A direct-injection turbo engine has two fuel pumps. The low-pressure fuel pump (LPFP) sits in the tank and sends fuel forward; the engine-driven high-pressure fuel pump (HPFP) raises that fuel to the very high pressure the injectors need. When a tuned engine asks for more fuel than either pump can supply, fuel pressure drops under load and the tune has to pull power to stay safe. An HPFP upgrade raises the high-pressure ceiling, and an LPFP upgrade makes sure the HPFP is fed properly. On big builds, and especially on ethanol, you usually need both.
This guide explains the two sides of the system, the signs that you have run out of pump, what Spool's FX names mean, and the options we carry for BMW, Mercedes-AMG and Audi engines.
Low-Pressure vs High-Pressure Side
The Low-Pressure Side
The LPFP is an electric pump in the fuel tank. It pushes fuel through the filter and lines to the engine at relatively low pressure. Its job is to keep the high-pressure pump supplied with a steady flow under every condition.
The High-Pressure Side
The HPFP is a mechanical pump driven by the engine, usually from a camshaft lobe. It takes the fuel from the low side and pressurizes it to the level a direct injector needs to spray straight into the cylinder. Some engines use one HPFP; others, like the S58 and the Mercedes-AMG M177, use two.
The two sides depend on each other. A strong HPFP cannot make up for a low side that cannot keep it fed, and the Spool S58 FX-350 listing recommends an upgraded LPFP so the pump runs in its optimal range and avoids cavitation damage.
Signs You Are Out of Pump
- Fuel pressure drops under load in logs, especially at high rpm in higher gears.
- The tune pulls power or the car feels flat near the top of the rev range.
- Fuel pressure fault codes or warnings after a tune or a move to ethanol blends.
- Problems only on ethanol: ethanol blends need noticeably more fuel volume than gasoline for the same power, so a pump that copes on gasoline can fall short on E85.
Data logging is the clearest way to tell which side is struggling. Low-side pressure falling first points to the LPFP; rail pressure falling while low-side pressure holds points to the HPFP. Our fuel pressure diagnosis guide and low-pressure supply test are written for VW and Audi TSI engines, but the logic of checking each side separately applies to any DI engine.
What the Spool FX Names Mean
Spool's high-pressure pumps are named for their pressure rating:
- FX-200: the earlier design. The listings describe a larger bore than stock for more flow, plug-and-play fitment and full E85 compatibility.
- FX-350: the newer design, which runs at higher pressure than the FX-200 (most listings quote 350 bar, or 35 MPa). Higher pressure lets the injectors flow more fuel within the same injection window and improves atomization. It uses stainless construction with a DLC-coated piston and bore.
Low-pressure upgrades are sold as stages. The Spool Stage 3 LPFP kits use a twin-pump layout: the factory-style pump runs normally and a second pump switches on at boost onset, which the listings say nearly doubles the flow of the stock pump. Some cars also need an activation harness, such as the Spool PWM activation harness for BMW.
Why the LPFP Usually Goes With the HPFP
Raising the high-pressure ceiling increases how much fuel the low side has to deliver. If the in-tank pump cannot keep up, the HPFP is starved, rail pressure falls and the upgrade is wasted. That is why the Spool FX-350 listings recommend an upgraded LPFP alongside them, and why owners moving to ethanol often do both sides together.
Options by Engine
| Engine | High-pressure options | Low-pressure options |
|---|---|---|
| BMW B58 Gen 2 | Spool FX-350 | Spool Stage 3 (G30 540i kit and others by chassis) |
| BMW S58 | Spool FX-350 (two pumps) | Spool Stage 3 for G80/G82/G83/F97/F98 |
| BMW N55 (F-series) | Spool FX-200 Stage 2 | Spool Stage 2 and Stage 3 |
| BMW S63 Gen 2 | Spool FX-350 | Spool Stage 3 for F90 M5 and M8 |
| Mercedes-AMG M177 | Spool FX-200 and FX-350 by model | Spool Stage 3 by model |
| Audi / Lamborghini 4.0T (EA825) | Spool FX-350 for RS6, RS7 and Urus | Spool low-pressure system upgrade |
Fitment is specific to model and chassis, so always check the listing.
Parts and Tools
- Spool FX-350 HPFP, BMW Gen 2 B58 — tune changes required
- Spool FX-350 HPFP Kit, BMW S58 — two pumps, supporting tune required
- Spool Stage 3 LPFP, S58 — boost-activated secondary pump
- Spool Stage 3 LPFP DIY Kit, G30 540i B58
- Spool FX-200 Stage 2 HPFP, F-Series N55
- Spool FX-350 HPFP Kit, Mercedes-AMG G63 & G550
- Spool Low Pressure Fuel System Upgrade, Audi EA825 4.0T
- Spool LPFP PWM Activation Harness, BMW
- 22-Piece Quick Disconnect Tool Kit — for releasing fuel line connectors
Browse every pump in Fuel System Upgrades or the full Spool Performance range.
Shop: High Pressure Fuel Pump (HPFP) Upgrades and Low Pressure Fuel Pump (LPFP) Upgrades.
Install and Tuning Notes
- Tune: the Spool B58 FX-350 listing says tune changes are required, and the S58 kit says a supporting tune is required. Plan the tune before the parts arrive.
- Fuel system safety: relieve fuel pressure before opening any line, work in a ventilated space away from sparks or flames, keep a fire extinguisher nearby and disconnect the battery if the instructions call for it. High-pressure lines hold pressure after the engine is off.
- Cleanliness: keep dirt out of open lines and fittings. Several kits include a 10-micron filter unit as an option.
- In-tank work: LPFP kits involve opening the fuel tank sender. See our fuel pump lock ring guide for the general technique.
- Professional help: the S58 kit recommends professional installation. If you are not comfortable with high-pressure fuel parts, have a shop do it.
Frequently Asked Questions
Do I need an HPFP or an LPFP?
Log fuel pressure on both sides. If low-side pressure drops first, start with the LPFP; if rail pressure drops while the low side holds, the HPFP is the limit. Big builds and ethanol blends often need both.
Can I fit an upgraded HPFP without a tune?
No. The FX-350 listings for the B58 and S58 state that tune changes or a supporting tune are required, and the pump cannot be used to its potential without one.
What is the difference between the FX-200 and FX-350?
The FX-350 is the newer, higher-pressure design (most listings quote 350 bar). The FX-200 is the earlier design with a larger bore than stock.
Are these pumps E85 compatible?
The Spool FX listings describe them as E85 compatible. Pair them with an ethanol analyzer so the tune knows what is in the tank. See our ethanol analyzer guide.