- in our testing of the 2021 bmw m550i xdrive, we expressed surprise at a slower-than-expected 4.1 second time to 60 mph. a retest dropped that time to 3.9 seconds, still short of bmw's 3.6-second claim.
- the issue hampering standing-start acceleration lies with a miscommunication between the engine computer and the car's stability control module, which results in a drop in turbocharger boost pressure.
- owners and dealers are being notified of the issue and that the fix will arrive this summer.
update 6/2/2021, 3 p.m.: bmw, in a release, said the problem explained in this article also affects all 2021 540i xdrive models, which will also get the software fix.
bmw will be informing 2021 m550i xdrive buyers and dealers this week that the 523-hp sports sedan isn't as quick as promised. in standing-start acceleration, the '21 m550i can't quite deliver on the claimed 3.6-second zero-to-60-mph time. earlier m550is are apparently not affected. the problem lies in a communication error between the stability control module and the engine computer that results in a drop in turbocharger boost pressure. bmw promises an over-the-air software patch will be coming this summer that will get the two modules to work better together and allow the car to meet bmw's 3.6-second zero-to-60 claim.
this news comes after a recent test of the 2021 bmw m550i xdrive left us puzzled by its slower-than-expected acceleration. while a 4.1-second leap to 60 sounds pretty fleet for a 4480-pound four door, a 5336-pound x5 m50i with an identical twin-turbo v-8 and eight-speed automatic hit 60 in 3.9 seconds. and, of course, there's the matter of bmw's 3.6-second time. we regularly beat, or at least match, bmw's quoted zero-to-60 times. also, the automatic and the m550i's all-wheel-drive system make a good launch nearly idiotproof. thinking that perhaps a lower-than-recommended octane went into the test car's tank, affecting the 556-hp twin-turbo v-8, or that some other issue had hobbled the car, we invited the m550i back for a retest.
in the second go-round, acceleration to 60 mph dropped from 4.1 seconds to 3.9, but clearly, something was amiss. thinking that the m550i we had in michigan might be an outlier, we requested one out of the los angeles press fleet. at that point, bmw informed us it was looking into the matter. a few days later, we learned that we wouldn't be getting an m550i in california because the issue wasn't limited to the michigan test car. we're hoping to try a fixed m550i as soon as the software patch lands.
this isn't the first time our testing has exposed cars that fell short of their promises. when the 1999 ford mustang cobra went on sale, its revised 4.6-liter v-8 promised 320 horsepower but delivered 60 mph in 5.5 seconds, a tenth slower than its 305-hp predecessor. the additional power had us expecting a run closer to 5.0 seconds, and we wrote about it. ford blamed a flawed and more restrictive intake. to fix the cars, the out-of-spec manifolds were replaced and a less restrictive exhaust went in to deliver the promised ponies.
two years later, the 2001 mazda miata's horsepower went from 140 to 155, but c/d testing showed the new car to be slower than the old one. the disappointing acceleration led mazda to confess that the 155-hp number came from a japan-market version that didn't meet u.s. regulations. customers of the early 2001 miatas were offered a buyback for their cars, but if they chose to keep their cars they'd receive free service for the remainder of the three-year/36,000-mile warranty. mazda also threw in a $500 gift card as a further mea culpa.