Reverse Engineering

Reverse Engineering

Forward thinking for Yamaha’s new YZ450F

AS PART OF AN INTEGRATED ENGINE/ chassis program of mass centralization, Yamaha’s newYZ450F engine reverses traditional layout. Its intake system is in the front, allowing it the steep intake downdraft angle that every sportbike engine uses to boost top-end power. To permit this, the cylinder is tilted slightly to the rear, and the exhaust comes out the back of the head. This change allows the light airbox (which is mostly empty space) to ride high in the chassis, while the heavier fuel tank and fuel slide down to a lower position mostly under the rider’s seat.

On air-cooled engines, exhaust always came out the front to make cooling air hit the hottest part of the head first. But motocross engines have been liquid-cooled for years now, so the exhaust port can be placed where it’s convenient. Indeed, 10 years ago, Cannondale ran a “backward” head on its MX400.

MX engines have always needed fuel systems that can operate in 0 g, so this 450F sends the sputtering carburetor and float bowl to the museum, replacing them with batteryless Keihin fuel injection via a 44mm throttle body and single 12-hole injector. The goal here is controllable power and a solid sense of “throttle connection” regardless of what the bike is doing-taking off, landing or bouncing through whoops.

For compactness, the engine has been shortened vertically and given a larger bore and shorter stroke (now 97.0 x 60.8mm). Japanese engineers find “free” power by cutting friction, so the F’s crank has been offset from the cylinder centerline 12mm to stand the con-rod more upright during the power stroke. This cuts friction by reducing the angle of the conrod to the cylinder axis, so the piston rubs less hard against the cylinder wall. Piston-ring tension has been reduced about 40 percent.

Higher torque is an eternal goal for MX engines, particularly at low and middle rpm. A slight increase in compression, a re-optimized combustion-chamber shape (so important in short-stroke engines) and increased cam lobe-center angle (now 105 degrees, up from 100) conspire to deliver those very goods. All four valves are titanium (36mm intake/30mm exhaust), and valve lifts and open durations are slightly increased. Valve-spring wire has been changed from round section to oval, permitting higher lift at no increase in spring length.

How do you tune a bike with no carburetor? You go to the GYTR accessory list and check off the “Power Tuner”-a hand-held device that lets you to program fuel and ignition maps at nine key points each, then upload those maps to your bike’s ECU. Kevin Cameron