"It's the only car I've been driving since '04," began John Betz, owner of Bahn Brenner Motorsport (BBM). "I beat the crap out of it, yet still it runs every time flawlessly!"
For over three years, John has driven his '96 GTI daily to work in Bend, OR. Under the hood rests a 2.0 8v motor with a base of 115hp and 135 lb/ft. But the engine is far from factory stock. It's equipped with BBM's latest stage 3 supercharger kit, which has been dynoed at 216whp and 223wtq.
With its power-to-weight ratio, the 2600 lb GTI is said to be capable of running down many of today's performance cars, including the BMW 3-Series, Nissan 350Z and Subaru STI.
BBM offers three stages for its supercharger kits, which are as easy to manage as they are effective, as John has demonstrated with his GTI. "These are real world kits that somebody can drive on a daily basis. It incorporates power, mileage and reliability." He stated. "If you look at cost-to-performance, you're going to have performance rivaling $30k cars."
Supercharger Selection
There's no denying the 2.0 8v is weak in stock form. The best way to gain more power is forced induction, with either a turbocharger or supercharger. During BBM's research, they felt a supercharger was better suited because of the immediate torque value.
John revealed some of his experiences with his stage 3 GTI. "At 2000rpm, the torque hits 200wtq. That's when most turbos start to spool and make roughly 150wtq," he explained. "Supercharged 8v are ideal because the engine isn't super high-revving, but is good for torque. So when you combine a charger with the engine, it's a nice match that raises the naturally-aspirated style power curve."
Before developing the product, BBM experimented with several different blowers in search for the ideal combination of off-idle torque and top-end power.
It's generally considered there are three types of superchargers: roots, centrifugal and twin-screw. The centrifugal blower is similar to a turbo, in the sense that as rpm rises, so does the boost. Although all blowers are typicallyrpm-dependent, centrifugal chargers produce max boost at the top of the rev range.
This design is essentially a turbo driven off the crank pulley, so they suit a high-spinning motor with a rev ceiling that can maximize the blower's efficiency. However, the 8v doesn't rev high enough to produce significant boost.
At the other end of the spectrum are roots and twin-screw chargers, often referred to as positive displacement blowers for their ability to increase power from idle to redline. Unlike the centrifugal units, these maintain boost from bottom to top, creating the effect of driving a vehicle with a larger displacement motor.
The internal design of these chargers consists of two counter-rotating rotors which form a helix-shape. As the screws turn with the crankshaft, air is pulled from the intake towards the exhaust. It then is compressed by the rotors and accelerated to create boost and power.
The problem with the roots blower is it creates heat after 10-12psi. Although the charger is making more boost, the heat can negate gains.
With the right set up, both centrifugal and roots units can make considerable power, but BBM opted for the twin-screw. This design has greater internal tolerances, making it more efficient. The clearance between the screws is basically the width of two human hairs, allowing instantaneous boost and less waste, continuing to build boost up to 20psi.