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Injectors

Electronic valves that spray fuel. Their flow rate and duty-cycle ceiling set the engine's fuel limit; dead time sets how precisely they meter at the low end.

What it is

A fuel injector is an on/off valve held open for a measured pulse width. Three numbers describe it:

  • Flow rate (cc/min) — how much fuel it passes when fully open.
  • Maximum duty cycle — the largest fraction of each engine cycle the injector can be held open (typically 0.85).
  • Dead time (minimum pulse width) — the shortest pulse that actually delivers fuel.

Capacity is flow × number of injectors × the maximum duty cycle. When the ECU's fuel demand reaches that capacity, delivered fuel is clamped and the mixture goes lean. The ECU adds dead-time compensation that grows at low battery voltage (referenced to 14 V, up to 2×) and at low fuel pressure (up to 5×).

Why it matters

Injectors bracket fuelling at both ends:

  • At the top end, when demand reaches capacity the injectors are maxed — they physically cannot flow more. The engine then leans out, which acts like a soft rev-limit. See Rev Limiter.
  • At the bottom end, idle needs tiny pulses. If the injectors are much bigger than the engine needs, those pulses approach the dead time, where metering is imprecise — idle mixture gets jumpy.

How to tune it

Size injectors to the power target with headroom, but no more. Bigger isn't automatically better: the larger the injector, the worse its low-pulse resolution at idle.

Rule of thumb

Leave 10–15 % headroom at peak power: demand should stay comfortably below the injector capacity. (The 85 % duty limit is already built into the capacity.)

Watch out

If the engine leans out and stops gaining power near the top of the rev range, the injectors are probably at their ceiling. Check: the throttle box FUEL cell stops rising while λ ACTUAL drifts lean at high RPM and load.

In the game

The game doesn't show injector duty cycle. Watch the FUEL cell in the throttle box (delivered fuel flow, in cc/min) and λ ACTUAL in the left readouts. (The hardware version is selected in Engine Setup → Fuel Injectors.)