Fuel Rail Pressure Compensation Calculator
Adjust injector flow for actual rail ΔP — essential for accurate tuning under boost.
How to Use
- Enter rated injector flow at its reference pressure (usually 43.5 psi / 3 bar).
- Enter new rail pressure and manifold (boost) pressure.
- Tool scales flow to actual ΔP across the injector.
Injector spec
Corrected flow
Formula
ΔP across injector
ΔP = P_rail - P_manifold
Flow scaling
Q_new = Q_rated · √(ΔP_new / ΔP_rated)
1:1 ref FPR
ΔP stays constant as boost rises
Frequently Asked Questions
Why does injector flow change with pressure?
Orifice flow scales with √ΔP. Double the pressure → 1.41× the flow. Rail pressure minus manifold pressure is what matters.
Why does boost drop injector flow?
Manifold pressure opposes injector. Returnless 1:1 base pressure regulators keep ΔP constant; static systems lose flow under boost.
Rated at 3 bar vs 4 bar?
European injectors often rated at 3 bar (43.5 psi). US aftermarket often 3.0 or 3.5 bar. Always confirm the spec.
Common Use Cases
Build tune
Know real injector flow before writing the VE table.
Returnless vs static
See the flow loss if your FPR is static and MAP rises.
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