Yamaha's innovative, new YZ450F motocrosser is perhaps a precursor of interesting developments on other models.
In the late 1990s Cannondale, the bicycle company, expanded into motorsports with a 440cc, four-stroke, single-cylinder dirtbike. In 2003 an international group began racing an inline-triple Superbike under the Foggy/Petronas FP1 name. What these two bikes had in common was their cylinder heads were reversed, with intakes at the front and exhausts at the rear.
While not the first time this idea had been tried, these were serious, professional designs. Neither was ultimately successful, but their problems had more to do with money and markets than with their innovative architecture. The concept of the reversed cylinder head languished.
The new Yamaha has taken up that innovation once again. The YZ450F has gone to a cylinder head with four titanium valves, rather than the five of its predecessor, to boost low to midrange power. The steep intake descends from the airbox near the steering head and above the radiators through a 44mm fuel-injection throttle body to the two intake valves. The exhaust exits at the back of the head, the pipe forming a spiral loop (which Yamaha calls Tornado) behind the shock to gain enough length for correct exhaust tuning before connecting to the muffler.
The cylinder is canted slightly rearward of vertical for better mass centralization. The lightweight airbox is forward, the heavier fuel tank directly above the cylinder head.
Cutaway of 2010 Yamaha YZ450F motocrosser shows forward intakes, rear exhausts, high-mount
Another notable innovation puts the crankshaft slightly behind the cylinder's central axis. This puts the connecting rod's centerline coaxial with the cylinder axis and perpendicular to the piston face at the point of greatest combustion pressure. There is less piston side-loading and thus less friction on the power stroke. It will be interesting to see real-world dyno figures on this engine.
Will we see future applications of these technologies for multi-cylinder streetbikes? Wouldn't a reversed cylinder head have the same advantages on, say, an inline-four? There the question becomes tricky. The Petronas triple had pretty much the entire area from its cylinder head back, under the seat and over the tire, taken up by its exhaust system. As on the YZ450F there was the need for extra length in the headers, but with three cylinders the exhaust system got very complex-the assembly was a work of art rendered in titanium. But with limited cooling airflow under the seat, the system ran hot enough that the surrounding panels sometimes caught fire! It was certainly hot for the rider.
Another problem with rear exhaust is fuel tank placement. Current inline-four sportbikes run downdraft intakes, putting the airbox above the forward part of the engine. On V-fours and V-twins, intakes in the V of the engine also put the airbox forward. This leaves the well behind the cylinders and under the front of the seat as space for the fuel to be carried. The roughly 5-gallon requirement for a sportbike is far greater than the sub-2 gallons of a dirtbike, and space is at a premium. If the area behind the cylinders is filled with exhaust plumbing, the fuel needs to be carried higher.
This is not to say there aren't other innovations that could bring this technology to the street. By routing the exhaust to a muffler box below the engine, like on Buells and the KTM RC8, it might be possible to manage exhaust heat and regain space for fuel.
Innovation is a curious thing, in that by making people think in different ways and by exciting and energizing new development paths, it leads to further innovations. It's a creative circle. Yamaha has taken up a technology that was essentially abandoned several years ago. Perhaps this time the idea will get its chance to shine.