It is possible MBUSA would still cover the car. Normally places like Renntech and Brabus offer their own warranties once the engine has been opened up. Replacing cams is not that intrusive. I know Renntech offers (or did) 2-3 years while Brabus was one year on converted engines.

Personally I second Kuans opinion, that 20 hp is simply not worth it. If you are doing it on an ML320, take the $5k and upgrade to a ML430. If you have the 430, take the money and buy the ML500, and so on to the ML55. If that is not enough, either one of the above mentioned tuners will bore the V8 to 6 liters.

IMO those tuners offer awesome full engine conversions. The engine mods, short of the full blown deal, are a waste of money. Chances are pretty good that you would never feel the 20hp. Also consider that all you have done is move the power up the rpm band with a cam change. In a heavy vehicle like the ML you need the torque down low. I wouldn't be surprised if you actually lost some performance in daily driving.

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1995 G320
1984 280GE
1971 Unimog 416