Ken, Remember if you buy a M104 you are also subject to a headgasket failure. Fortunately it looks as though mine has already been done.

On the Renntech engines, I am not sure how they configure those engines. I know that in the sedans you can opt from a couple of power levels. Passing on the more expensive one forgoes the extra head porting and keeps the torque peak as low as stock (rpm) yet with a substantial power boost.

I have also found the M104 in the G to be rather peaky. I get a substantial "kick" over 3500 rpm, below that it is not much better than the M103.

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1995 G320
1984 280GE
1971 Unimog 416