I too am wondering about this... I replaced my 2.8L M104 with a 2.9L 602 Turbo but am still using the original tranny and diff that came with the gasser. It's not yet finished but will be soon... so I don't know if I'll have issues with the gearing. Predictions anyone?

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85 190E 2.3(SOLD)
86 230E (-->300D) sold
87 300D (-->300TD) sold
68 250S w/ a 615 and manual tranny (RIP)
87 300TD (SOLD)
95 S280 "The KRAKEN" (Turbo 2.9 602 transplant) traded
86 190E 2.3... current project