Fuel Injector Size Calculator
Find the injector size (cc/min & lb/hr) for your target horsepower.
Get injectors sized & tuned?
How it works
Undersized injectors cap your power and run the engine lean at high load — the fastest way to melt a piston. Oversized injectors hurt idle and low-load fuelling. This calculator sizes them from your target power, the number of injectors, the fuel’s brake-specific fuel consumption (BSFC), and a safe maximum duty cycle (we default to 85%).
BSFC is how much fuel the engine burns per horsepower per hour. Petrol turbo builds sit around 0.50; E85 needs roughly 30% more fuel, so ~0.64. Never plan for 100% duty cycle — the injector needs headroom, so we size to 85%.
The result is a guideline. Real injector choice also depends on fuel pressure, injector characterisation (dead time / short-pulse behaviour) and your ECU. On a DMS build we pick the injector, characterise it properly and tune it on the dyno — the number here just gets you in the right range.
These figures are engineering guidelines to help you plan. The reliable number always comes off the dyno — that's how every DMS build is proven.
FAQ
What BSFC should I use?
For a petrol turbo build, ~0.50 is a safe default. Naturally aspirated petrol is a bit lower (~0.45). E85 needs ~30% more fuel, so ~0.64. Diesels differ entirely. This calculator defaults to safe petrol/E85 values.
Why 85% duty cycle and not 100%?
Injectors need headroom above your peak demand for transient enrichment and cold starts, and they lose linearity near 100%. Sizing to ~85% keeps them in their accurate, reliable range.
Do bigger injectors ruin my idle?
They can, if the ECU can’t control them at very short pulse widths. Modern injectors with proper dead-time/short-pulse data (which we characterise when tuning) idle cleanly even when large.