eBay Motors reviews the Dinan Signature Series S3 BMW 550i. The respectable tuner takes a standard BMW 550i and turns into a vehicle that could compete against the M5. The  S3 upgrade package ups the power to 542 horsepower and 587 lb.-ft. torque, and delivering a top speed of 208 mph. In stock configuration, the twin-turbo V8 makes 400 horsepower, peaking from 5500 to 6400 rpm and 450 lb.-ft. from 1750 to 4500 rpm.

The factory twin-turbochargers were also replaced with larger units that create additional boost at high rpm and reduce intake heat and carbon fiber air intake ducts provide maximum airflow to a pair of high performance air-to-water intercoolers.

The company also spent significant time developing the Stage 6 Performance Engine Software that increases the power while maintaining the car’s reliability.

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Here is an excerpt from their review:

Handling the extra power requires taking the wheels, brakes and suspension to the next level. Our 550i test vehicle was equipped with the optional S3 550i Suspension System that includes sport-tuned springs and shocks. Larger 20-inch Dinan/HRE wheels replaced the stock 19-inch alloys. Six-piston Brembo brakes built to Dinan specifications were also installed on the S3 setup to enhance stopping power.

The Dinan S3 550i Suspension System improves upon the factory setup with a set of springs that have slightly stiffer spring rates and custom bump stops that allow front shocks to retain stock travel height. Ride height is lowered ½ inch all around, body roll is decreased, and center of gravity is lowered. Even with its lowered stance, we like how the original ride quality is uncompromised.

The F10 550i boasts a multilink setup with double track control arms and BMW’s Integral-V rear axle with a swing arm and three track arms, all made from aluminum. Looking for ways to improve upon the factory setup, Dinan replaced the front camber arms with Negative Camber Control Arms that have an immediate effect of reducing understeer and improving turn-in response. The rear controls arms that Dinan uses have precision ball joints and machined aluminum housings. Thus, rear suspension stability improves and responsiveness is a tick quicker.

We drove the Dinan 550i over a lot rough roads, in and out of corners, flying down long straightaways, and all day long the car’s composure was impeccable throughout.

How fast is the Dinan S3 550i? Well it feels incredibly fast, that’s for sure. On NorCal roads, the Dinan 550i easily roasts its tires in first and second and third and sometimes fourth. We can say that once you hit moderate speeds, this 5 Series is delightfully neutral, and will happily exit any corner tail-first should you ask it to. The long wheelbase makes drifts slow and controlled, and the brilliant steering reduces your stress factor to zero. Altogether, Dinan has kept the 5 Series’ underlying cross-continental comfort and amplified the car’s potent German engineering.

Full review at eBay Motors