Fuel Injector Size Calculator
Estimate the fuel injector size you need from horsepower, fuel type, BSFC and duty cycle, or find how much horsepower your current injectors can support.
How the fuel injector size calculator works
This calculator uses the same core equations professional tuners use to size fuel injectors. It supports gasoline, E85 and methanol, and lets you work in either direction:
- Injector size from horsepower – plan a new build or upgrade.
- Horsepower from injector size – see how far your current injectors can go.
Formula for injector size from horsepower
Step 1 – Total fuel flow required
\\( \text{Fuel flow (lb/hr)} = \text{HP} \times \text{BSFC} \\)
Step 2 – Injector size per injector
\\( \text{Injector size (lb/hr)} = \dfrac{\text{HP} \times \text{BSFC}}{\text{Number of injectors} \times \text{Duty cycle}} \\)
Duty cycle is entered as a fraction (e.g. 85% → 0.85) in the formula.
Converting lb/hr to cc/min
Injector manufacturers often rate injectors in cc/min at a given fuel pressure, while tuning math is usually done in lb/hr. This tool converts both ways using a fuel-density based factor:
- Gasoline: ≈ 10.5 cc/min per lb/hr
- E85: ≈ 9.8 cc/min per lb/hr
- Methanol: ≈ 7.9 cc/min per lb/hr
The exact factor depends on fuel temperature and blend, so treat results as a guideline.
Typical BSFC values
| Engine / fuel | Typical BSFC (lb/hp/hr) |
|---|---|
| Gasoline NA street / mild performance | 0.45 – 0.55 |
| Gasoline boosted (turbo / supercharged) | 0.55 – 0.65 |
| E85 performance | 0.60 – 0.75 |
| Methanol race engines | 0.80 – 1.00 |
If you are unsure, choose a slightly higher BSFC to be conservative. That will suggest a larger injector with more headroom.
Step‑by‑step: sizing injectors for a build
- Choose “Injector size from HP”. Enter your target horsepower and whether it’s crank or wheel HP. If you enter wheel HP, the calculator will estimate crank HP using the drivetrain loss.
- Enter cylinder count. For port injection this is usually the same as the number of cylinders. For throttle‑body or staged injection, use the number of injectors actually supplying fuel.
- Select fuel type. This sets a reasonable BSFC starting point, which you can fine‑tune in the BSFC field.
- Set maximum duty cycle. 80–85% is a good target for most builds.
- Click “Calculate injector size”. Review the recommended lb/hr and cc/min per injector, and compare to injector data sheets from your preferred brand.
Example calculation
Suppose you have a turbocharged 4‑cylinder gasoline engine and want 450 wheel HP. You expect about 15% drivetrain loss, so crank HP is:
\\( \text{Crank HP} = \dfrac{450}{1 - 0.15} \approx 529 \text{ hp} \\)
Assume BSFC = 0.60 lb/hp/hr and duty cycle = 85% (0.85):
\\( \text{Fuel flow} = 529 \times 0.60 \approx 317 \text{ lb/hr} \\)
\\( \text{Injector size} = \dfrac{317}{4 \times 0.85} \approx 93.2 \text{ lb/hr per injector} \\)
Using ≈10.5 cc/min per lb/hr for gasoline, that’s roughly \\( 93.2 \times 10.5 \approx 978 \text{ cc/min} \\) injectors. The calculator performs this automatically.
Safety margins and real‑world tuning
- Always verify AFR with a wideband O2 sensor under load.
- Ensure your fuel pump and lines can supply the required flow at pressure.
- Consider future power upgrades and choose injectors with some extra capacity.
- Check injector dead time / latency data for your ECU and fuel pressure.
FAQ
How do I calculate fuel injector size from horsepower?
Use the formula: \\( \text{Injector size} = \dfrac{\text{HP} \times \text{BSFC}}{\text{Injectors} \times \text{Duty}} \\), where duty is a fraction (e.g. 0.85). The calculator handles this and converts to both lb/hr and cc/min.
What BSFC value should I use?
For a typical naturally aspirated gasoline street engine, 0.50 lb/hp/hr is a good starting point. For boosted gasoline engines, 0.60 is common. E85 and methanol require more fuel, so BSFC values are higher. When in doubt, choose a slightly higher BSFC.
Why is injector duty cycle important?
Running injectors at 100% duty cycle leaves no time for them to close, which hurts control and reliability. Most tuners limit peak duty cycle to 80–85% at maximum power so the injector can still respond accurately and has some headroom for transient enrichment.
Can I use this calculator for both port and direct injection?
Yes, as long as you know the correct BSFC for your engine and fuel. Direct‑injection engines often have different BSFC and pressure characteristics, so consult OEM or tuner data and treat the result as a guideline rather than an absolute requirement.
Does fuel pressure affect injector size?
Injector flow ratings (lb/hr or cc/min) are always specified at a particular fuel pressure (e.g. 3 bar / 43.5 psi). If you run a different pressure, actual flow will change with the square root of the pressure ratio. This calculator assumes you are using the injector’s rated pressure; always check the manufacturer’s data sheet.