Sizing fuel injectors

The question is always “will these injectors feed it”, and it comes apart into three pieces that get confused with each other.

How much fuel the engine needs

Power comes from burning fuel, so the fuel flow follows from the power and how efficiently the engine turns it into work:

fuel (lb/hr) = power (hp) × BSFC

BSFC — brake specific fuel consumption — is pounds of fuel per horsepower per hour: about 0.48 naturally aspirated on petrol and 0.58 on boost, because a boosted engine is run richer and later to keep it alive.

Divide by the number of injectors, then divide again by the duty cycle you are prepared to run, and that is the injector you need to buy.

Why the fuel changes everything

E85 does not have petrol’s energy. Making the same power means burning more of it, and the amount more is the ratio of their energy densities:

BSFC(fuel) = BSFC(petrol) × LHV(petrol) ÷ LHV(fuel)

Petrol carries about 43.4 MJ/kg and E85 about 29.2, so E85 burns roughly 49% more fuel by mass — a boosted BSFC near 0.86 rather than 0.58. It is also denser, so by volume, which is what an injector and a pump actually move, you need about 40% more.

That is the whole reason a fuel system that was comfortable becomes marginal on a fuel change with nothing else touched. Methanol is worse again: it more than doubles the fuel flow.

The commonly quoted E85 BSFC of 0.75–0.78 is low enough to under-size an injector by a whole size. The energy argument is the one to trust.

What an injector actually flows

The number on the box is the flow at the pressure it was rated at, almost always 43.5 psi (3 bar). An injector is an orifice, so flow follows the square root of the pressure across it:

flow = rated × √(actual Δp ÷ rated Δp)

Doubling the fuel pressure buys about 41% more flow, not 100%. Turning the pressure up is a smaller lever than it looks, and it costs you pump headroom and injector latency in exchange.

The pressure that counts is the difference

An injector sprays into the manifold, so what drives flow is fuel pressure minus manifold pressure. A regulator with a vacuum line to the manifold raises fuel pressure as boost rises and holds that difference constant. One without a reference does not — and at 20 psi of boost, a 43.5 psi system is really flowing at 23.5 psi, which is about 27% less flow at exactly the moment the engine wants the most.

That single detail has leaned out more engines than injector sizing ever has.

Duty cycle, and why 85%

Duty cycle is the fraction of the time the injector is held open. The usual ceiling is 85%, and it is a real limit rather than superstition: past there, a saturated injector’s opening and closing times stop being a small fraction of its cycle, so flow stops being linear with pulse width. The map you tuned below that point no longer predicts what comes out. At 100% the injector is simply open, and there is nothing left to give.

Size to 80–85% at your power target and you have somewhere to go. Size to 100% and you have already arrived.

What this does not cover

Injector latency — dead time — is not modelled here. It matters enormously for idle and low-load fuelling, and it varies by injector and by voltage; it is a table your ECU wants, not a number a calculator gives you.

Nor does this size the pump or the lines. An injector that can flow it is no use behind a pump that cannot, and pump curves fall with pressure the same way injector flow rises with it.