Dinan 550xi S2, the 200-mph M5 that BMW doesn’t build: Motoramic Drives

If you’ve ever owned a BMW, you’ve probably pined for some Dinan upgrades. The company, founded by Steve Dinan in 1979, occupies the sort of space that AMG once did for Mercedes, before it was brought in-house — while Dinan isn’t officially affiliated with BMW, the Germans clearly don’t hate what he’s doing. And what he’s doing is taking fast BMWs and making them faster.

Amidst the chaos of Pebble Beach, I get a chance to head out for a drive with Dinan in his 550xi S2, an all-wheel-drive 5-series with reworked suspension and brakes. And also more power; instead of the stock 550’s 400 hp, the Dinan car makes 508 horses and a staggering 584 lb-ft of torque. The speed limiter is jettisoned, giving the car a claimed top speed of more than 200 mph. How far away are the Bonneville Salt Flats, anyway?

Even with a turbocharged V-8, adding that much power involves more than an ECU reflash. Dinan hardware includes larger intercoolers, a new exhaust and a cleverly reworked intake. Where the stock 550 uses diagonal chassis braces at the front corners of the engine bay, Dinan installs hollow braces that act as a secondary pathway for air to enter the intake. He says the system increases flow by 20 percent.

I’m honestly not going to be able to test the suspension upgrades in the environs of Pebble Beach — 17-Mile Drive is a poor venue for seeking max g-forces — but I do get plenty of chances to dip into the throttle and feel that wave of torque at work. With all-wheel-drive, the 550xi S2 just lays down the power and crushes you back in the seat. It already feels, to me, much like what you’d expect out of an AWD M5. But if you want to get even closer to that ideal, you could opt for the S3. That one includes bigger turbos and makes 542 hp, which would be close to an M5, if BMW didn’t thoroughly sandbag its factory horsepower numbers (an M5 is really more like 600 hp).

While the 550xi is an interesting exercise in “What if?” I’m kind of more interested in just shooting the bull with Steve Dinan as we drive. This guy knows BMWs on a level that’s probably deeper than anyone outside the factory. Really, he is his own secondary factory at this point. “We’re more like a small car company than a tuner,” he says. “We have 70 people, and a lot of them handle the programming.” Indeed, in the car I’m driving, the motor, transmission and suspension are all reprogrammed, and all work together flawlessly. “When I started, it was easy for a few guys to modify cars. But I don’t know that I could start a company like this now.”

I want to know, given his experience, which BMWs he thinks are best. Critics, myself included, tend to view older BMWs like the E39 M5 through rose-colored glasses. I used to have an E36 M3, and I still contend that BMW steering never got any better than that. But Dinan, engineer that he is, can be pretty dispassionate about old cars. “By the time they come out with a new model, I’ve already torn the old one down so many times that I know exactly what they fixed the next time around,” he says. He enjoyed the E39 in its day—“That was a great car for us,” he says, but he’s onto new things. You can’t get too nostalgic when there are 2014 M5s out there, waiting to become 750-hp M5s.

As far as his clientele, the increasingly insane performance of factory BMWs hasn’t hampered Dinan’s influx of customers. A stock M5 is arguably overpowered as it is, but people want more. “The M car customers are our customers too,” Dinan says. “They’re prequalified on two fronts — they want power and they’ve got money to spend.”

Thus Dinan isn’t competing with BMW’s M division; he’s taking it places that it won’t go for years to come. Consider, for instance, an all-wheel-drive M5. BMW will probably build one. And you could wait. But for $16,291, the cost of the S2 package, the future is now.

Dinan 550xi S2

Stage 5 Performance Engine Software with Boost Upgrade


Free Flow Exhaust (Rear Mufflers)


High Performance Air-to-Water Intercooler


Carbon Fiber High Flow Air Intake/Strut brace


Total for S2 Engine Tune


Maximum Horsepower

508 @ 5000 rpm

Maximum Torque

584 lb-ft @ 3500 rpm

Performance Suspension System




Low Compliance Rear Control Arms


Front Camber Arms


Total for S2 Suspension Tune:


S2 Accessories


Dinan S2 Package Total: