Injector Size Calculator
Enter the crank horsepower you are aiming for, the number of cylinders, the type of induction or fuel and the maximum duty cycle. The calculator gives the injector flow needed and the nearest common size.
- cc/min and lb/hr
- NA, turbo and E85
- Duty cycle check
Use crank horsepower. Wheel horsepower is about 15% lower on most cars.
Injectors must flow enough fuel at full power without being held open all the time. Each horsepower burns about half a pound of petrol an hour in a naturally aspirated engine, and more with boost or E85. Divide that by the number of injectors and allow headroom so they stay under about 80% duty.
Injector size chart
cc/min at 80% duty and 3 bar. Search for a power.
| hp per cylinder | NA petrol | Turbo petrol | E85 |
|---|---|---|---|
| 25 | 164 | 197 | 246 |
| 50 | 328 | 394 | 492 |
| 75 | 492 | 591 | 738 |
| 100 | 656 | 788 | 984 |
| 125 | 820 | 984 | 1230 |
| 150 | 984 | 1181 | 1477 |
| 200 | 1313 | 1575 | 1969 |
Power per cylinder is total crank power divided by cylinders. A 400 hp V8 is 50 hp per cylinder.
Power and injector size
Four cylinders at 80% duty. Slide the power.
The spray cone in the drawing widens as flow rises. Past about 1,000 cc, idle quality needs careful tuning.
How to size injectors
Fuel per hour first. Step through it.
Set target power
Crank horsepower.
1 lb/hr equals about 10.5 cc/min of petrol. Divide cc/min by 10.5 to go the other way.
Petrol, turbo or E85
Fuel demand changes. Compare them.
The leanest safe mixture at full power.
Richer mixture to control heat.
Ethanol has less energy, so more flows.
Running rich under boost keeps turbo engines cool, which is why they need more fuel per horsepower.
Before swapping injectors
Fuel system checks. Tick each item.
Tick each item as you go.
Bigger injectors only help if the pump can feed them. Check the pump flow at your boost and fuel pressure.
What you get
Injector sizing. Each tile explains one result.
cc/min.
US rating.
Next one up.
At that size.
At full power.
Duty at the chosen size shows how hard the injectors will work at full power.
Frequently Asked Questions
No question matches that search.
What size injectors for 300 hp?
On a turbo 4-cylinder, about 590 cc/min. On a V8 NA, about 245 cc/min.
How do I convert lb/hr to cc/min?
Multiply by 10.5.
What duty cycle is safe?
Up to about 80% for continuous full-power use.
What size injectors for 500 hp?
On a turbo 4-cylinder, about 985 cc/min. On a turbo 6, about 660.
How much bigger for E85?
About 30% to 40% more than petrol.
Does fuel pressure change injector flow?
Yes. Flow rises with the square root of the pressure ratio.
Can injectors be too big?
Yes. Very large injectors can idle poorly without careful tuning.
Fuel for the power you want
Multiply power by the fuel factor, divide by injectors and duty, and pick the next size up. Match the pump and get a tune, and the engine will make its power safely.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.