Spool Performance B58 Gen 2 Auxiliary Fuel System

The Spool Performance B58 Gen 2 Auxiliary Fuel System is designed for B58 Gen 2 owners pushing beyond the limits of the factory fuel system who want a solution that’s properly engineered, easy to install, and built for long-term reliability.

From the start, our focus was on correct fuel architecture and real-world usability. Every component and layout choice was made to deliver stable, predictable fueling at higher power levels while keeping installation clean and OEM-friendly.

Why a Properly Designed DI + PI Fuel System Matters

As power levels increase on the B58 Gen 2 platform, fueling challenges typically arise when direct injection (DI) and port injection (PI) are supplied from the same fuel feed.

When DI and PI share a feed:

The pulsed nature of DI can introduce pressure fluctuations that disrupt fuel flow to the PI injectors
Port injection requires boost-referenced fuel pressure to maintain consistent injector differential under load
At higher boost and airflow, maintaining precise and repeatable fueling becomes more difficult

By separating the DI and PI supply, each system is able to operate independently and exactly as intended. The result is smoother fuel delivery, improved consistency, and greater confidence as power increases.

Designed to Install Cleanly

A significant amount of development time went into making this system straightforward to install without unnecessary modifications.

The OEM fuel pump top hat is retained using additional precision-machined aluminum fuel fittings that integrate directly into the factory top hat, eliminating the need for repinning or modifying factory wiring
Optional billet top hats are available separately for those who prefer them
A plug-and-play wiring harness is supplied for direct compatibility with the Reflex controller, simplifying installation and control

These design choices keep the install clean, reduce complexity, and maintain a factory-like fit and finish.

Scalable Pump and Control Options

The system is built to grow with your setup:

Dual Walbro 450 fuel pumps included standard
Optional Walbro 525 upgrades available for higher horsepower goals
Choice of:
- Standard relay configuration
- Solid-state relay option for full PWM pump control available for purchase separately. NOTE: 2 harnesses required per AFS Kit - Link to Harness: https://spoolperformance.com/products/untitled-may18_19-53-33?_pos=1&_sid=2a7c2a7b8&_ss=r

This flexibility allows you to match the fuel system to your current build while leaving room to expand.

High-Quality Fuel Plumbing

All fuel lines included in the kit feature triple-layer PTFE construction:

Inner PTFE core for full fuel and ethanol compatibility
Middle stainless steel reinforcement for pressure strength
Outer nylon protective layer for abrasion resistance and durability

This ensures long-term reliability in high-heat, high-vibration environments.

Compact, Purpose-Built Fuel Pressure Regulation

The system includes Spool Performance’s in-house designed compact fuel pressure regulator, developed specifically for auxiliary fuel applications:

Boost-referenced for accurate pressure control under load
Stable regulation at high flow rates
Compact design for clean engine bay packaging
Built to withstand demanding conditions

Built to Support Serious B58 Gen 2 Builds

Whether you’re building a strong street car or planning a higher-power setup, the Spool Performance B58 Gen 2 Auxiliary Fuel System delivers the fueling control and capacity needed to push the platform with confidence.

With properly separated DI and PI supply, OEM-friendly integration using precision aluminum fittings, Reflex compatibility, high-quality plumbing, and scalable pump options, this system is designed to perform reliably as your build evolves.

If you’re ready to upgrade your B58 Gen 2 fueling with a system that’s thoughtfully engineered and genuinely easy to live with, this is the solution built to get you there.

Fitment

This auxiliary fuel system is designed for BMW vehicles equipped with the B58 Gen 2 engine, including:

BMW M340i / BMW M340i xDrive – 2020+
BMW M440i / BMW M440i xDrive – 2021+
BMW X3 M40i – 2020+
BMW X4 M40i – 2020+

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