BMW's turbocharged 335i is rated by the manufacturer at 300 HP and 300 lb-ft of torque at the crankshaft, which seemed a bit low to us considering the 255 HP rating of its naturally-aspirated brethren. Sure, the turbo system was designed to maximize the usable powerband instead of creating moonshot-like peak power, but a mere 17% bump in the power ratings just didn't compute.
Investigating the matter further, Automobile Magazine strapped down a 335i to a Dynojet chassis dyno. Such a device measures power at the rear wheels, and translation into crankshaft numbers requires that one account for drivetrain losses. Taking into account the loss of 15% or thereabouts that we'd expect to see from a Dynojet run (don't get hung up trying to determine a precise "loss" number, because there is much more to it than just an estimation of friction), something in the neighborhood of 255 HP would be expected at the rear wheels from 300 HP at the crank - but that wasn't the case. The 335i instead laid down 276 HP, as well as spinning the rollers with 300 lb-ft of torque as measured at the rear wheels. That's about 37% healthier than the 330i.
So, what's up? Yes, the new SAE J1349 dyno standard is supposed to eliminate this sort of rating tomfoolery, but it's only accurate under one specified set of conditions. Dynometers, such as the Dynojet, attempt to correct the results back to a set of conditions, but they're slightly different than those used for J1349. That likely accounts for only a few percentage points of difference, though. More likely is that the Bimmer's engine management system, rather than simply establishing a limit for boost, is constantly attempting to maximize airflow while controlling detonation. Such a system cannot easily be corrected, and putting such a vehicle on a dyno under optimum conditions (usually with particularly cool ambient air temps) can yield results well in excess of the manufacturer's ratings, and this likely explains why the measured torque peak is disproportionate to the horsepower peak.
All this power - accessible at nearly any engine speed, judging by the broad torque curve - gives the 335i performance that nearly rivals that of the previous-gen M3, with the quarter-mile disappearing in 13.6 seconds and with a trap speed of 104 MPH. That sort of performance puts it in the running as Sleeper of the Year.
[Source: Automobile Magazine]