The rear torque-driver spring also has a very specialized function which is to maintain enough tension on the belt to keep it from slipping. This also is a bit of balancing act because too much belt tension translates to inefficiency. It’s kind of like over-tightening the chain on your bicycle- power is wasted by over-stretching the belt. On the other hand, too little tension and the added power of your recent engine modification will vanish as heat generated by belt slipping and won't find its way to the rear tires. The trick is to find the just the right tension without overdoing it. A secondary influence of the torque driver spring is that it has a slight impact on shift speed since the belt tension needs to be reacted against by the front sheaves. The higher belt tension tends to push the belt deeper into the front sheaves making the ratio higher, and revving the motor higher. Therefore, for a given desired rpm you would need to compensate with a higher roller weight to bring the rpms back down. After a torque driver spring change, it’s good practice to re-optimize the roller weights.