Thirty years ago, turbos were everywhere. They dominated in Formula 1 and rally cars, all the best vacuum cleaners and even completely unrelated consumer products bore their name. Honda, Kawasaki and Suzuki all made turbocharged motorcycles. So why did then then, and why don't they anymore?
What is a turbo? It's a centrifugal compressor that forces air into the inlet of an engine – forced induction. The special thing about a turbocharger though, is the way that it is driven. The power to drive a turbocharger comes from a turbine that sits in the exhaust manifold and extracts power from the engine's exhaust gas flow.
The turbine and compressor are attached together to form a single unit. The turbine driver the compressor via a bearing mounted shaft that runs through the center of the unit. They're usually mounted as close as possible to the engine in the exhaust manifold, where the exhaust gases have the most heat and flow energy (enthalpy) available to drive the turbine. With sufficient exhaust flow, the turbine is able to drive the compressor to very high speeds – up to 300,000rpm on smaller turbos. The compressor then forces air through some pipes, and sometimes an inter-cooler, into the engine's inlet.
Despite their small size, turbocharger compressors can flow vast amounts of air, much more than the engine they are feeding. This surplus of air in the inlet increases in pressure and forces its way into the engine giving it a bigger charge to burn, enabling the engine to make more torque and more power. The end result is that a turbocharged engine performs like a normally aspirated engine of larger capacity, but without the extra weight and friction of a larger engine. The old adage, 'There 'aint no substitute for engine size' is not entirely true. Turbochargers do have some unfortunate downsides, especially when fitted to motorcycles. Despite being small, along with their associated pipework they can take up a lot of space, plus they don't work at all well at very low rpm. Without sufficient flow in the exhaust, the turbine can't drive the compressor, whereupon it becomes impending power output. The minimum engine speed at which the turbo is able to deliver effective boost pressure is know as the boost threshold. Many people incorrectly refer to this poor, low rpm performance as turbo lag, but turbo lag is a separate issue. Turbo lag occurs when the throttle is opened quickly at high rpm.
The extreme high speeds required by a turbo to deliver boost pressure takes time to achieve. This 'lag' or 'spool-up time' makes the engine feel unresponsive and sluggish. After a few seconds, exhaust gas flow increases, driving the turbo up to operating speed and enabling it to deliver boost pressure. The result is a surge in torque; fine for full throttle drag motorcycles but not ideal for road or track motorcycles which require predictable, progressive throttle response and power delivery.
Turbo motorcycles from the '80s were pretty poorly executed marketing gimmicks, buoyed by the success of the staggeringly powerful Formula 1 cars of the era. These motorcycles had excessive turbo lag, high boost thresholds and tricky maintenance, without any serious performance enhancement compared to larger capacity machines. Despite this, I'm sure we will see them again, and soon, but not on high performance motorcycles, rather on lower powered, low revving, high-tech, high efficiency petrol or ethanol powered machines.