Bolt-on drum charger boosts motorcycle power and torque up to 25% - at a fraction of the cost of a turbo

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Alter Ego's Drum Charger: CAD render showing where the charger would fit on a KTM RC390 sportsbike(Credit: Alter Ego Hardware)

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This refreshingly simple forced induction system for motorcycles won't give you the huge power output of a turbo system – but it can give you a boost of up to 0.6 bar (8.7 psi) and some 15-25 percent more power, torque and efficiency at "about a tenth" the price of a turbo setup.

If you want more power, torque and efficiency out of your motor, one of the best ways to get it is to force more air into the combustion chamber, then give it more fuel to make a bigger bang with. There's a few different ways to make this happen. You can go really fast and funnel the ambient air you're riding through into the airbox – that's ram air, and it's a staple of most sportsbike designs.

Then there's the turbine-driven methods, superchargers and turbochargers, which use spinning compressors to squash air down to high pressures. Superchargers power their compressors directly off the engine crank, sapping a little power to return lots more. Turbos power their compressors using small turbines that harness the speed of exhaust gases. Both these methods are very effective, but complex and expensive, and both tend to produce their best results at higher RPMs.

If you look at the car world, turbos are where it's at right now. As manufacturers fight to meet ever-tightening emissions regulations and fuel economy standards, car engines are generally getting smaller, using fewer cylinders and lower displacement, along with turbochargers to keep power and torque figures up in the fun zone.

But in the motorcycle world, things seem to be going the other way. There's only one forced-induction motorcycle on the market, and it caused a huge sensation when it was launched. The ball-tearing supercharged Kawasaki H2 is an incredible machine, as I found out when I left a trail of steaming Pollock-style terror stains across the roads of Southern California last year. But it's nearly twice the price of a "regular" superbike; heck, you could probably buy a new ZX-10R and throw a turbo kit on yourself for less money.

The H2 isn't built to use forced induction in a practical way, it's a hero bike designed to drop jaws, a trump card in bench racing arguments. But Italian ideas factory Alter Ego believes there's room in the market for a simpler, cheaper and more practical design.

The Drum Charger is superficially similar to a turbo; it harvests leftover exhaust gas energy to pressurize the air intake. But instead of using complex, fast-moving turbines and compressors, it does so in a very mechanically simple way.

The drum charger, at its heart, is a plain old disc-shaped membrane – you could think if it a bit like a speaker cone. On one side of this membrane, it's exposed to the exhaust gases as they pass through towards the muffler, via a closed channel that comes off the main pipe. On the other side is the air intake.

When the cylinder fires, a pulse of hot exhaust gas is sent down the pipe, part of which exits out through the muffler, but part of which goes down the drum charger's closed channel and pushes against the membrane. As the membrane gets pushed away from the exhaust, it reduces the volume in the cold intake chamber, increasing the pressure before that air is sent to the airbox through a series of reed valves.

As soon as the exhaust pulse finishes, the membrane returns to its original position thanks to a leaf spring, bouncing the pressure wave back out into the exiting exhaust, but leaving the gas in there - so this channel never heats up beyond 50 degrees celsius. As it's driven by the exhaust pulses, it's totally synchronized to the cylinder's combustion cycle – so, provided the drum is the optimal distance from the exhaust header (some 60-80 cm, depending on the model) it will always develop its pressure charge at exactly the right time.

The result? A boost of around 0.3 bar (4.35 psi) with a single drum charger, or 0.6 (8.7 psi) with a dual-membrane unit. This from a simple plastic drum with a carbon fiber membrane, a spring, some valves and no other moving parts. That doesn't compare with the kind of boost you can get from a turbo or supercharger – heck, the Kawasaki H2's standard blower makes up to 1.41 bar (20.5 psi) – but it's an incredibly cheap and easy way to get the same effect at a lower and more practical level where you don't have to start worrying about what grade fuel you're running.

The key downside is fairly obvious; the drum charger is a big ol' chunk of plastic that needs to sit somewhere on your bike. That membrane can't do much work unless it's a decent size – around 220 mm diameter is enough for small capacity bikes and scoots where each cylinder is less than 250 cc, and larger cylinders up to 400 cc require a 270 mm diameter membrane to get enough pressure generated.

And this is all per-cylinder. Each drum charger works exclusively on a single cylinder, making this technology extremely cumbersome once you move beyond a twin. So you're not going to be seeing it pop up on an inline four superbike any day soon. In fact, an 800cc twin is about as large a motor as Alter Ego thinks the drum charger will be able to work for.

At this year's EICMA expo in Milan, Alter Ego rolled out a KTM RC390 – a single cylinder sportsbike with two single-membrane drum chargers bolted on. "With little or no engine calibration," according to a press release, this bike received an instant 15-odd percent power and torque boost across the entire rev range, with a smoother power curve that the company claims leaves the bike very usable. Here's a dyno chart.

Even though it's a chunky little blighter, at a price "about ten times lower than the cost of a turbo," the drum charger will certainly raise some eyebrows, particularly among small-capacity single cylinder sportsbike owners. The RC390 seems like pretty much the perfect bike to throw one onto as an aftermarket kit.

Such a kit would consist of the drum charger itself, plus a different exhaust pipe, a different airbox and some kind of alteration to the fuel mapping. The company says it would be simple enough for any mechanic to throw on a bike.

But Alter Ego's real intention is to sell the drum charger concept straight to manufacturers who can use it as an OEM component. A bike designed around the idea of drum charging could theoretically get rid of the airbox altogether, freeing up space for one or more chargers mounted in less inconvenient spots than "right where my leg wants to be."

The Rome-based company feels that there's an opportunity for the motorcycle world to go smaller while still generating strong power and torque figures. It's not going to revolutionize the top end of the sportsbike market, but the drum charger represents a clear opportunity to beef up what's happening in the middle and lower end sections of the market.

That's a worthy goal, and with a design this simple it's got a chance to work. We'll keep an eye on how these guys progress in the coming months.

More information: Alter Ego

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