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Modifying Closed Loop Cars that Need LOTS More Fuel! Part 2

The design and hardware

by Julian Edgar

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At a glance...

  • Dual pressure fuel system
  • Returnless and conventional fuel systems
  • Pumps, pressure regs, solenoids and one-way valves
  • System design
  • Part 2 of a 3-part series
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Last week in Modifying Closed Loop Cars that Need LOTS More Fuel! Part 1 we discussed the difficulties in modifying the mixtures of a car that always stays in closed loop. In a previous article (Altering Closed Loop Mixtures) we’ve looked at how this can be done if the overall required fuel flow doesn’t need to change a lot. But what if you’re adding a turbo to a previously naturally aspirated engine? Then you’ll need to both enrich the high load air/fuel ratios and also flow a lot more fuel to cater for the increased power. And in that case, doing as we did previously (disconnecting the oxy sensors then tweaking the airflow meter output signal) won’t work.

So how can it be done? The answer is to use two fuel pressures and switch between them. If the oxy sensor input(s) are disconnected at the same time as the fuel pressure is switched from high to low, the ECU won’t know what’s going on and the so will run the same injector pulse widths as it would normally when it loses the oxy sensor signals. The higher fuel pressure will flow more fuel through the same injector pulse widths and the result is an enriched mixture, even at much higher than normal airflows.

Returnless System?
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Before you can run dual fuel pressures, you need to be able to access the standard fuel pressure regulator. In cars that mount the fuel pressure regulator in the engine bay and use a return line to the tank, that’s straightforward. But if your car runs a returnless fuel system and the reg is mounted within the tank, it’s more difficult.

If the latter is the case, you’ll need to modify the internal fuel pressure reg, fit an external fuel pressure regulator and add a return line to the tank. While this is a little easier than it first appears, it’s still not 10 minutes of work! How to do it is covered in a series starting at Modifying Returnless Fuel Systems, Part 1.

Dual Fuel Pressure System

Click for larger image

This diagram shows how the dual fuel pressure system works. Understanding it is critical if you’re to successfully install the system. Starting at the fuel tank, fuel flows from the standard pump (here it’s an in-tank one) through the fuel filter. At this point a second, high pressure fuel pump is installed. This pump runs only when high fuel pressure is needed. So that fuel can easily bypass Pump 2 when it is not running, a one-way valve is installed in parallel with this pump. (Some companies installing upgrade second fuel pumps don’t fit the bypass one-way valve, but all fuel system authorities suggest the valve should be used.)

The fuel then flows forward through a solenoid to the first, low pressure regulator. (This is normally the standard fuel pressure reg.) In parallel with the first fuel pressure regulator is another reg, set to higher fuel pressure.

In normal low, fuel pressure operation, Pump 2 is off and the solenoid is open. Reg 1 is set to (say) 45 psi and Reg 2 to 85 psi. The fuel is pressurised by Pump 1, flows through the fuel filter and – via the one-way – valve around Pump 2. Because Reg 2 is set to a much higher pressure than Reg 1 and the solenoid is open, the fuel takes the easier path through Reg 1 that sets the fuel pressure at 45 psi.

In high pressure operation, Pump 2 is switched on and the solenoid is switched off. In this situation, Pump 1 acts as a supply pump for Pump 2. Because of the greater pressure being developed at the outlet of Pump 2, the one-way valve is forced closed and so the fuel provided by Pump 2 flows to the pressure regulators. The solenoid blocks the flow to Reg 1 and so Reg 2 sets the fuel pressure at 85 psi.

It’s important to note that if required, the solenoid can be placed in the return line of Reg 1, rather than being in front of it. Closing the solenoid will still prevent fuel flow through Reg 1, however, with the solenoid located in the return line to Reg 1, this regulator’s diaphragm is subjected to the high fuel pressure provided by Reg 2. That’s why it’s better to have the solenoid before the reg, but in many cars that use a regulator mounted on the fuel rail, it will be difficult to place the solenoid ahead of the regulator.

The solenoid and Fuel Pump 2 are operated simultaneously by the one electrical relay, allowing the solenoid to close at the same time as Pump 2 comes into operation. In practice, the fuel pressure change is immediate and precise – the pressure jumps from one regulated fuel pressure to the next regulated fuel pressure.

Switching Between Pressures

To allow the solenoid and the fuel pump to be simultaneously switched (one off and one on), a single pole, double throw relay is used. (Sometimes these are known as changeover relays.)

Click for larger image

The wiring diagram looks like this. The ground of the pump and the solenoid and connected together and then to chassis. (Some solenoids are polarity conscious, ie have positive and negative connections.) The positive terminal of the solenoid is connected to the normally closed output terminal of the relay and the positive of the pump is connected to the relay’s normally open output terminal.  The relay input terminal is connected (via a fuse) to a 12V source. Therefore, depending on the status of the relay, power is fed either to the pump or solenoid and when the relay is not activated, it’s the solenoid that gets the power.

Note that on this diagram the relay is triggered by a manual ‘high/low fuel pressure switch’. In actual fact, this changeover occurs automatically, based on the action of a boost pressure switch, voltage sensing switch, etc. We’ll cover this aspect in more detail next week when we install the system.

