Itll feel more powerful for the main reason of, the car weights less then the x5.

Another reason is the engine is tuned different then for the X, as for the X it needs more torque down low to get it going faster then the 645 or gearing ratios too.

645 also is RWD only and not awd, may have a difference in how power delivery feels.

Theres some things you can do that will make the motor feel more powerful and responsive. i have the n62 4.8 but 4.4 is same thing with less displacement and power.

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2006 X5 4.8is Fully Loaded