Steve Dinan has been enhancing BMW models since 1979. But don't throw his company into the ring with the dozen or so other tuners who tweak, tinker and piggyback upgrades on the famed German marque. Dinan is a tuner, but it's also an engineering firm that writes its own software, builds its own parts and then backs everything it does with a factory-grade warranty. That sort of fastidiousness comes at a price, but most of its customers – including the powerhouse of BMW Motorsport – rely on Dinan to help them come out on top.

In stock form, BMW's 550i is a formidable four-door with a twin-turbocharged 4.4-liter V8 rated at 400 horsepower and 450 pound-feet of torque. While those figures allow it to run with quick company (0-60 in 5.0 seconds, according to the automaker), Dinan puts the sedan's kettle on full boil with its S3 package. Starting with the engine, the performance engineering firm bolts on larger turbochargers, air-to-water intercoolers, a trick strut tower brace cold air intake, a quad-pipe free flow exhaust and its own engine management software. Pump in some premium fuel, and the result is 542 horsepower and 587 pound-feet of torque. Power is sent through the stock eight-speed automatic to a limited slip rear differential (Dinan will upgrade xDrive all-wheel drive models, too).

The Dinan S3 also features and extensive suspension upgrade that includes new front camber arms and low compliance rear control arms (engineered to reduce understeer and improve turn-in). The stock dampers are retained, but new bump stops are installed along with new springs. Overall, the car rides about a half-inch lower than stock to improve roll rate. To reduce unsprung weight, forged 20-inch HRE Performance wheels are fitted at all four corners (wearing Michelin Pilot Super Sport tires - 275/35ZR20 up front and 295/35ZR20 in the rear). Lastly, the company remaps the factory Electronic Damper Control (EDC) software with its own Dinan Shockware to work in conjunction with the new enhancements.

The base price of the 2013 BMW 550i is $62,625. Add another $38,773 for the full complement of Dinan upgrades on our test vehicle, and the BMW Performance carbon fiber body components. Expensive, but withhold your judgement until after you've stepped on the accelerator pedal.

Driving Notes:
  • Without a doubt, the Dinan engine package is the most seamless aftermarket upgrade I have ever experienced. The impressive increase in power is velvety smooth and natural in its delivery, devoid of the negative characteristics (hiccups, peaks and flat spots) that are common to most other tuned vehicles. The 550i lacks launch control, so the twin-turbo S3 requires a delicate right foot, or the rear tires quickly liquefy. Drive it properly, with a light touch followed by heavy throttle a few feet off the line, and the sedan takes off like a missile - quickly embossing your spine into the leather seatback. The smooth ZF eight-speed shifts with authority, yet without any of the harshness normally found with a dual-clutch. Dinan doesn't publish acceleration figures, but my tuned derrière dyno says it slices upwards of a second off BMW's published 550i time to 60 mph... and it runs unrestricted to over 190 mph.
  • Dinan touts its S3 550i as "a more luxurious version of the M5" with softer shocks and active roll control that give its car the edge when it comes to ride quality. They aren't fibbing, as the enhanced 5 Series provides a very comfortable ride when the EDC is set in Comfort, and sharp response when configured in Sport and Sport Plus. There is less understeer, thanks to the new camber and control arms, and nothing seemed to unsettle the chassis. The stock adaptive suspension worked seamlessly with the physical and software upgrades - the wide range of suspension settings was impressive.
  • If asked what I didn't like about the augmented 5 Series, I'd have to mention the steering. Three years ago, when I reviewed the BMW 550i, I commended the steering's accuracy but complained about its numbness and isolation. That said, Dinan doesn't touch the electric steering, so don't expect it to be improved. Again, this is more of a wish for improvement than a deal breaker.
  • Dinan's superb S3 BMW 550i obviously competes nearly directly with the factory's own flagship M5. In terms of power output, the Dinan feels every bit as burly as the 2013 M5 (keep in mind that BMW boosted performance of its M5 for the 2014 model year with the optional Competition Package), but its personality is softer and bit more tame - less high strung would be another good description. Both are excellent examples of the 5 Series. However, if asked which one I prefer, I'd select the hardened M5 for competitive track events but choose the polished Dinan as my daily driver. Where do you spend more of your time?
Related Gallery2013 Dinan S3 BMW 550i