Three-Timer

T E S T I N G D E P A R T M E N T ~ By Tony Quiroga F I N D I N G F A U L T S

Th i rd T i m e’s th e C h a r m

Your M550i xDrive was missing some ponies. We helped find them.

Our test of the 2021 BMW M550i xDrive earlier this year revealed a slowerthan-expected 4.1-second time to 60 mph. While that beats the time a Ferrari F40 set 30 years ago, these days that’s slower than the 3.9 seconds it took us to get to 60 in an X5 M50i that weighs 856 pounds more than the M550i. They have identical 523-hp powertrains, so that raised some eyebrows. It’s rare, but turbocharged cars are sometimes slower than expected, and the culprit is often low-octane fuel. A retest on a fresh tank of 93 octane after the initial California 91 dropped the time to 3.9 seconds, still short of BMW’s 3.6-second claim (plus, we often outperform manufacturer claims).

When BMW learned of our test results, engineers went to work and discovered that a software glitch was causing a miscommunication between the engine computer and the car’s stability-control module. In a standing-start launch, the stability control would intervene and direct the engine to cut boost, keeping it from making full power and adding tenths to the 60-mph time, a problem for all 2021 M550i and 540i xDrive models.

This summer BMW sent out a statement informing owners that there would be a fix. The update can be done over the air or at a dealer service department. Now, several months later, we’ve tested the fixed version of the M550i and hit 60 mph in 3.5 seconds and the quarter-mile in 11.8 seconds at 119 mph. It appears that all 523 horses are present and able to run. When the 1999 Ford Mustang SVT 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. We expected a run closer to 5.0 seconds. Ford blamed a flawed and more restrictive intake. To find the missing power, the out-of-spec manifolds were replaced and a less restrictive exhaust was bolted in.

In 2001, the Mazda Miata’s horsepower jumped from 140 to 155, but C/D testing revealed the new car wasn’t as quick as 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. Affected customers were offered a buyback, but if they chose to keep their car, 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.