HP EFI Sub-Harnesses - Smart Coil

The Holley HP EFI Sub-Harnesses Smart Coil (SKU HLY558-312) is a precision ignition coil wiring harness designed for Holley HP and Dominator EFI fuel injection systems. This universal harness integrates smart coil technology, delivering reliable spark delivery across a range of engine configurations and displacement classes. The sub-harness approach allows builders to maintain system modularity while ensuring consistent ignition timing and voltage regulation—critical for engines running high-boost forced induction or naturally aspirated applications where fuel delivery and ignition synchronization directly impact power output and drivability.

Smart coil technology employed in this harness eliminates traditional distributor-based ignition limitations by allowing individual coil packs to fire each spark plug independently, reducing electrical noise and enabling precise timing control across the entire RPM range. The harness assembly measures 10.4 × 2.3 × 10.7 inches and weighs 1.13 lbs, engineered for compact routing within modern engine bays. Termination connectors and wire gauge are optimized to handle the current demands of multi-coil ignition systems without voltage drop—essential for sustained ignition energy in high-RPM operation. Universal fitment across Holley HP and Dominator platforms means this harness functions as a direct replacement or upgrade component without requiring custom fabrication or extensive ECU recalibration.

This harness is specified for engine builders, performance tuners, and off-road enthusiasts integrating or servicing Holley EFI systems on custom engines, street rods, race platforms, and powersport applications. Technicians upgrading from older carbureted or points-based ignition setups will find this sub-harness simplifies the transition to full EFI control, reducing troubleshooting variables and enabling more aggressive tuning on turbocharged or supercharged builds. OEM replacement shops and professional garages stock this harness as a critical service item for Holley system diagnostics and warranty replacements.

The sub-harness modularity of Holley EFI architecture allows builders to source specific functional components—such as the smart coil wiring—rather than replacing entire engine harnesses, reducing costs and downtime. Compatibility with Holley's ecosystem of fuel injectors, intake manifold controllers, and aftermarket tuning software positions this harness within a proven platform trusted across bracket racing, street performance, and marine engine rebuilds.