Injector Calculators

Injector Duty Cycle

How much of the available time your injectors spend open. At 6,000 RPM a four-stroke engine completes one cycle in 20 ms — a 10 ms pulse width means 50% duty. Keep it below 80–85% for a safe margin; at 100% the injector is static (wide open) and can lean out the engine. Flip “solve for” to work backwards — the longest safe pulse width at a given RPM, or the max RPM your current pulse width allows.

Solve for

Duty cycle Max pulse width Max RPM

Engine type

4-stroke 2-stroke

Engine RPM *

Injector pulse width (ms) *

Time per cycle 20.0 ms

Injector duty cycle 50.0%

In the safe range — injectors have time to close fully between pulses.

Injector Flow Rate

What size injector your build needs: (Engine HP × BSFC) ÷ number of injectors, at your target duty cycle. BSFC is how much fuel the engine uses per hour for each horsepower it makes — see the reference values below. Flip “solve for” to run it in reverse: the HP a given injector supports, or how many injectors your build needs.

Solve for

Injector size Engine HP # of injectors

Engine HP *

BSFC *

# of injectors *

IDC as a % *

lb/hr flow rate 39.1 lb/hr

cc/min flow rate 410 cc/min

Supported engine HP — hp

Injectors needed —

Minimum size per injector. Round up to the next available injector size.

Injector HP — Max Capacity

How much horsepower your current injectors can support: (injector flow rate × number of injectors × IDC) ÷ BSFC. Enter your injector flow rate (IFR) in either unit.

Flow measure

lb/hr cc/min

IFR — per injector *

# of injectors *

IDC % *

Max injector HP @ BSFC 0.5 538 hp

MAX HP FOR BSFC .4	672
MAX HP FOR BSFC .5	538
MAX HP FOR BSFC .6	448
MAX HP FOR BSFC .7	384
MAX HP FOR BSFC .8	336
MAX HP FOR BSFC .9	299

Injector BSFC

Working backwards: given your injectors and the power the engine makes, what BSFC does that imply? Useful for sanity-checking whether your current setup matches how the engine is fueled.

Flow measure

lb/hr cc/min

IFR — per injector *

# of injectors *

Engine HP *

IDC % (assumed)

BSFC 0.54

In the forced-induction / nitrous range (0.6–0.75 forced induction, 0.6–0.65 nitrous, 0.45–0.5 naturally aspirated).

General gasoline BSFC values to use: naturally aspirated engines 0.45 – 0.5 · nitrous oxide engines 0.6 – 0.65 · forced induction engines 0.6 – 0.75. Conversion used throughout: 1 lb/hr ≈ 10.5 cc/min (gasoline). If your duty cycle is running high, the fixes are larger injectors, more fuel pressure (within safe limits), or tuning adjustments — and if your injectors are flowing below spec, cleaning often recovers the difference.

Injector Duty Cycle

Injector Flow Rate

Injector HP — Max Capacity

Injector BSFC

Dirty injectors don’t flow their rated numbers