11 THINGS YOU NEED TO KNOW
Mounted close to the engine, it is responsible for delivering fuel most of the time. They’re normally angled to aim fuel at the back of the inlet valve.
These are mounted further from the inlet valve (generally in the top of the airbox), and are angled so fuel is injected parallel to the air flow. This helps to better mix the air and fuel, improving performance at high rpm.
Velocity stack (variable height)
The length and taper angle of velocity stacks is important. The shape of the intake lip gets air into the tube more efficiently than a straight tube with a square end. Longer stacks enhance torque at low intake velocities (lower rpm) by increasing the volume of air in the inlet tract. Shorter velocity stacks work better at higher rpm. Variable height stacks give the best of both worlds by allowing the ECU to switch between the two.
Throttle body diameter
Too small, and the engine won’t be able to breathe freely at high-rpm (like trying to breathe fast through a straw), restricting power. Too big and the air velocity through the throttle body will be too low (like trying to whistle with your mouth wide open). Air velocity carries the atomised fuel droplets into the cylinder and gives the air momentum, which helps cram more into the cylinder.
These measure throttle plate angle, allowing the ECU to work out how much fuel to inject. On ride-by-wire systems, the ECU uses this sensor to check the throttle plates are giving you what you asked for.
Stepper motors are used to mechanically open the throttle plates in ride-by-wire and secondary throttle systems. The most important factor is their response speed (how quickly the system reacts to what you’ve asked for), and its ability to maintain a constant position.
The primary throttle plate restricts airflow into the engine, governing the torque the engine produces. Even on a closed throttle, the throttle plate is just cracked open, allowing enough air past to let the engine idle. On ride-by-wire bikes, there is no mechanical connection between the throttle grip and the throttle plate. Instead, it is controlled by the ECU, according to rpm, gear and throttle grip angle.
Secondary throttle plates
Before ride-by-wire, secondary plates were used to restrict rider input. A second set of throttle plates were mounted upstream of the primaries. Secondary throttles were only ever used to open the throttle under engine braking conditions.
A manifold air pressure (MAP) sensor monitors the vacuum in the inlet tract behind the primary throttle plates. The engine can use this to calculate the amount of fuel required.
Some bikes appear to have two MAP sensors. They don’t. Instead, one sensor monitors only the pressure in cylinder one – removing the need for a cam position sensor as the engine can work out when cylinder one is on the ‘suck’ stroke.
Air bypass valves
An electro-mechanical valve that allows air to bypass the primary throttle plates. They can be used to control the engine idle speed and engine braking force.