Too small and your injectors max out and lean the engine at the top end. Far too big and idle, cold starts and low-speed drivability get harder to tune. The goal is enough fuel for your power target with some headroom. Here's the standard way to size them.
The formula
Injector size (lb/hr) = (Horsepower × BSFC) ÷ (Number of injectors × Max duty cycle)
- Horsepower — your target crank horsepower.
- BSFC (brake specific fuel consumption) — how many pounds of fuel per hour the engine burns per horsepower.
- Max duty cycle — how much of the time the injector is open at peak. Plan around 80–85% (0.80–0.85) so you aren't running injectors flat out.
To convert to cc/min, multiply lb/hr by about 10.5 (for gasoline).
Typical BSFC values (rules of thumb)
| Engine / fuel | Approximate BSFC |
|---|---|
| Naturally aspirated, gasoline | 0.45–0.50 |
| Turbo or supercharged, gasoline | 0.60–0.65 |
| E85 | Roughly 30–40% more fuel than gasoline |
These are starting points, not guarantees. Your tuner's data on your actual combination always wins.
Worked examples
Turbo 4-cylinder, 400 hp, pump gas
(400 × 0.60) ÷ (4 × 0.80) = 75 lb/hr ≈ 790 cc/min. A common pick would be in the 850–1,000 cc range for headroom.
Same engine on E85
Add roughly 30–40% → about 1,050–1,100 cc/min. That's why E85 builds often step up to 1,300 cc and larger.
NA V8, 450 hp
(450 × 0.50) ÷ (8 × 0.80) ≈ 35 lb/hr ≈ 370 cc/min.
Fuel pressure changes the numbers
Injector ratings are given at a specific pressure. Flow rises with the square root of the pressure change: running 4 bar instead of 3 bar gives about √(4/3) ≈ 15% more flow. Many setups use a rising-rate or boost-referenced regulator so effective pressure climbs with boost.
Don't forget the rest of the fuel system
- Fuel pump — must supply the total flow at your pressure. See fuel pumps.
- Fuel rail — big injectors and E85 often need a larger rail. See fuel rails.
- Regulator — see fuel pressure regulators.
- Filter — protect expensive injectors with a good fuel filter.
- Connectors — aftermarket injectors may need plug adapters.
- Tuning — new injectors need correct injector data (flow and latency) in the tune.
Shop injector sets
Browse fuel injector sets from Injector Dynamics, DeatschWerks, Bosch, FAST, Grams Performance and AEM, or by cylinder count: 4-cylinder, 6-cylinder, 8-cylinder. Questions about fit? Call (604) 809-7398.
Reusing injectors?
If your current injectors are big enough, have them cleaned and flow tested first so you know they're balanced — $15 per injector at our Surrey shop. Book a flow test.
Related: why injector balance matters · testing used injectors