Why not Just a Rising Rate Reg?

If you’re wondering why we don’t just use a rising rate fuel pressure regulator and forget all the switching, refer back to last week’s article - Modifying Closed Loop Cars that Need LOTS More Fuel! Part 1.

Different Fuel Systems

In a conventional fuel system that uses a return line, in-tank pump and in-line filter, you need to buy only Pump 2, the one-way valve, solenoid and Reg 2. All these components are readily available, although you may need to do some telephoning before you find some of them. We’ll cover their sourcing in a moment – but first, more on returnless systems.

As described in the ‘Returnless System?’ box above, if your car has the fuel pressure reg located in the fuel tank and just the one line comes forward to the injectors, you’ll also need to fit an external fuel pressure reg and so will need to add Reg 1 to the shopping list. (The system shown in this article was fitted to a car that originally had a returnless fuel system. That’s why a new Reg 1 is needed – it replaces the function of the reg which is mounted in the tank.)

Click for larger image

And there’s another complexity. Returnless fuel systems don’t use manifold-referenced fuel pressure regs. In other words, the fuel pressure doesn’t hold constant headroom above manifold pressure. Instead, the fuel pressure might always be just 45 or 50 psi. (Note: this is higher than the fuel pressures used in conventional EFI systems.) To achieve this fixed, high fuel pressure with a new external Reg 1, you can use an aftermarket regulator or do as we did in  Modifying Returnless Fuel Systems, Part 1 and use the pictured D-Jetronic regulator which is cheap, well made and can be adjusted to give a fixed fuel pressure up to 50 psi.

So that’s Reg 1 – but what about the high pressure reg, Reg 2? Does the second high pressure reg need to be a fixed fuel pressure design or one referenced against manifold pressure? The short answer is that either approach can be followed, but in a turbo engine, referencing Reg 2 against manifold pressure will probably provide a better spread of on-boost air/fuel ratios. All aftermarket adjustable fuel pressure regs are manifold pressure sensed, so sourcing one is easy.


Let’s summarise where we’re going. To build a dual fuel pressure system you’ll need these bits:

Standard Fuel System with a return line and regulator mounted in the engine bay

Returnless Fuel System with in-tank regulator

Regulator 1

Already fitted

New external one must be fitted, together with tank return line

Regulator 2

Required, should be boost sensed in forced aspirated cars

Required, should be boost sensed in forced aspirated cars




Solenoid location

Probably in Reg 1’s return line

Before Reg 1

Pump 2 and one-way valve



The Components

  • High Pressure Pump (Pump 2)

The pump you source must be capable of supply adequate fuel for the engine at the higher than standard fuel pressure. This pressure requirement automatically means a high quality pump – don’t assume that because the standard pump can provide 80 psi when the fuel pressure return line is restricted, it will reliably do so whenever the increased fuel pressure is needed! To consistently provide more than about 50 psi, you’ll need a dedicated pump.

Click for larger image

In our application, we used a Bosch 0 580 254 910 pump, as used in K-Jetronic Mercedes models. This pump is capable of running 90 psi without problems and can push fuel as hard as 130+ psi. Automotive Service Solutions provided the pump at a cost of AUD$179.

Click for larger image

To allow a hose to be attached to its outlet, an adaptor was needed. South Coast Diesel Injection provided this fitting at a cost of AUD$28.

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An adaptor was also needed on the pump inlet to match the outlet hose diameter of Pump 1. Hose Suppliers Australia provided this fitting which cost AUD$2.70.

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  • One-Way Valve

The one-way valve consisted of a  5/16inch in-line aluminium-bodied valve sourced from South Coast Diesel Injection at a cost of AUD$20.

Click for larger image
  • High Pressure Regulator (Reg 2)

For Reg 2 we used a Malpassi fuel pressure regulator. The model we chose is a rising rate design – that is, the fuel pressure is referenced against manifold pressure and fuel pressure rises disproportionately with boost. If the manifold connection is made before the throttle body, the fuel pressure will rise only with boost – it will be unaffected by vacuum. If the manifold connection is made after the throttle body, fuel pressure will fall and rise with vacuum and boost, respectively. Automotive Service Solutions provided the regulator at a cost of AUD$170.

Click for larger image
  • Fuel Solenoid

Fuel shut-off solenoids are used in carby cars being converted to LPG, so these solenoids are available from LPG specialists. The one we bought cost AUD$50 and came from Performance Autogas.

Add in some high pressure fuel hose, lots of brass T-pieces and stainless steel clamps, and you can see that the bill for these parts adds up to about AUD$500. That’s not cheap, but remember it also includes a substantial upgrade in fuel system flow (depending on the in-tank pump, enough for about 350hp) as well as a major increase in pressure.

Next week we’ll install the system in a car.

Parts Suppliers

Automotive Service Solutions +617 5527 9999

South Coast Diesel Injection +617 55325427

Hose Suppliers Australia +617 5532 7833

Performance Autogas +617 55311399

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