If you're building a dual-injection setup—port injection running alongside direct injection—there's a temptation to simplify the plumbing by teeing off a single low-pressure supply to feed both the PI rail and the inlet of the high-pressure fuel pump. One line, less complexity, done.

Don't do it.

This isn't a "best practice" recommendation or belt-and-suspenders conservatism. It's a fundamental mismatch between how these two systems actually work, and combining their feed paths creates problems that you genuinely cannot tune your way out of.

The HPFP Isn't Just Consuming Fuel—It's Also Pushing It Back

Here's the part that trips people up. A cam-driven HPFP doesn't work like a simple pump that pulls fuel in and sends it to the rail. It controls high-side pressure by deciding how much of the incoming fuel to actually compress. The rest gets bypassed—spilled back out the low-pressure side.

This happens continuously. At idle, part throttle, during transients, during pressure regulation events—the HPFP is constantly returning fuel to the low-pressure circuit as a normal part of its job. The amount changes with load and commanded rail pressure, but the spill flow itself never really stops.

So the HPFP isn't a passive consumer sitting at the end of your fuel line. It's an active source of reverse flow and pressure disturbances on that same line.

What Happens When You Tee Them Together

Port injectors are simple devices. They open for a commanded pulse width and deliver fuel based on differential pressure across the injector. The ECU's fuel model assumes that supply pressure is stable—it calculates injector flow from a characterization done at known, steady conditions.

The moment you introduce HPFP spill flow into that same supply, you've violated that assumption. Now the PI supply pressure is fluctuating in response to whatever the HPFP is doing. During a hard transient—say, a quick stab of throttle—HPFP activity spikes, spill flow changes, and you get short-duration pressure events in the PI supply that have nothing to do with the PI system's own demand.

The injectors don't know any of this is happening. They just open for their commanded pulse width and deliver the wrong amount of fuel.

You'll also pick up a thermal problem. Fuel that's been worked through the HPFP comes back warmer than it went in. Fuel temperature affects density, and density directly affects injected mass. So now your PI system's delivered fuel mass is drifting with HPFP activity in a way that's neither steady nor predictable from the PI system's perspective.

Why a Pulsation Damper Doesn't Solve It

The other attempted fix is installing a pulsation damper on the PI supply side—the idea being that the damper absorbs the pressure disturbances coming from the HPFP spill before they reach the PI rail.

In a system where the disturbances are small and infrequent, a damper can do useful work. But that's not what's happening here. HPFP spill is continuous and load-dependent, and the disturbances it generates span a range of frequencies and magnitudes that change with operating conditions. A damper is a passive device with a fixed compliance—it's sized and tuned for a specific pressure range and disturbance character.

On top of that, the pressure waves coming off HPFP spill events travel through the fuel fast—faster than the damper's diaphragm or accumulator volume can respond. By the time the damper is reacting, the disturbance has already reached the PI rail. You're not absorbing the wave, you're just adding a compliant dead end somewhere downstream of where the damage is already done.

And none of this touches the thermal problem. Warmer spill fuel is still mixing into the PI supply regardless of what's happening to the pressure signal. You might take the edge off some of the pressure variation and still have fuel density drift undermining your injected mass.

The damper treats the symptom. The symptom is just more complicated—and faster—than the damper can handle.

What You'll Actually See in Data

If you're logging a car with a shared feed, look for these:

AFR hunting or oscillation during transients that doesn't respond cleanly to fuel trims, cylinder-to-cylinder spread under load that seems to shift around rather than stay consistent, and reduced repeatability when you're trying to dial in a tune—conditions that looked good one day behave differently the next. These almost always get blamed on injector variation or tune slop. Usually it's the hydraulic coupling.

The Right Way to Plumb It—And Why We Built the Spool Auxiliary Split Fuel System

Keep the feeds separate. The HPFP gets its own dedicated low-pressure supply. The PI rail gets its own. Return routing stays upstream of your pressure regulation, and HPFP spill flow never sees the PI supply path.

That's the architecture we designed around when we developed the Spool Performance Auxiliary Split Fuel System. The whole point of the kit is to take the guesswork out of this. Rather than asking a builder to source fittings, figure out routing, and hope their plumbing layout actually achieves true hydraulic isolation, the system is engineered from the start to maintain dedicated feed paths for both circuits. The HPFP gets what it needs, the PI rail gets a clean, stable supply, and the two never interact.

It's more plumbing than a simple tee, yes—but that's exactly why we packaged it the way we did. The complexity is handled on our end so the installation is straightforward and the architecture is correct from day one.