Enter your injector flow rate, number of injectors, fuel type (BSFC), and target duty cycle to estimate supported horsepower — or flip the tab to size injectors for a horsepower target.
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Formula
Supported HP from injectors:
HP = (Flb/hr × N × DC) ÷ BSFC
where Flb/hr = flow per injector (lb/hr), N = number of injectors, DC = max duty cycle (decimal, e.g. 0.80), BSFC = brake specific fuel consumption (lb/hp·hr).
Injector size needed (per injector):
Flb/hr = (HP × BSFC) ÷ (N × DC)
Convert: cc/min ≈ lb/hr × 10.5 (gasoline, ~0.0965 lb/cc·min).
Interpretation
The result is the maximum crank horsepower your fuel system can feed at the duty cycle you chose. Running injectors above ~85% duty leaves no headroom for cold starts, altitude, or transient enrichment, which is why 80% is the industry-standard sizing target. If your injectors land above that number in real-world logging, you are out of fuel — not out of engine.
- Under 70% duty at peak power: comfortable margin, safe for tuning.
- 70–80%: ideal sizing zone.
- 80–90%: marginal; flow becomes non-linear and AFR control suffers.
- Over 90%: static — injectors are effectively open, engine leans out under load.
BSFC choice matters more than most people think: swapping from 0.50 (NA gas) to 0.75 (E85 boost) raises fuel demand by 50% for the same horsepower.
BSFC Reference Table
| Fuel / Application | BSFC (lb/hp·hr) |
|---|---|
| Diesel | 0.38 – 0.42 |
| Naturally aspirated gasoline | 0.45 – 0.50 |
| Nitrous / mild boost gasoline | 0.55 |
| Turbo / supercharged gasoline | 0.60 – 0.65 |
| High-boost gasoline (race) | 0.65 – 0.70 |
| E85 naturally aspirated | 0.65 – 0.70 |
| E85 forced induction | 0.75 – 0.80 |
FAQ
Should I enter flywheel (crank) horsepower or wheel horsepower?
Use crank horsepower. BSFC is defined against engine output, so if you only have a dyno wheel number, add roughly 15% (manual) or 20% (auto) to estimate crank HP first.
My injectors are rated at a different fuel pressure. Does that matter?
Yes. Injector flow scales with the square root of pressure. A 550 cc/min injector rated at 43.5 psi flows about 637 cc/min at 58 psi. Use the flow rating at the pressure your regulator actually runs.
Why is E85 BSFC so much higher?
E85 has lower energy density, so the engine burns roughly 30–40% more fuel by mass for the same power. That is why E85 builds often need injectors one or two sizes larger than a comparable gasoline setup.
What if I only know cc/min and the calculator asks for lb/hr?
Just pick cc/min in the unit dropdown — the calculator converts internally. If you need to convert manually: lb/hr = cc/min ÷ 10.5 for gasoline.
