Originally Posted by drewski554
N51 SULEV has the 3-stage intake manifold, gets better MPG, and produces the same horsepower AT A LOWER COMPRESSION RATIO, meaning it has more potential than the N52 engine.
Interesting take but a bit off. You have more parts to change on the N51 to create power. In addition, a 3 stage intake is more effective on a higher compression engine, i.e. the 2006 330i and the N52 in the X3 that has less restrictive emissions parts. You are really kind of pigeon holed on the N51, especially when you go for smog. SULEV engines don't really get better mpg, it is made for better emissions and gains in mpg are through the non dct transmission which keeps revs down, if you drive in a spirited manner and keep the rpms in the power band, you wont yield higher mpg.