Horsepower goal in, injector size out, in lb/hr and cc/min, sized at a safe 80 percent duty cycle with the right fuel math for gasoline or E85.
| Injector (lb/hr) | 24 | 30 | 36 | 42 | 60 | 80 |
|---|---|---|---|---|---|---|
| Max HP | — | — | — | — | — | — |
Brake specific fuel consumption is how many pounds of fuel an engine burns per horsepower, per hour. Multiply by your horsepower goal for total fuel demand, then split it across the injectors with duty cycle headroom. That is the whole formula.
E85 carries less energy per gallon, so the engine needs roughly a third more volume for the same power. That is baked into the E85 BSFC numbers here. It is also why E85 builds run out of injector and pump long before gasoline builds at the same power.
Oversized injectors make idle and cruise pulse widths so short the injector cannot meter consistently, and drivability suffers. Size for the goal with room to grow modestly, not for a fantasy number.
On a V8 running gasoline, naturally aspirated: about 39 lb/hr, so 42s are the pick. Boosted on gasoline: about 48 lb/hr, so 60s. On E85 boosted: about 66 lb/hr, so 80s. All at 80 percent duty cycle. Run your exact combo above.
Oversized injectors idle poorly because the pulse width at idle gets too short to meter accurately. Size for the goal with 80 percent duty at peak, and let the pump and regulator keep up. Room to grow means one size up, not three.
Yes. Flow rises with the square root of the pressure ratio: an injector rated 42 lb/hr at 43.5 psi flows about 48 lb/hr at 58 psi. This calculator assumes rated pressure. If your system runs higher base pressure, your injectors effectively run one size larger.
Pistons & Ink is a digital product studio based in Ocala, Florida, building web and mobile products for the automotive enthusiast community. We come from the custom car and truck world — lowriders, minitrucks, C10s, restomods — and we build tools we'd use ourselves.
This fuel injector calculator is one of 11 free tools in the Pistons & Ink Digital Toolbox, built for the aftermarket auto industry. If it helped you, the rest of the toolbox is right here.
Visit Pistons & Ink →