Next to the turbos, I think I can say without hesitation that the best mod I've ever done to a car is to put the true Sportline steering box (124 460 71 01) on my 300CE. The car handles so much more like the E500 that it's almost like a different car. I don't know exactly how many turns shorter the ratio is, but man it turns like lightning.

I highly recommend you find one if you have a W124.

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I never go far without a little Big Star

1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k