High Impedance Injectors
Technical guide to high-impedance (saturated) fuel injectors on Honda PGM-FI engines, explaining their electrical properties, driver types, and compatibility.
High-impedance injectors, also known as saturated injectors, have a relatively high internal coil resistance, typically ranging from 12 to 16 ohms.
Overview
In fuel injection systems, the ECU controls fuel delivery by grounding the injector circuit to open the injector's internal solenoid valve. High-impedance injectors are named after the way the ECU's driver circuit operates: the driver applies a constant 12V signal to the injector coil, and the current "saturates" the coil, holding the valve open for the duration of the pulse width.
Because of their high resistance, the electrical current flowing through the injector is low (approximately 1 amp). This low current generates minimal heat in the ECU's internal injector drivers, allowing the ECU to control the injectors directly without any inline resistors or specialized driver circuits.
Evolution on Honda Engines
Honda's use of injectors evolved across different OBD generations:
- OBD0 MPFI (1988â1991 D/B-series): Utilized low-impedance injectors (2â3 ohms) from the factory, which required an inline resistor box to limit current and prevent damage to ECU drivers.
- OBD1 (1992â1995 D/B/H-series): Civic and Integra models transitioned to high-impedance (saturated) injectors, plugging directly into the wiring harness without a resistor box. The Prelude H22/H23 remained low-impedance with a factory resistor box.
- OBD2 (1996â2001 D/B/H/F-series): All Honda models transitioned to high-impedance saturated injectors.
- K-Series (2001+): Employs modern high-impedance injectors with updated plug connectors.
Specifications
High-impedance injectors are widely used in factory and aftermarket setups. Below are the specifications of common Honda factory high-impedance injectors:
| Engine / Vehicle | OBD Generation | Flow Rate (cc/min) | Nominal Resistance |
|---|---|---|---|
| D16Z6 (Civic EX/Si) | OBD1 | 240cc | ~12.5 ohms |
| B18C1 (Integra GS-R) | OBD1 / OBD2 | 240cc | ~12.5 ohms |
| H22A4 (Prelude VTEC) | OBD2 | 290cc | ~13.0 ohms |
| K20A2 (RSX Type-S) | K-Series | 310cc | ~12.0 ohms |
| S2000 (F20C/F22C) | OBD2 | 360cc | ~12.5 ohms |
Wiring Reference
High-impedance injectors are wired in a direct layout. Each injector receives a switched +12V power source, and its ground return line connects directly to a dedicated pin on the ECU.
Important
There are no inline resistor packs or external driver boxes in a saturated injector circuit.
Advantages and Build Guidelines
Advantages
- Simpler Wiring: No resistor box is needed, reducing failure points and simplifying the engine harness.
- Thermal Efficiency: Generates less heat in both the injector casing and the ECU drivers.
- Modern Compatibility: Most high-performance aftermarket injectors are high-impedance, allowing for high flow rates (1000cc/min+) while maintaining excellent idle control and response times.
Conversion Tip
Caution
If converting an OBD0 chassis (which uses a factory resistor box) to run an OBD1 ECU with high-impedance injectors, you must remove the resistor box and bridge the power supply wires. Failing to bypass the resistor box will result in excessive resistance, leading to weak injector activation and severe lean conditions.