Why Port Injection Is the Key to Big Power on E85 (B58 & S58 Explained)

Every tuned B58 and S58 build eventually runs into the same wall, and it isn’t the turbo — it’s fuel. These engines are direct injected from the factory, and while DI is brilliant for efficiency and emissions, it becomes the limiting factor the moment you start chasing serious power on ethanol. Here’s why, and what the fix actually looks like.

The problem with direct injection at high demand

Direct injection fires fuel straight into the cylinder at very high pressure, but the injectors only have a tiny window of time to do it — while the piston is in the right part of its stroke. As RPM climbs and boost rises, that window shrinks while the fuel demand grows. Add E85 to the mix — which requires roughly 30–40% more fuel volume than gasoline for the same power — and the factory high-pressure system simply cannot keep up. You’ll see rail pressure drop in your logs, and your tuner will be forced to pull timing and boost to keep the engine safe.

What port injection does

Port injection adds a second set of injectors upstream in the intake ports. These injectors spray at low pressure and aren’t constrained by the DI timing window, so they can supply enormous additional fuel volume. The factory DI system keeps doing what it does best — precise combustion control — while port injection handles the extra volume that boost and ethanol demand.

On the S58, adding port injection traditionally meant sourcing an expensive custom intake manifold. The Precision Raceworks S58 Port Injection Kit solves that with a direct-fit solution for the G8X M2, M3, and M4 — no manifold swap required.

The second weak link: the low-pressure system

Here’s the part people miss. Once you add port injection, all that extra fuel has to come from somewhere — and the factory in-tank pump was never sized for it. The low-pressure fuel system becomes the new bottleneck.

That’s what a standalone auxiliary fuel system (AFS) is for: a dedicated pump setup that feeds the port injection system independently, so the factory pump keeps feeding the DI side exactly as designed. Precision Raceworks builds direct-fit AFS kits for the platforms we care about:

Kits are offered in tiers (AFS.460, AFS.640, AFS.800 and so on) sized to your horsepower target — the number roughly corresponds to supported fuel flow, so bigger builds pick bigger tiers.

Small details that matter

Fuel pumps at full duty are hard on stock relay wiring. The Solid-State Relay Upgrade Kit replaces simple on/off relay control with solid-state switching, improving reliability and pump control — cheap insurance on any AFS install.

The takeaway

If your goal is a big-power E85 build on a B58 or S58, the recipe is well established: flash tune, port injection, and an auxiliary fuel system sized for your target. Skip the fueling and you’ll be tuning around a fuel ceiling forever; do it right and the straight-six will happily take everything you throw at it. Questions about sizing a setup for your build? Get in touch — we’ll spec it with you.