The purpose of the drivetrain is to transfer the engines power to the wheels and plant that power to the pavement. But like what 78novaman said it doesn't make the power but you need to set it up right to deliver the power effectively. If you have an engine that builds it's power at 4,000 rpm's you don't want a drivetrain that will put the engine at 8,000rpm's. Vise-versa if you have an engine that maximizes it's power at 8,000 rpm you'll never see that power if your engine can't rev over 4,000 rpm because of your 2.73 gears and jumbo tall tires. Imagine a crotch rocket style bike that could never rev above 2,500 rpm, it would be a dog cause their engines powerband is at high rpm's. Yet Harley Davidsons are geared to run low rpm's because that's where their powerband is. Hope I made sense.