conversion about 2 years ago, and I've been through 4 different setups, the original had the battery (4S 3700 LiPo) mounted on top of the upper plate, used a Nemesis 6XL motor, and quark 125 speedo. I kept the mechanical brake, and 2 speed was easy to make work. All I did was take a 48 pitch 5mm pinion that fit snugly inside the bearing hole in the clutchbell, and JB welded it in there, and it worked fine. The hardest part was shaving the brake disc down to get the motor to fit and the pinions to mesh w/ the spurs. The speedo went in between the chassis and upper plate (not very good it always got too hot after 3-1/2 minutes and thermaled) Allthough my initial setup was fast, 78 mph on GPS and it got there in 2.2 seconds, it just wasn't right. It took me almost a year to get it where it is now, I have removed the 2 speed (the newer brushless motors have more than enough torque and the power you think you might be saving is lost with all the weight of the 2 speed setup), switched to a NEU 1515 motor, a Castle 1/8 Monster speedo, removed the mechanical brake, and have had a 1/2 dozen LiPo packs custom made (4S2P 30C cells 4400mah total) to drop into the center of the chassis and upper plate that I have made into a tub type design. I have lowered the CG and have gotten the car to be only 2-1/2 ounces over the legal limit of a 1/8 nitro car. I am using readily available 32pitch plastic spurs and aluminum pinions, they work flawlessly and I have an unlimited amount of gear ratios. In preliminary testing I ran for 6 minutes and used 2200 mah of battery power, this was not on a race track and the outside temps were in the 50's, but it seems like it is gonna be a good setup. The car was only 69 mph, but the acceleration is instantaneous, I can gear up even more to get more speed or increase timing or even go to a little hotter wind motor. I will post some pics this weekend of my setup, I'm starting a new project this weekend with a MRX4X and have a bunch of new ideas that should make the car able to compete with the top nitro cars out there.