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Pri­mary in­jec­tor

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.

Sec­ondary in­jec­tor

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.

Po­si­tion sen­sors

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

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.

Throt­tle plates

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.

MAP sen­sor

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.

Pres­sure sen­sor

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.

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