FUEL INJECTOR CALCULATOR
Calculate the exact injector size for your VW/Audi build. Select your engine, set your power target, and get real injector recommendations with duty cycle analysis.
BUILD PARAMETERS
RESULTS
= (400 × 0.50 × 10.5) ÷ (4 × 0.80) = 656 cc/min
INJECTOR RECOMMENDATIONS
Sorted by fit for your build. Green = ideal headroom (15-40%), Yellow = usable, Red = undersized.
| Status | Injector | Type | Flow (cc) | Flow (lb/hr) | Headroom |
|---|
WHAT IS BSFC?
Brake Specific Fuel Consumption (BSFC) measures how efficiently an engine converts fuel into power. It's expressed in lb/HP/hr — pounds of fuel burned per horsepower per hour.
- Gasoline: ~0.50 lb/HP/hr (stoichiometric AFR 14.7:1)
- E85: ~0.65 lb/HP/hr (stoichiometric AFR 9.8:1 — needs ~30% more fuel)
- Methanol: ~0.90 lb/HP/hr (stoichiometric AFR 6.5:1 — nearly double gasoline)
DUTY CYCLE EXPLAINED
Duty cycle is the percentage of each engine cycle that the injector is open and flowing fuel. At 100% duty cycle, the injector is held open continuously — it can't flow any more fuel.
- Below 80%: Safe. Injector has cooling time and margin for transient demands.
- 80–85%: Usable but tight. No room for hot days or fuel pressure drops.
- 85–90%: Danger zone. Injector coil overheating risk, poor spray pattern.
- Above 90%: Do not exceed. Risk of injector failure, lean condition, engine damage.
PORT VS DIRECT INJECTION
Port injectors spray fuel into the intake manifold at 3–4 bar (43.5–58 PSI). Direct injectors spray directly into the combustion chamber at 150–200+ bar.
Higher fuel pressure means the same size injector flows more fuel. DI injectors are rated at their specific operating pressure — cc/min ratings between port and DI injectors are not directly comparable without pressure correction.
SIZING GUIDELINES
Always size injectors with 15–30% headroom above your calculated requirement:
- Street cars: 20–30% headroom for daily reliability
- Track cars: 15–20% headroom (tighter is OK with good tuning)
- Flex fuel (gas + E85): Size for E85 requirement — it's always larger
- Oversized injectors can cause poor idle and low-load drivability issues