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Fuel Pressure

The pressure across the injector sets how fast fuel flows. It rises and falls with the square root of pressure — and if it drops too low, the mixture goes lean and closed-loop bows out.

What it is

An injector's flow depends on the pressure difference pushing fuel through it. From Bernoulli's relation, flow scales with the square root of pressure:

\[\text{flow} \propto \sqrt{\text{pressure}}\]

So doubling pressure doesn't double flow — it multiplies it by about 1.41. The ECU assumes a reference pressure when it computes injector pulse widths; if actual pressure differs, real flow differs.

The system. An electric pump primes for about 2 seconds when you switch the ignition on, then builds to the target: 3.5 bar on the I4 and V12, 4.0 bar on the V8. Pump efficiency depends on battery voltage (nothing at 8 V or below, full at 14 V or above). A pump too small for the demand lets pressure sag under high load — the V8 and V12 have larger pumps (3400 and 4000 cc/min) than the I4 (1500 cc/min).

Why it matters

Real thresholds the ECU acts on:

  • Injector flow scales with √(P ÷ P_target), so a sagging rail makes each pulse deliver less fuel than the ECU expects and the engine runs lean.
  • Below 20 % of target pressure, no fuel is delivered at all.
  • Below 70 % of target, closed-loop trim switches off (badge OPEN (FAULT)) — it won't try to correct a mixture error caused by failing hardware.
  • Injector dead-time compensation grows as pressure falls, compounding the error at idle.

How to tune it

There is no pressure setting to change — the target comes from the engine base and the pump size is part of the build. What you control is demand: a build that asks for far more fuel than its pump can supply will sag.

Watch out

A sudden lean condition under high demand, with injectors not maxed, often means the fuel system can't hold pressure. Watch the FUEL PRESSURE bar before re-tuning fuel tables.

In the game

The FUEL PRESSURE bar in the left readouts shows the rail pressure in your chosen pressure unit. Watch it fill when you switch the ignition on and hold steady under load; a drop there explains a lean readout that the tables don't.