The 621t is suppose to have more torque which makes it perfect for a truggy. It also has the new Cinderella head. My buddy bought one and put it in his MBX6 buggy with a 14t bell and still had plenty of power to clear any jump. We mated it to the Radical pipe with one big and one small spacer, a P4 glow plug and was getting 10:30 minutes a tank.