Modifying Closed Loop Cars that Need LOTS More Fuel! Part 3

As we’ve covered in the previous two parts of this series (see Modifying Closed Loop Cars that Need LOTS More Fuel! Part 1 and Modifying Closed Loop Cars that Need LOTS More Fuel! Part 2), modifying a car that’s always in closed loop to provide both richer full-load mixtures and cater for a lot more fuel flow can be extremely difficult. However, by using an additional fuel pressure regulator, solenoid valve, one way valve and fuel pump, it’s possible to have a system that can be rapidly switched from standard to high fuel pressure. If this switching occurs at the same time as the oxy sensors are disconnected, the ECU cannot learn its way around the mixture change.

The approach can be used in cars that have conventional fuel return line systems and also, with some further modification, in cars with the newer returnless fuel systems. The car on which the system is being installed in this series initially had a returnless fuel system and so needed modification of the in-tank regulator, the addition of an external fuel pressure reg, the fitting of an external fuel filter, and the installation of a return line back to the tank.

Fitting it all In

So while many people will need to find room only for the new fuel pump, one-way valve, solenoid and fuel pressure regulator (and usually some of those components will mount under the bonnet and some near the fuel tank), in this car, a Toyota Prius hybrid, room needed to be found for:

• Low pressure Bosch regulator
• High pressure Malpassi regulator
• Bosch fuel pump
• One-way valve
• Solenoid valve
• Mazda fuel filter

If it was humanly possible, I wanted to avoid mounting any of these components in the boot, and initially arranged them on a flat plate which was intended to go above the beam rear axle, attached to the underside of the floor. However, no matter how the parts were laid out, this proved to be impossible – there were just too many components to fit in the available space.

An alternative plan was then adopted. The two pressure regs and the solenoid were mounted on a plate positioned across an under-body cavity (a similar space to the muffler cavity but on the opposite side of the car). Located above the beam axle on a separate bracket were the fuel filter, fuel pump and one-way valve. In order that all the components could be mounted on the plate, an extensive series of brackets needed to be made.

The pressure regulator assembly looks like this. The Bosch D-Jetronic regulator is at top right. This pressure regulator replaces the original in-tank reg of the returnless fuel system and in this application is set to 50 psi, the same level as the factory fuel pressure.  (As with the original in-tank pressure reg, it’s not manifold pressure referenced.)

To the left of the D-Jetronic reg is the fuel solenoid, which when closed, stops fuel getting to the inlet of the D-Jetronic reg. In this closed state, fuel is forced to flow through the Malpassi adjustable high pressure reg (bottom left). This pressure reg is set to a much higher fuel pressure (eg 85 psi) and furthermore, as boost pressure rises, fuel pressure rises at an increasing rate. The manifold pressure connection is highlighted with the green arrow.

The Bosch K-Jetronic high pressure fuel pump can be seen at right. It has been sheathed in thin foam rubber before being held to its mounting plate with two powder-coated steel plumbing clamps (about 75 cents each from a plumbing supplies shop!). The rubber helps dampen any vibration from the pump. The one-way valve allows fuel supplied from the in-tank pump to bypass the high pressure pump when the high pressure pump is switched off. At left is the fuel filter which uses the bracket from a Mazda 121 (‘bubble’) and an aftermarket Ryco filter. The T-piece that supplies the fuel to the fuel pressure regulator panel can be just seen at the purple arrow. The green arrow highlights a heat shield that prevents the end of the pump from becoming too warm through radiant heating from the nearby exhaust.

The red hose used on both assemblies is high quality and is rated to 290 psi.

Set Up and Testing

It is absolutely vital that you use a fuel pressure gauge when setting up and testing the system. The gauge doesn’t have to specifically designed for fuel – any good quality gauge that reads up to (say) 100 psi will be fine. As shown here, the gauge is Tee’d into the line between the injectors and the last fuel pressure regulator. It should be mounted on a long hose so that it can be read when the car is moving – it’s safest to use some electrical tape to stick the gauge temporarily to the outside of the windscreen.

If your system uses two new fuel pressure regulators (ie you’re modifying a returnless system), the first step is to set the low fuel pressure. This should match the factory spec. Make sure the solenoid is open and the new high pressure fuel pump is off. Start the engine, check for fuel leaks, then read off the fuel pressure, adjusting it to the required value by altering the low pressure reg’s setting.

The next step is to set the high fuel pressure. The final value that you’ll use will be found in road testing, but at this stage you want to confirm that the fuel pressure switching system is working properly. Start the car, switch off the solenoid and turn on the high pressure pump (a manual single pole, double throw switch lets you do both these steps in one go).  Check for leaks and read the fuel pressure gauge. If the second regulator’s adjustment screw has been wound in, you should read a higher fuel pressure value.

