You might want to give more info about what you are looking for, where you race, and how big you are. Understand, no kart will make you WIN, you win a race in the shop before you show up in prepping tires and preparing your kart. If you are a small guy you could run a 115cc or a set of dual yamaha's are very fast and versatail. If you are a big guy you should look at a like a MX engine, the 250cc 2 strokes that Tim Taft is building the bottom ends for are very lite and very powerful. The 450's make big torque but are a little heavy and be a little rougher to setup depending on your kart and body weight. If you are a medium size guy you might want to look at a stroked Sudam or a BRC150, but we all need to know more info to steer you in the right direction.