Automobile Magazine compares BMW’s single-turbo twin-scroll N55 engine to the twin-turbo N54. Which engine has less lag? What about power output? Let’s have a look.
“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.
The new engine, the N55, is different from the N54 in that it uses only one larger turbocharger in place of the N54’s two smaller turbos. To keep lag to a minimum, BMW used a twin-scroll turbo and its Valvetronic throttle-less, variable-valve-lift system. On the street, after driving N54 and N55 cars back to back, it’s definitely safe to say that the N55 suffers from even less turbo lag than the N54 did. And that engine set the benchmark.
BMW’s quoted output for the N55 remains 300 hp and 300 lb-ft, which raises a question: what if the N55 actually does only make 300 hp? And, given the simultaneous appearance of a 335is (which uses an even more potent, 320-hp version of the N54) we couldn’t help but wonder: what if the 335is didn’t actually make any more power than the old, underrated 335i?
There’s only one way to find out, right? Call the dyno shop.
To answer the questions quickly: BMW isn’t hiding anything. The 335is put down 293 hp and 343 lb-ft, beating every previous N54 we’ve tested by a big margin. And if you look at the torque curve, the midrange overboost function BMW promised (rated at 369 lb-ft) is definitely alive and well.”