This S420 is a very nice 4 valve rev motor, but not a good torque motor. The 500 is a torque motor, the 600 just huge. They do not have the same issue. I find that up to 65-70mph the car drives better in 4th gear. 5th gear only when over 70. I would not do forced induction on a motor with such high compression, nor juice it. From memory the 420 is a 10-11 to 1 comp. Motor. I also drive within the city most of the time where gears would increase my mileage. I also noticed an increase on the highway according the the vac. Gauge mpg meter.