Dinan S2 M5

"So, what's your favorite car?" The question invariably rears its ambiguous head every time someone learns what I do for a living. You'd think after 15 years in the business I'd have an answer, or at least narrowed it down to a few vehicles. The truth is, I don't have one favorite--I have a bunch of them. Yeah, that's a weak response, but it's the truth.

Last week it was Ford's RS200 (it tied with the Lancia Delta Integrale). The week before, it was Audi's A4 Avant. Yesterday it was Editor Brown's Porsche C4S.

Today, however, my favorite car is the Dinan S2 M5. Although more than 2 months have passed since I drove it, its memory has been indelibly etched on my mind. You cannot forget a car so fundamentally good.

The S2 is the product of Steve Dinan's meticulous approach to automotive tuning. Every gear, length of tube, screw or bit of software has been tried, tested and tortured to the point of disintegration. Time-consuming? You bet. Expensive? Sort of. But considering Dinan is the only North American tuner recognized by BMW with fully warranted products, a Dinan BMW is a bargain.

Although a stock M5 is by no means an underachiever, the S2 manages to leave it wanting. Shod with what are perhaps the broadest shoulders of all sport sedans, Dinan coaxed significantly more performance from the M5 with a program largely based upon bolt-on components. The engine has been massaged to improve breathing, or more accurately, give the M5 a pair of god-like lungs. The airbox was augmented with Dinan's cold air intake system, featuring dual intake tubes wrought from carbon fiber. The filters are relocated outside the engine compartment to ensure the motor gets plenty of cold, dense air as well as providing insulation from engine compartment heat. The stock plastic air mass meters have been replaced with larger diameter machined units, increasing flow by some 25%. The eight throttle bodies' internal diameters have been increased for a substantial boost in airflow. The stock velocity stacks have also been replaced by shorter, larger diameter aluminum units that have been tuned to optimize their performance at higher rpm. Lastly, a CNC-machined plenum spacer increases plenum volume by 20%, and the camshaft timing was reprogrammed to take advantage of the intakes' increased capacities.

On the exhaust side lies perhaps the most gorgeous set of headers to ever grace a BMW. The intricate curves and bends leave these stainless-steel, ceramic-coated 4-2-1 anti-reversion headers with an organic appearance, like the guts of some metal beast. Matt Taylor, a 20-year employee with Dinan, artfully designed these units, which replace the restrictive stock exhaust manifolds. The headers alone netted a gain of some 28 hp. The S2 M5 exhales through Dinan's stainless-steel free-flow exhaust, providing a roarty tone and a significant weight savings over the stock parts.

Dinan programmers made significant revisions to the M5 ECU, increasing its parameters to keep up with the engine's newfound capabilities. In addition to timing, VANOS timing and fuel maps, the Dinan ECU also increases the rev limit to 7300 rpm and removes the top-speed governor. And to make certain the S2 M5 continues to perform even under huge loads, the system that monitors the intake air temperatures for the ECU was modified to reduce the effects of heat soaking. The Dinan system provides a more accurate computer signal, ensuring timing is not reduced based upon a higher than actual temperature reading. The summation of Dinan's work brings the stock output of 394- to 470 bhp, with torque increasing from 368- to 419 lb-ft. And all that power is delivered with a curve nearly identical to BMW's, only Dinan's is much higher on the graph.

The S2 M5 incorporates several driveline mods designed to improve efficiency by reducing rotational weight to take full advantage of existing torque. The factory's dual-mass flywheel has been lightened by 7 lb, resulting in a 40% reduction in rotational mass. To ensure the engine remains stable during idle and clutch engagement, the idle speed has been increased to 900 rpm.

The stock 3.15 final drive gearing has been shortened to a 3.45, increasing the torque delivered to the ground by nearly 10%. It's a great method to extract even more response from the hugely capable motor. Whereas a stock M5 launches with neck-snapping bravado, the S2 M5 feels as though you were shot from a cannon. The torque is so powerful, you actually strain to keep your hand on the shifter. And with six forward selections, that's a sizable but hugely rewarding chore. The S2 goes through the gears with frightening brevity, each shift punctuated by the rev limiter. It is amazing a V8 can rev up so fast.

Dinan had just finished development of a new suspension comprised of Dinan's proprietary performance springs, adjustable rear anti-roll bar, front fixed camber plates and JRZ dampers. The JRZ units feature both compression and rebound adjustments and have been specially tuned to Dinan specifications.

"I try to tell people this suspension is markedly firmer than the factory's and yet more comfortable," said Steve Dinan. "They don't get it. They can't understand how a suspension can be both. They've got to experience it for themselves before realizing you don't always need to compromise for greater performance."

I know of JRZ primarily through its involvement in racing. I also know there is a lot of engineering behind the technology and it is not inexpensive. Without question, the S2 M5 was the most remarkable handling mid-size sport sedan this writer has ever driven. The S2 cuts through both tight corners and long, high-speed sweepers with preternatural smoothness. It's hard to believe the car weighs some 2 tons because it behaves with all the dexterity of a much smaller, lighter vehicle. Although the steering was a tad heavy (a result of wider tires), the S2 responds like the world's biggest formula car--turn-in is crisp and immediate and, depending upon how brave you are, the BMW will wag its tail with throttle-induced oversteer. Getting to that point, however, is not easy, as the car's overall grip is formidable. Although Dinan cars are expected to behave in such a manner, the S2's street civility is surprising. It still feels every bit the luxury cruiser BMW designed it to be. Just poking around town revealed a perfectly compliant ride, neither too stiff nor too soft.

The S2 M5 rides on Dinan's lightweight one-piece forged wheels: 9x18 in front and 9.5x18 in the rear. Weighing a paltry 17 lb (a bit more out back), these wheels are quite possibly the lightest, strongest road wheels available. Michelin Pilot Sports measure 275/35ZR18 and 285/35ZR18, respectively, a 30/10mm increase in the stock width.

Yesterday my favorite car was Dinan's S2-M5. Today, it's still the S2. I have a feeling it will be the same tomorrow.

You don't forget a car this good.