Home › Calculators › Max horsepower calculator
Calculator · SizingMax horsepower from your injectors
The reverse of the sizing question: you already have injectors — how far will they go before they run out of flow? Enter their rated flow and this tool returns the crank horsepower the set can honestly support at your chosen duty-cycle ceiling.
Rated flow only counts at its rated pressure — usually 43.5 psi (3 bar). Running higher rail pressure raises real flow; see the pressure calculator below.
The formula
Max HP = (injector flow in lb/hr × cylinders × max duty cycle) ÷ BSFC
BSFC assumptions match our size calculator: 0.50 naturally aspirated, 0.60 boosted, gasoline. On E85 the engine needs about 35% more fuel volume for the same work, so the supported horsepower is divided by 1.35. The cc/min to lb/hr conversion is the standard gasoline factor of 10.5.
Worked example (pre-filled above): eight 42 lb/hr injectors at 80% duty in a naturally aspirated gasoline engine support (42 × 8 × 0.80) ÷ 0.50 = 537.6 hp. A 42 lb/hr injector is a 441 cc/min injector, so you can enter it either way and get the same answer.
Reading the result
- It is a ceiling, not a promise. The engine also needs air, a pump that keeps up (pump flow calculator) and a tune that holds the target air-fuel ratio.
- Close to the line? If your goal sits within 10% of the ceiling, step up a size — the size calculator works the problem from the horsepower side.
- Old injectors flow less. Varnish and a clogged basket filter quietly eat flow. A serviced injector with a fresh micro filter gets much closer to its rating.
Sized them? Now service them
New injectors still need fresh seals and filters. The Universal EV1 Rebuild Kit — 200-Injector Service Tray + Removal Tool, $72.49 covers 200 injectors with matched O-rings, pintle caps and micro basket filters, and the Micro-Filter Basket Removal Tool, from $14.99, pulls the old basket filter without damage.
Free tracked USPS shipping in the US · 30-day returns · secure card checkout by Stripe · sold by Partsapiens.
Not sure which kit your injectors take? See kits by injector part number or the fitment guide.
More injector calculators
- Injector flow converter — cc/min ↔ lb/hr.
- Injector size calculator — injector size for a target horsepower.
- Max horsepower from your injectors — the reverse calculation.
- Flow rate at a different fuel pressure — the square-root rule.
- Injector duty cycle calculator — pulse width and RPM to duty %.
- Fuel pump flow requirement — LPH your engine actually needs.
Last updated October 20, 2026 · Partsapiens, 5325 Alton Pkwy, Suite C300, Irvine, CA 92604 · partsapiens@gmail.com