Screw it! Yes, you read that correctly, screw it! That's what you do to a car that was built to handle better than it accelerates.
You may recognize the '04 BMW 330i featured here, as it was our former project car that made the cover back in et 6/05. We'd enlisted the expertise of Turner Motorsport to build the ultimate dual-purpose car; a comfortable, high performance street car that doubled as a weekend track car. With two Speed World Challenge touring car championships, and four winners titles in the '06 Grand Am Cup, plus extensive R&D done on track, TMS was able to apply its knowledge with impeccable ease.
The highlights on the car included race-developed H&R RSS coilovers and sway bars, Turner camber plates, adjustable rear trailing arms, solid bushings and reinforced suspension mounts. All were developed from the race program, but the killer component was the 3.64:1 limited-slip diff lifted straight from one of the TMS racecars to our 330i.The final chassis mods included a StopTech Big Brake kit and 19" BBS LMs shod with Michelin PS2 tires.
The engine wasn't ignored either. It boasted Schrick cams, TMS software and underdrive pulleys, Supersprint headers, sports cats and exhaust system.
The result was a car that put down lap times faster than most M3s. The combination of non-existent understeer or body roll with razor-sharp turn-in and smooth power delivery made this car addictive to drive.
An avid reader of the magazine, Captain Justin Miller, jumped on the opportunity to own the car and purchased it sight unseen. However, being stationed in England with the Airforce he had to wait a year before actually enjoying it.
During the course of the year he looked for ways to put more power to the ground. Although we'd put together an exceptional machine, when you're used to flying jets, you become accustomed to reaching triple-digit speeds at a remarkable rate. So in his quest for more, Justin decided to "screw it."
In this instance "screw it" refers to a new option for BMW owners looking to supercharge their motor. Twin-screw superchargers are different from the more common centrifugal superchargers because they offer instant torque, a flat torque curve and greater efficiency.
A centrifugal supercharger is like the compressor side of a turbo, but driven by a belt from the crankshaft. A gearbox is used to step up the speed of the compressor wheel to tens of thousands of rpm, while boost pressure rises nonlinearly with rpm. Max boost is produced towards the top end of the rev range. Since they operate at high-speed, centrifugal compressors can move a lot of air relative to their physical size, which makes them easier to fit in an engine bay.
The twin-screw, however, is a positive displacement supercharger. It fills a fixed volume chamber with air at atmospheric pressure and moves it to the high-pressure side. This is accomplished using two rotors. The Lysholm-type, which is referred to here, uses rotors that have a different number of lobes, each with their own specific shape. Within each compressor, there are male and female rotors that typically have either three and five lobes, or four and six lobes, respectively. The twin-screw will continue to compress air as long as it's turned by the crank. Since twin-screw compressors move a fixed volume of air per revolution, they create constant boost pressure across the rev range.
Because the twin-screw internally compresses the air, it results in up to 80% adiabatic efficiency (low heat transfer) over a broad range. And while centrifugal compressors can achieve similar efficiency, it's typically over a much narrower rev range.
The instant torque delivery, flat torque curve and high efficiency sold Justin on the twin-screw. At the time, a couple of companies were working on applications for the E46, one being Active Autowerke in Miami.
Although Active is the recognized world leader in BMW turbo technology, the company only turned to superchargers a few years ago. It got involved through a collaboration with one of the pioneers of BMW twin-screw applications, Stephen Andersen of Hurtig Motorwerks. Back in '01, Stephen started investing countless R&D hours in his home garage, selecting the correct blower size, developing multiple prototypes and test-fitting the hardware.
Active's partnership involved tuning the system and finalizing the kit. The company was also able to fund the production, distribution, support and warranty elements.
The Hurtig/Active kit is built around the Opcon Autorotor 1.7L twin-screw compressor. It claims to offer the best operating efficiency of any supercharger across the rev range, and the positive displacement even creates boost from idle.
This particular compressor was chosen for its OEM level of reliability and efficiency. Furthermore, to protect from heat soak, it uses an air-liquid chargecooler incorporating Opcon/Laminova cores.
The resulting kit is set for 9psi and produces a flat torque curve from 2500rpm to redline. A good analogy of the twin-screw in a BMW is to think of a 3-Series V8 with the sensitivity and high revs of an inline six.
Kits are currently offered for the E46 325i/330i, as well as all '93-99 E36s, plus the Z3 with the M52/S52 engines.
Three different stages are available for the E36, each one offering more boost and power. Active has made this possible with its proven tuning ability, providing the appropriate software for both OBD1 and OBD2 engine management systems.
Justin's 330i was only the second E46 to have the twin-screw installed. With the combination of our cams and free-flow exhaust, the car put down a tire-blistering 344hp and 285 lb/ft to the wheels on Active's Mustang dyno.
With this newfound power added to an already exceptional car, you can bet this TMS/AA-equipped 330i will serve as the ultimate weekend warrior, but should still remain predictable and streetable for daily use. Are you ready to screw up your BMW?
Active Autowerke(305/233-9300, activeautowerke.com)
Turner Motorsport(800/280-6966, turnermotorsport.com)