Enter your power target, engine type, and duty cycle to find the minimum injector flow rate required. Outputs in both cc/min and lb/hr.
Injector sizing is a function of three variables: how much fuel the engine consumes (BSFC × power), how many injectors share that load, and how long each injector can be open (duty cycle). Under-sizing causes lean conditions at high load - an engine-killing failure mode.
BSFC is how many pounds of fuel the engine burns per horsepower per hour. Naturally aspirated gasoline engines typically fall between 0.45-0.55. Forced induction engines run richer (higher BSFC) at peak load - use 0.55-0.65 for turbocharged builds, 0.65-0.70 for E85.
Duty cycle is the fraction of time an injector is held open. At 80% duty cycle, the injector is open 80% of each engine cycle. Running above 85% causes injector nonlinearity and poor atomization - the engine loses fuel control. Always size for 80% max as a safety margin.
An injector's headline rating is its static flow - the rate when it is held fully open at its rated fuel pressure, usually 3 bar (43.5 PSI). That number assumes 100% duty cycle, which you should never actually run. Dynamic flow is what you really get once you account for the opening and closing portion of each pulse and a realistic maximum duty cycle. This is why you size off static flow but then apply the 80% duty margin built into the formula above.
You can squeeze more out of a given injector by raising base fuel pressure, but the gain follows a square-root law, not a linear one:
So bumping a 3 bar injector to 4 bar multiplies flow by only √(4/3) ≈ 1.155 - about 15% more, not 33%. Raising rail pressure is a fine way to find a little extra headroom, but it is not a replacement for correctly sized injectors, and very high pressures shrink the usable idle pulse width.
Direct injection (DI) engines like the BMW N20, N55, and S55 have the injector spraying directly into the combustion chamber at very high pressure (200+ bar). Port injection injectors operate at 3-6 bar. These are not interchangeable - this calculator covers port injection sizing. DI injectors are rated differently by the OEM and cannot be swapped for larger units without ECU support.
Suppose you are building a 500 WHP gasoline turbo V8 - eight injectors. A moderately boosted gasoline engine runs a BSFC around 0.60 lb/hp·hr, and you want to plan for an 80% maximum duty cycle. Start with total fuel demand at the crank:
Spread that across eight injectors, each capped at 80% duty:
So you need at least 492 cc/min per injector. The nearest sensible common size is 550 cc/min, which leaves roughly 12% headroom - a comfortable margin for hot-soak conditions and the inevitable next power upgrade. If you later switched this same engine to E85, fuel demand would rise by 30 to 40 percent and 492 cc/min would no longer be enough, pushing you toward 750 cc/min or larger.
Flow per injector (lb/hr) = (HP × BSFC) / (number of injectors × max duty cycle). Multiply lb/hr by 10.5 for cc/min. A 400 hp gasoline turbo build (BSFC 0.60) on 6 injectors at 80% duty needs about 50 lb/hr (525 cc/min) per injector.
Size for 80% maximum duty cycle. Above roughly 85%, injectors behave nonlinearly and atomization degrades, costing fuel control exactly when the engine is under the most load.
Static flow is the rate when the injector is held fully open (100% duty) - the headline rating, for example 1000 cc/min. Dynamic flow accounts for the fact that an injector spends part of every pulse opening and closing rather than fully open, and that real engines run below 100% duty. Always size off static flow and apply a duty-cycle margin; never plan to run an injector at its full static rating.
Yes, but only with the square root of the pressure ratio. Flow scales as the square root of (new pressure / rated pressure). A 550 cc/min injector rated at 3 bar flows about 550 × √(4/3) = 635 cc/min at 4 bar - roughly 15 percent more, not 33 percent. Raising rail pressure is a useful top-up, not a substitute for correctly sized injectors, and it costs you some dynamic range at idle.
For gasoline, divide cc/min by about 10.5 to get lb/hr, or multiply lb/hr by 10.5 to get cc/min. So a 550 cc/min injector is roughly 52 lb/hr. The 10.5 factor depends on fuel density, so it is specific to gasoline; ethanol blends are denser and the same volumetric flow carries a slightly different mass.
E85 has roughly two-thirds the energy density of gasoline, so the engine has to burn far more of it - the stoichiometric ratio drops from 14.7:1 to about 9.8:1. In practice you need on the order of 30 to 40 percent more fuel volume for the same power, which is why a build that is fine on gasoline injectors can run out of injector at the same horsepower on E85.
Related Calculators