Looks like the first M135i mods would be a limited slip diff and better rear subframe bushings.
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2021 X3 M40i: M Sport Differential, Adaptive M Suspension.
Previous BMWs: 1971 BMW 2002, 1973 BMW R75/5, and 2011 BMW 335d
Previous Other Vehicles: '67 Cougar, '70 911e, '86 Jetta Diesel, '05 and '12 Foresters, et al.
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