Injector sizing is one of the few engine-build numbers where being slightly oversized is correct. Static flow ratings on the box and actual flow into a running engine are not the same number — dead time, voltage, and duty cycle all chip away at usable capacity.
The sizing formula
Injector Size (lb/hr) = (HP × BSFC) ÷ (Cylinders × Max Duty Cycle)
- BSFC (brake-specific fuel consumption): 0.45–0.50 for NA gas, 0.55–0.60 for boosted
- Max duty cycle: 80% for street, 85% max for dedicated race
Exceeding 85% duty cycle leaves no headroom for enrichment during transients or hot weather. That's where engines go lean and break.
Static rating ≠ actual delivery
Published injector flow ratings are static — measured at fixed fuel pressure with the injector held wide open. In a running engine, injectors pulse open/closed rapidly, and actual delivery is affected by:
- Dead time: lag between the electrical signal and mechanical opening. Lower voltage = more dead time = less effective flow.
- Battery voltage: at 12V vs 14V, the same injector behaves differently. Always datalog voltage during tuning.
- Fuel pressure: roughly +3–4% flow per PSI above the rated pressure. A 42 lb/hr injector at 60 PSI (rated for 43.5 PSI) flows closer to 50 lb/hr.
E85 and alternative fuel sizing
E85 has a stoichiometric AFR of ~9.8:1 vs gasoline's 14.7:1 — it requires roughly 30% more fuel by volume to hit the same Lambda. If your engine needs 42 lb/hr injectors on gas, you need ~55 lb/hr on E85.
Many builders size for E85 from the start — running gasoline just means lower duty cycles, which is perfectly fine. Undersized injectors on E85 max out at 100% duty cycle and go lean under load. That destroys pistons and bearings in seconds.