Big power from small displacement.

Just over four decades ago, Japanese motorcycle manufacturers began experimenting with forced induction. Honda was first to hit the market with a forced-induction production bike, with the 1982 CX500 Turbo (hang on Kawasaki Z1R TC fans, I’m getting to that). Later that same year came the Yamaha XJ650 Turbo. The following year Suzuki introduced the XN85 Turbo, and in 1984, Kawasaki joined the pressurized-intake club with the GPZ750 Turbo. 

While Kawasaki was the last to join the party with a mass-produced turbo model, the company actually offered the aforementioned turbocharged Z1R TC in 1978. However, the TC was not a regular production model, but rather a turbo conversion. New Z1Rs were sent to Turbo Cycle Corporation in the U.S., where turbochargers were installed before the bikes were sold through Kawasaki dealers. The turbo craze was short-lived, and by 1986 the Big Four had abandoned the technology — it was discovered that the promise of big performance was greatly exaggerated. What those bikes suffered from was copious amounts of turbo lag and added weight. 

Turbochargers date to the early part of the 20th century, and they were widely used in aircraft during the Second World War. An easy way (not so easy, really) to get more power from an internal combustion piston engine is to get more air and fuel into the combustion chamber. A turbocharger forces air into the intake tract of an engine, thus pressurizing it and getting more fuel mixture into the combustion chamber. The result is a bigger bang than a naturally aspirated engine of equal displacement. 

Power is never free, however, and there are some drawbacks to using a turbo. The pressurized fuel mixture has a higher chance of detonating inside the combustion chamber, which can be destructive. To counter this, the static compression ratio is lowered. The CX500 Turbo, for example, had a compression ratio of 7.2:1, compared to 10.0:1 for the non-turbo CX500. Without the boost pressure of the turbo, the turbocharged CX500 engine would be sluggish and underwhelming — which it was, at lower revs. This is because it takes some time before the turbocharger spools up and builds intake pressure.

A turbocharger is a simple device: it has a single shaft riding on bearings with an impeller at each end. One impeller is driven by exhaust gases, which in turn spins the other impeller to produce pressure. At low engine speeds, exhaust flow is not high enough to spin the impeller fast enough to produce pressure, and this is what causes turbo lag. This lag wasn’t an issue on WWII aircraft, since once the throttles were applied during takeoff, the engines would maintain steady rpm until the plane landed. 

Motorcycles, on the other hand, are constantly accelerating and decelerating, exacerbating this flaw. Also, when the turbo spins at high revs, it can produce too much boost, which in turn can lead to catastrophic engine failure. To prevent this, a wastegate opens at a preset pressure to bleed off excess exhaust. These drawbacks brought on the demise of forced induction in production bikes. 

Another way to pressurize the intake is via a supercharger. Superchargers use different types of shaft-mounted pumps to pressurize the intake tract (the Kawasaki H2 uses an impeller), but they differ mainly from turbochargers in the way they are driven: by the crankshaft. This direct drive has the benefit of reducing lag (no waiting for spool up), and high boost is controlled via a bypass valve.  The tradeoff is parasitic power loss due to friction and impeller drag. 

The first non-competition two-wheeler to utilize a supercharger was the Peugeot JetForce Compressor scooter in 2004, boasting 20 hp from a 125-cc single. More recently Kawasaki introduced the supercharged H2, primarily to flex its horsepower muscle. However, advances in engine technology, electronics, and emissions standards have made turbos and superchargers commonplace in the automotive industry, mainly because more power can be produced from smaller displacements, resulting in reduced fuel consumption and exhaust emissions. An extreme example of this is the Chevy Silverado full-sized pickup that’s powered by a turbocharged 2.7-litre inline-four that claims 310 horsepower. 

The latest bike-maker to talk of reintroducing turbocharging is Honda. While details are few, the concept shown at EICMA last November features a turbocharged 75-degree V3 of unknown displacement. Honda gets around turbo lag by powering the turbo by electricity. This puts little demand on the engine, while allowing the turbo to spool up via ECU programming. The technology has already been used in F1, and on the road by Audi and Mercedes-AMG. The drawback is that cars use a 48-volt system to power the turbos. The smaller size of the Honda turbo will likely require more conventional 12-volt power, making it a possibility in the near future.