A few years ago, we were among the first to get a brand-new BMW 335i on the dynamometer. We don't often dyno cars, but we couldn't resist. Before the first 335i was delivered to a paying customer, the rumor mills were bubbling about the 335i's twin-turbo straight-six being underrated.
It turns out those rumors may have been right. The 2007 BMW 335i's N54 twin-turbo six is rated by its manufacturer at 300 hp and 300 lb-ft of torque. And when we put the first 335i coupe on the rollers, it laid down a very impressive 275 hp and 300 lb-ft of torque. In awful conditions, too: 92 degrees of humid, Fahrenheit heat.
Remember -- the power measured at the 335i's rear wheels should be lower than the engine's rated power. Some of the engine's output is lost to friction in the transmission, driveshaft, differential, axles, wheel bearings, and tires. The dynamometer we used to measure the 335i's output is a DynoJet model, and DynoJets tend to read higher than other dynos, but the output was a good bit higher than we expected.
So a few weeks later, when a second 335i coupe came to our office, I dragged it right to the dyno shop to find out whether that first car's output had been an anomaly. It wasn't. The second car put down an equally impressive 282 hp and 285 lb-ft.
One thing to remember-always, when looking at dyno numbers-is that the peak numbers tell only part of the story. Even though these two motors had slightly different peak outputs, their curves looked largely the same. So the difference between the two is easily merely just normal variation from engine to engine.
That the N54 is a consistent engine in testing was highlighted three years later when I tested a 135i coupe on the dyno. That engine's output was right in that same brawny ballpark: 276 hp and 285 lb-ft of torque.
The 135i used a later version of the N54 that includes a slightly different wastegate design. The only real difference between it and the earlier cars was the turbo's response at low revs (look at the torque curve at the beginning of the test to see the late N54 produce more torque, earlier.)