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Injector calculators: run the numbers before you order parts

Choosing injectors — or judging the ones already in the rail — is mostly arithmetic, and getting it wrong is expensive in both directions: too small and the engine leans out at the top end, too big and it will not idle cleanly. We keep a set of six free calculators on our sister site, InjectorTool, and each one shows the formula it uses so you can check the reasoning, not just the result.

• Flow unit converter — a flow bench speaks cc/min, a tuning suite speaks lb/hr. This translates between them with the standard gasoline factor, so a bench report and a catalogue listing can be compared like for like.
• Sizing for a power target — starting from a horsepower goal, it works out the delivery each injector has to provide, allowing for cylinder count, whether the engine is boosted, the fuel in the tank (including the extra volume E85 demands) and a sensible duty-cycle ceiling.
• What your current set can support — the same maths turned around: feed in the rating of the injectors you already own and get the crank power they can feed before they saturate.
• Flow corrected for rail pressure — ratings are measured at one pressure; your regulator may be set to another. This applies the square-root relationship between pressure and flow, in psi or bar.
• Duty cycle from a data log — turn a logged pulse width and engine speed into a duty percentage, and see plainly when the injectors are being asked for more time than the engine cycle contains.
• Pump capacity check — works out how many litres per hour the engine actually consumes at full power, then adds a safety margin so the pump you pick is not already at its limit on day one.

All six live together on the InjectorTool calculators hub. Once the numbers point at a size, the matching service parts are here and there: micro-filter removal tool and filter bundles on this site, repair kits by engine family, and the full range of kits indexed by injector part number on InjectorTool. If you would rather read than calculate, the learn library covers connectors, pigtails and filter service, and the rebuild guides walk through complete jobs.

Last updated October 20, 2026 · Partsapiens · partsapiens@gmail.com