Modifications
Drivetrain
Reduced final drive ratio
The Drexler LSD was ordered with a new rear axle, creating the ideal opportunity to alter the final drive ratio. I decided to order the LSD with a rear axle from the E46 330i automatic, featuring a final drive ratio of 3.46:1. The new ratio provides a 7 percent shorter gearing compared to the standard 3.23:1 final drive of the Roadster S. The reduction in final drive ratio is a nice performance upgrade, especially for a stock Alpina without supercharger. Top speed was not affected with the stock engine, but obviously the shorter final drive is a limiting factor with the supercharger. The rev limit in sixth gear is set to 6700rpm as opposed to 7200rpm in lower gears, limiting top speed to about 275km/h. With a power output of 420HP at 6700rpm the Roadster S Supercharged should be easily capable of speeds in excess of 300km/h when equipped with the appropriate gearing.
Since I don't do a lot of high speed Autobahn runs, I can be perfectly happy with a top speed of 275km/h. The in-gear acceleration is extremely impressive at any speed and it is incredible how easily you can pull away from other cars even at speeds well above 200km/h.
I would advice against a final drive reduction of more than 7 percent as it would increase engine revolutions too much at cruising speeds.
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