HEI Ignition Harness

The Holley HEI Ignition Harness (558-304) is an EFI wiring adapter designed to bridge older GM small cap HEI distributors with modern Holley Avenger, HP, and Dominator EFI fuel injection systems. This ignition harness eliminates the need for complete distributor replacement when retrofitting classic or vintage vehicles with contemporary fuel management technology, preserving original engine bay aesthetics while unlocking modern engine control benefits.

The adapter harness functions as the critical signal interface between your existing small cap HEI distributor and Holley's EFI platforms. When upgrading to Holley Avenger, HP, or Dominator EFI systems, engine control modules require clean ignition trigger signals. This harness translates the original distributor's firing pattern into the digital input format the ECU expects, enabling seamless fuel injection timing and spark advance control without compromising drivability or starting reliability. Compact construction (7.2 x 3.9 x 7.3 inches) fits neatly in tight engine compartments; the 0.3 lb weight adds negligible mass.

Ideal for enthusiasts performing EFI conversions on small block or big block Chevrolet engines equipped with original HEI ignition systems. Common scenarios include street rods, restorations, and performance builds where modern fuel management is desired but original interior engine configuration must be maintained. The 558-304 eliminates the expense and complexity of sourcing a period-correct distributor that already supports Holley EFI systems, making this harness a cost-effective solution for precision engine management upgrades.