And now to the N55: well, it doesn't, in fact, make as much power as the others N54s we've tested, but it does make slightly more torque. Peak numbers were 266 hp and 302 lb-ft. Before you start typing your "N55 sucks!" forum posts, let's remember that while the N55 put down 10 less horses than two of the N54s, a third N54 produced 10 more horsepower than the others. Meaning: 10 hp is within the noise when you're at this power level -- it's a variation of less than 4%. And you can easily see 4% variance between runs on the same engine.
(Case in point, while the N54 is a supremely consistent engine on the dyno -- especially for a turbocharged engine -- the variation between the best run and the weakest run on the three N54s we tested ranged from 1.0 to 2.6 percent. That's between runs done on the same dyno, on the same day, with the same exact car, in the same conditions. These engines were tested on four different cars in two different states in temperatures ranging by 30'F over four years and three of them were within 4%. Okay?)
[Obviously, the N54 "is" engine is vastly more powerful in the midrange and slightly more so up top.]
In fact, the only real difference between the regular N54s and their successor, the N55, is that the N55's output drops off more as it approaches its maximum speed. This isn't a surprise-the N55 has only one turbo with which to force-feed the engine with air. To keep turbo lag at a minimum, BMW likely used the smallest turbocharger possible that could achieve the (modest) boost levels that the N55 runs on. A slightly larger turbo might have increased high-rpm boost (and thus, power) slightly, but it'd come at the expense of greater lag. And besides, we're talking about an engine that over-delivered to begin with.
And remember, only a fool looks at peak numbers. It's the shape of the curve -- and the area under it -- that really determines how fast a car is and how fast it feels in everyday driving. Looking at the graphs, it's immediately clear that there's not much of a difference between the regular N54s and the N55.
The end result? We can finally and directly compare an early N54, late N54, high-output N54, and an N55. The N55 exhibits less turbo lag in normal street driving, and even though its peak output was near the bottom of the group of engines we've tested, it's still well within its rated peak horsepower output -- and well above its rated peak torque output. Let the N54 vs N55 war continue!
Previous N54 Dyno Features:
2007 BMW 335i Dynamometer Run
2007 BMW 335i Dynamometer Revisited: We Test Another One!