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.
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
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
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
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.)
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.
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
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
New external one must be fitted, together with tank return line
Required, should be boost sensed in forced aspirated cars
Required, should be boost sensed in forced aspirated cars
Probably in Reg 1’s return line
Before Reg 1
Pump 2 and one-way valve
- 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.
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.
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.
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
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.
- High Pressure Regulator (Reg
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
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.
Automotive Service Solutions +617 5527 9999 firstname.lastname@example.org
South Coast Diesel Injection +617 55325427
Hose Suppliers Australia +617 5532 7833 email@example.com
Performance Autogas +617 55311399