In the auto industry, single-cylinder four-stroke engines are mainly found in laboratories, where they dutifully serve development and combustion-research engineers. In the motorcycle industry, “thumpers” are the heart and soul of motocross and enduro bikes, and their engine technology changes virtually every model year. The top design priorities for both types of competition machine are effective power delivery and low overall mass, as exemplified by Yamaha’s 2010 YZ450F.
Astute AEI readers will notice from the accompanying image that the liquid-cooled 450-cm3 engine’s cylinder is canted a few degrees backward from vertical in the bike’s frame; even keener-eyed readers will note that the DOHC cylinder head is “backward” from traditional motorcycle practice. Its exhaust port faces rearward and its intake system is in front.
This switch from convention (also used most recently by PA-based Cannondale on its ill-fated 400-cm3 off-road bikes) was driven by the Yamaha design team’s strategy to centralize the motorcycle’s mass, thus providing riders with a better handling, straighter-tracking machine. (Anyone who has watched Supercross racers fly through the air and across extreme terrain at high speed can imagine why a sweet-handling bike is critical.)
The rearward-canted cylinder and reversed four-valve head optimize space for a long, straight intake tract, a 44-mm Keihin throttle body, and single 12-hole fuel injector. On the back side, the Yamaha’s long exhaust header is bent almost in a circle, also to concentrate mass and provide some critical exhaust-wave tuning.
The engine configuration also permits packaging of a large airbox behind the steering head, ahead of the plastic fuel tank.
The YZ’s development team also minimized engine height with ultra-oversquare (97 x 60.8-mm) bore and stroke dimensions and oval-section valve springs which, combined with a 12.5:1 compression ratio, also provide numerous benefits in the engine’s ability to rev, breath freely, and produce useful power and torque.
A focus on reduced reciprocating mass is evident throughout the engine, particularly in the hollow camshafts and titanium valves. With its lightweight and stiff chassis, the Yamaha weighs 247 lb (112 kg) ready to race.