2009 BMW M3
There's a saying in London that you wait 30 minutes for a bus, then three turn up together. That's what's happened with the E90/E92 supercharger market. For a while, it looked as if we might not get any, and now they're sprouting from everywhere (see Bimmerfest X in this issue).
Before the show, we were given the opportunity to sample a prototype system from Active Autowerke - the forced induction BMW specialists from Miami. And while the supercharger would work equally well on the E92 Coupe, the E90 sedan is more versatile and able to slip under the radar.
Adding Active's supercharger wouldn't alter that perception much. If you keep the engine at low revs, you might not even realize it was there - after all, how many wives explore the upper reaches of an 8400rpm rev limit?
At low revs, the Active M3 is a pussycat. A spitting, snarling pussycat, admittedly. But it idled cleanly and pulled from low revs without complaint. What's more, the blower is almost completely silent. Where the deception might come undone is a) the triple blow-off valves, b) the F1 exhaust and c) if you use the full rev range.
Keeping the revs low does minimize the pisssshing sound from its three blow-off valves, and Active sells a quieter version of its titanium-tipped stainless exhaust system. For our drive it had Active's "race" system that retains the stock headers but an X-pipe with two resonators deletes the four cats. The rear is Active's regular cat-back Signature system that's far more subtle.
All the subterfuge will be in vain if you can't resist opening the throttles with the wife alongside. Even with earplugs to disguise the noise, she's going to feel the unearthly surge of acceleration this 600hp conversion offers
Until now, Active has generally used Rotrex blowers for its applications, but with nothing big enough a prototype GTS-8550 centrifugal supercharger was purpose-built by HKS Japan. It builds boost with engine revs like a turbo, yet acceleration is totally linear, climbing to redline. In fact, Karl Hugh from Active considered increasing the redline to 8600rpm since it would still make power up there. As it stands, power delivery is turbine smooth and of the magnitude that never ceases to amaze, no matter how many times we tried it.
Running 6.5-6.8psi, Karl has seen as much as 650hp at the flywheel, but de-tuned it to around 602hp for us because it needs a larger air intake. Right now he's looking for reliability before putting it on the market in 10/09, although we're sure he'll inevitably creep towards 700hp as he gets more familiar with the blower and electronics.
In the meantime, he's perfecting the software and completing castings for the brackets, pulleys and tensioners, while fabricating plumbing for the 940cfm front-mount intercooler.
He also created the huge plenum chamber to balance air supply to the eight throttle bodies, fed by individual trumpets. Two blow-off valves in the plenum prevent a build-up of damaging back pressure, while a third is located by the supercharger for the same reason. A set of 520cc Bosch injectors provide the necessary fuel.