What is "nitro" and what percentage of "nitro" should I run in my engine?

Nitromethane is the ingredient in the fuel that ignites inside the engine. A higher "nitro" content generally equates to better performance. Engines are designed to run best within a range of nitro percentages. How the engine is ported, the size of the combustion chamber and other factors determine how much nitro can be efficiently used in the engine. The TRX 2.5R / TRX 3.3 Racing Engines respond exceptionally well to 20% nitro, returning cooler temps, more power, and a smoother throttle response. For those who want to run higher nitro, 33% is the optimum nitro percentage for Traxxas engines. Increasing the nitro beyond 33% can introduce the need for engine modifications (ports, head shimming, etc.) to avoid starting and tuning difficulties. There are limits to how much nitro an engine can effectively use to make more power. The Pro.15 responds well to 20% nitro Top Fuel. 33% nitro is perfectly safe for the Pro.15 but extra care should be taken to richen the fuel mixture, monitor the tuning, and watch for overheating in hot climates.