> That would suggest that we should compare turbocharged engines to turbocharged engines, and NA engines to NA engines, whichever cycle the engine is operating on.

I agree completely.

But if one is talking about the X5, you have to go with what's available, so the X5 comparison is a NA gasoline engine vs. a twin-turbo diesel with urea injection.

However, I suspect that when the 2010 X5 model comes out, the base gasoline engine in the X5 may very well be a twin-turbo version. BMW then will raise the base price a bit to cover the increased cost, and the premium differential to get a diesel over the base engine will come down to a more reasonable level, e.g., BMW has this all planned-out when they put the rather high price for the diesel in-place earlier this year.