Injector Static Flow at a Different Rail Pressure
Injector flow at a rail pressure other than the one it is rated at.
Example
You enter
- Rated injector flow (cc/min) 550
- Rated at pressure (psi; 43.5 = 3 bar) 43.5
- Actual rail pressure (psi) 43.5
- Manifold pressure (psig; + boost, - vacuum) 15
- Fuel system returnless
You get
- Differential across the injector 28.5 psi
- Flow ccmin 445.185
- Flow lbh 42.3986
- Pct change -19.0573
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method.
Details, formula, and sources
ScopeAn injector is an orifice, so flow follows the SQUARE ROOT of the differential - pressure is a weak lever, needing 4x the differential to double flow. The trap is the fuel system: a return system's manifold-referenced regulator holds the differential constant under boost, but a returnless system holds the RAIL constant, so 15 psi of boost on a 43.5 psi rail drops a 550 cc/min injector to 445 - 19% LESS fuel exactly when the engine wants more.
Formula or tableflow_new = flow_rated x sqrt(dP_new / dP_rated); returnless dP = rail - manifold pressure, return (manifold-referenced) dP = the regulator's base setting; lb/h = cc/min / 10.5.
Source and editionOrifice flow scaling with the square root of the pressure differential - public physics, no table and no fitted constant. The cc/min to lb/h factor matches the landed injector-size tile (gasoline about 0.72 specific gravity).
Free accessThe square-root orifice relation is elementary fluid mechanics; injector flow ratings and their test pressure are published on every injector's data sheet.
What governsEstimate. AHJ and licensed professional govern.
API field namesrated_flow_ccmin, rated_pressure_psi, rail_pressure_psi, manifold_pressure_psig, system_type, effective_dp_psi, flow_ccmin, flow_lbh, pct_change
- Square-root orifice law flow scales with sqrt of the differential; derived, not fittedorifice flow physics
- System type sets the differential returnless = rail - manifold; return = constant regulator settingfuel-system plumbing
- Static flow only full-open capacity; dead time and pulse-width delivery are not modeledstated scope limit