2009 Indy MotoGP: Four-Stroke Future

Goodbye million-dollar two-strokes, hello multi-million-dollar four-strokes?

By Kevin Cameron

July 2009

Next year, the 60-year-old 250cc Grand Prix class will be replaced by "Moto2," which will be prototype chassis powered by a 600cc four-stroke spec engine. Honda has been chosen to supply engines which, we are told, will make about 150 horsepower.

When the FIM World Championships began in 1949, each class stood on its own merits because motorcycles of all sizes sold well. Today, the 125 and 250cc classes are just the farm league supplying new riders to the "real" attraction: MotoGP.

The 1949 250cc title was won by Bruno Ruffo on a 25-hp Guzzi four-stroke Single. Power had doubled by 1964 when Phil Read took the title on a Yamaha RD-56 two-stroke. Power doubled again by the mid-1990s as two-stroke technology matured.

In about 1976, veteran tuner Kel Carruthers would transform Kenny Roberts' new but mysteriously slow Yamaha 250 into a race winner by last-minute porting and exhaust-pipe shortening. This kind of grass-roots innovation ruled through the 1970s.

As racing became more specialized and power harder to get with hacksaw, torch and die-grinder, teams instead vied with one another to be allowed to buy expensive factory "power-up" kits. Teams then got more benefit from politics than from engine development. Most recently, a season on a top Aprilia 250 is said to be $1 million, including bikes, parts and technical support—equal to the cost of 500cc factory lease bikes of the two-stroke era. GP engines, which had once been such a hands-on enterprise, have evolved into a "black box."

If that is so, why not just admit it, standardize it, source it from a single manufacturer and take it out of the equation?

One hundred and fifty horsepower from a 600cc Four requires the engine to peak at 16,500 rpm. If Honda uses the present CBR600RR engine as a basis, with stock bore and stroke of 67.0 x 42.5mm, peak piston acceleration becomes 7500 g. At the current 15,000-rpm redline of the stock CBR, the figure is 6700 g. Peak piston acceleration in Formula One and MotoGP is currently 9000–10,000 g, so the Moto2 engine is conservative by comparison.

The increased revs will require upgraded valvetrain components and perhaps a conversion from chain to gear camshaft drive. The cylinder head and piston shapes would be developed, as well.

Will 150 Moto2 horsepower be more difficult for aspiring riders than the 100-plus hp of the 250s? No, as the smoothness of a four-stroke will replace the "hit" and narrow power of two-strokes. Pundits will say that Moto2 is a better preparation for four-stroke MotoGP than any two-stroke class could be.

Will Moto2 bring fascinating chassis innovation? Maybe, but don't hold your breath. Racing has always preferred a quarter-of-a-percent improvement in conventional design to any revolution.

Will Moto2 cut costs? When the manager of an AMA Daytona SportBike team was asked the price "to put another 600 in the truck," he replied, "$250,000." But that's for a close-to-stock machine. In Moto2, you also have to build or buy a "prototype" racing chassis, as well as engines. In a year or two, that Aprilia 250 may look pretty attractive.