What is a “forward-positioned intake” and “rearward-facing exhaust?”
The cylinders of single-cylinder engines were usually positioned nearly parallel with the ground or with a forward incline. Conventional fuel-injected production bikes also mounted their fuel injectors where the carburetor used to be. However, the 2010 YZ450F abandoned this approach.
It was developed to concentrate the bike’s mechanisms in everything from the engine to the chassis itself to contribute toward the machine’s overall performance in a quest for mass centralization and easy-to-use power. To do this, Yamaha created a new fuel-injected engine format with a straight, forward-positioned intake, a rearward-inclined cylinder and rearward-facing exhaust port.
Because the forward-positioned intake uses a duct at the front of the bike to route fresh air to the engine, the air is not heated by the engine and stays cooler. At the same time, dust kicked up by the bike itself is less likely to enter the duct. This unique layout is now used on all Yamaha 4-stroke YZ and WR models.
A particularly standout feature first seen on the 2014 YZ450F and YZ250F is the exhaust pipe that wraps around the cylinder like a serpent. Of course, an engine will run without an exhaust pipe, but it won’t produce as much usable torque. When the engine is running, the exhaust gas expelled by the engine produces pressure waves inside the pipe. When these waves reach the end of the pipe, they reflect back to the engine and this actually helps not only accelerate the flow of exhaust gas out of the engine, but also helps improve the flow of intake air into the engine, helping the engine deliver its full potential. This unique wraparound exhaust pipe is key to drawing out the engine’s full performance.