How do I convert lb/hr to cc/min?

Multiply by about 10.5 for gasoline. 50 lb/hr is about 525 cc/min.

What duty cycle is too high?

Plan for no more than about 85 % at peak power. Close to 100 %, the injector barely closes between pulses and can't add fuel. efi2000 uses 85 % as the default ceiling.

Do I need to retune after changing injectors?

Recalculate Required Fuel (or update injector flow on rusEFI) and set the new dead time. Then check idle and light load, where dead time matters most, and confirm full load with a wideband.

How does efi2000 calculate Required Fuel?

From your engine, injectors and fuel in Vehicle properties. It shows the suggestion right on the Required Fuel setting, so you can apply it in one click.

All guides

Guide · updated 23 October 2026

Injector sizing, dead time and Required Fuel

How much injector an engine needs, the settings that describe an injector to the ECU, and what to change when you fit new ones.

Sizing injectors for a power target

Injector flow is rated in cc/min or lb/hr (1 lb/hr is about 10.5 cc/min of gasoline), at a stated fuel pressure. The usual sizing formula is:

flow per injector (lb/hr) = power (hp) × BSFC ÷ (number of injectors × maximum duty cycle)

BSFC, brake specific fuel consumption, is the fuel used per horsepower per hour. Common planning figures for gasoline are about 0.45 to 0.50 lb/hp/hr for naturally aspirated engines and 0.55 to 0.65 for turbo engines.
Maximum duty cycle is the share of each cycle the injector is open at peak power. Plan for about 80 to 85 %, which leaves margin.
E85 needs roughly 30 to 40 % more flow than gasoline for the same power.

Example: 300 hp from a turbo four-cylinder at 0.6 BSFC and 85 % duty needs 300 × 0.6 ÷ (4 × 0.85) ≈ 53 lb/hr, about 560 cc/min per injector.

Fuel pressure

Flow changes with the pressure across the injector: new flow = rated flow × √(new pressure ÷ rated pressure). Raising pressure from 3 to 4 bar adds about 15 %.

With a manifold-referenced regulator, rail pressure rises with boost and the pressure across the injector stays the same. With a returnless system, rail pressure is fixed, so the pressure across the injector falls as boost rises and the injector flows less.

Dead time and duty cycle

An injector takes time to open, so part of each pulse delivers no fuel. That delay is the dead time, or injector latency, and it gets longer as battery voltage falls. The ECU adds it to every pulse.

Dead time matters most at idle and light load, where pulses are short. A wrong value shows up as fuelling that is off at idle but fine at full load, and changes with battery voltage.

Duty cycle is pulse width divided by the time available per cycle. At 100 % the injector never closes and can't deliver more, so a tune that needs more fuel than the injectors can supply runs lean.

Required Fuel

Speeduino and MegaSquirt start every fuel calculation from Required Fuel: the pulse width, in milliseconds, that delivers a stoichiometric mixture at 100 % VE. TunerStudio's Required Fuel calculator works it out from displacement, cylinder count, injector flow and the fuel's stoichiometric AFR.

When you change injectors, recalculate Required Fuel and update the dead time. The VE table should then need only small changes. For example, a 2.0-litre four-cylinder on gasoline gives about 7.6 ms with 440 cc/min injectors and about 3.4 ms with 1000 cc/min injectors.

How common ECUs set up injectors

Speeduino

Required Fuel in ms, a base injector open time (dead time) with battery voltage correction, and an injector layout of paired, semi-sequential or sequential. The layout and number of squirts change how Required Fuel is split across pulses.

rusEFI and epicEFI

No Required Fuel. You enter injector flow in cc/min or g/s, the pressure that flow was measured at, how fuel pressure is regulated (manifold-referenced, returnless or sensed rail pressure), and a dead time table against battery voltage. Fuel is worked out from displacement and flow.

MegaSquirt MS3

Required Fuel from the ReqFuel calculator, and injector dead time with battery voltage correction.

Using efi2000 for injectors

Vehicle properties holds your injectors. Pick yours from the component library to fill in flow, dead time and the rest from its data, or enter them by hand.

efi2000 suggests Required Fuel from your engine, injectors and fuel, so you don't need a separate calculator.
Spotter warns when the VE table asks for more fuel than your injectors can deliver.
The table generator warns when your power target outgrows the injectors.
Autotune never adds fuel where the injectors have run out.

When you fit new injectors, update them in Vehicle properties and apply the new Required Fuel suggestion on Speeduino or MegaSquirt, or the new injector flow on rusEFI and epicEFI. Every check uses the new injectors straight away.

Questions

How do I convert lb/hr to cc/min?: Multiply by about 10.5 for gasoline. 50 lb/hr is about 525 cc/min.
What duty cycle is too high?: Plan for no more than about 85 % at peak power. Close to 100 %, the injector barely closes between pulses and can't add fuel. efi2000 uses 85 % as the default ceiling.
Do I need to retune after changing injectors?: Recalculate Required Fuel (or update injector flow on rusEFI) and set the new dead time. Then check idle and light load, where dead time matters most, and confirm full load with a wideband.
How does efi2000 calculate Required Fuel?: From your engine, injectors and fuel in Vehicle properties. It shows the suggestion right on the Required Fuel setting, so you can apply it in one click.