Holley 12-879 Billet Bypass Return Style Fuel Press Reg Boost Ref. 4-65 Psi Adj.

Holley 12-879 Billet Bypass Return Style Fuel Pressure Regulator

Achieve precise fuel pressure control for both carbureted and EFI setups with Holley’s billet bypass return style regulator. Built for performance, this unit blends a rugged 6061-T6 billet aluminum core with a sleek black anodized base and a clear anodized top for durability and standout aesthetics in any engine bay.

Key features

• Adjustable fuel pressure range of 4–65 psi for exact tuning in varied applications
• Boost Reference – 1:1 to maintain proper fuel pressure under boost conditions
• Bypass return design with a return port back to the tank to manage excess fuel and stabilize pressure
• All-metal construction: billet aluminum with a black base and clear top for durability and style
• Three 3/8" NPT ports: Inlet, Outlet, and Return; plus a gauge port (1/8" NPT)
• Includes manifold reference fitting for accurate boost or manifold reference setup
• Carbureted baseline spring (4–15 psi) installed at purchase
• Second pressure spring included to extend range up to 65 psi for EFI applications
• Port versatility: available in 3/8" NPT or 6AN configurations to match your fuel line fittings
• Diesel and E85 compatibility for broad fuel compatibility

What it does and how you’ll use it

This regulator is designed for systems that return excess fuel to the tank. Its 1:1 Boost Reference ensures fuel pressure tracks boost pressure, helping maintain stable fueling under boost and preventing lean conditions during acceleration. With 4–65 psi adjustability, you can dial in exact pressure targets for carbureted setups now and smoothly transition to EFI later without replacing the regulator.

Compatibility and use cases

• Suitable for carbureted engines that’ll eventually upgrade to EFI, providing a future-proof solution
• Ideal for engines with a return line back to the tank, helping stabilize fuel pressure across RPM and load changes
• Compatible with diesel fuels and E85, expanding its applicability across different builds
• Available in 3/8" NPT or 6AN port options to fit a wide range of existing lines

Technical details at a glance

• Ports: 1 × 3/8" NPT Inlet, 1 × 3/8" NPT Outlet, 1 × 3/8" NPT Return
• Gauge port: 1 × 1/8" NPT
• Boost Reference: 1:1
• Spring configuration: factory-installed 4–15 psi carbureted spring; second spring included for EFI up to 65 psi
• Construction: 6061-T6 billet aluminum; black anodized base; clear anodized top
• Fittings included: manifold reference fitting
• Fuel compatibility: diesel and E85

What you get

• Holley billet fuel pressure regulator (12-879) with 4–15 psi carbureted spring installed
• Second pressure spring to extend range up to 65 psi for EFI
• Manifold reference fitting
• 1/8" NPT gauge port
• Port options available in 3/8" NPT or 6AN configurations