(Note: if the second fuel pressure reg hasn’t been set to a higher pressure than the low pressure reg, the low fuel pressure reading will always be too low – and adjusting the low pressure reg won’t change it! That is, the high pressure reg must always have a starting point which is higher than the low pressure reg’s fuel pressure setting. And another thing: if the high pressure reg is manifold pressure referenced and the car is forced aspirated, the measured fuel pressure will rise with boost. You’ll be able to see this when road testing.)

When you switch to a higher fuel pressure, the idling engine’s note will change – depending on how quick the self-learning is, it may even stall or run obviously very rich. (In the hybrid Prius, the self-learning is so quick that barely a change is detected, even when switching from 50 to 85 psi!) Try switching the fuel pressure up and down a number of times – in each case, the fuel pressure should very rapidly jump from the one regulated pressure to the next, without any overshoots or undershoots.

The next step is on-road testing. It is best to have a volunteer who can read the gauges, or alternatively, can drive. The air/fuel ratio should be monitored – either with a professional wide-band meter, or at a pinch with a Mixture Meter or multimeter working off the standard oxy sensor. (Monitoring the output of the factory sensor isn’t nearly as good as using a professional meter, but as Real World Air/Fuel Ratio Tuning shows, it is vastly better than no mixture monitoring at all.)

In addition to electrical control of the high pressure pump and the solenoid, you’ll also need a way of switching-out the oxy sensor(s). At this point in the testing, this can be as simple as using a switch to disconnect the input signal(s) from the sensor(s). (You can see why you need a second person in the car!)

Drive the car at low fuel pressure and with the oxy sensor(s) connected and check the closed loop system is working correctly. This is indicated by an air/fuel ratio of about 14.7:1 and oxy sensor outputs that oscillate back and forth. Put the car under load (eg by climbing a hill) and then switch out the oxy sensor(s) and at the same time, switch-in the increased fuel pressure. The mixtures should go much richer.

Set the high fuel pressure regulator to provide the required air/fuel ratio when the fuel pressure is high – for example, in a turbo car, 12.5:1 under full load.

If you can accurately measure the air/fuel ratio, make sure that each time the oxy sensors are switched out and the fuel pressure increased, the same air/fuel ratio is obtained. This is very important, as it will show the car is not trying to learn around the changes that you have made. (If you leave the oxy sensor(s) connected when changing the fuel pressure, it’s very likely that after a while you’ll see that when you raise fuel pressure, the mixtures don’t go as rich as they once did.) Do this testing over an hour and perhaps 50-80 kilometres – it’s vital to make sure that self-learning isn’t occurring and this may not be immediately obvious. (This is NOT something to do in a few quick runs on a dyno.)

Depending on whether the high pressure fuel reg is manifold pressure referenced, and what degree of rising rate it is (eg 3:1 or 2:1 or 1.7:1), there will be some variation in high fuel pressure air/fuel ratios. For example, if the fuel pressure is increased at low loads, it’s likely the mixtures will be richer than if the fuel pressure is increased only at high loads. However, normally the enriched mixtures will be needed only at high loads, with the system otherwise staying in closed loop (ie low fuel pressure) operation.

Triggering the Switch

A vital part of getting the system working well is the calibration of the switch-over point – that is, when the fuel pressure increases and the oxy sensor(s) are disconnected.

Let’s assume that the car is a normally naturally aspirated machine that’s been turbo’d. If you make the switchover as soon as the intake manifold is showing (say) 2 psi boost, there are likely to be many times when at part throttle, the mixtures are far richer than needed. This is because of two things - firstly, the mixtures probably won’t go leaner than 14.7:1 at 2 psi boost anyway, and secondly, if the turbo will well-matched to the engine, it’s easily possible to be running 2 psi boost on light throttle, when the enrichment is not needed. So making the switchover with a boost pressure switch is a bad idea.

Another approach is to use The Simple Voltage Switch, which can be used to monitor any voltage-outputting engine sensor. Obvious sensors to use are the airflow meter or throttle position sensor.

It would seem a good idea to monitor throttle position – if you mash your foot to the floor, the mixtures go rich as early as possible... which is good for power. However, while this may work fine in some cars, in others the heightened fuel pressure at such low loads can result in mixtures that are too rich – and there may also be another problem. In the Prius, to gain the correct high-pressure mixtures, the engine must have been running in closed loop for at least a short time before the switch-over is made. This may also apply in other cars.

Perhaps the best approach is to monitor the output of the airflow meter. The trip-point of the Simple Voltage Switch (or, for frequency outputting airflow meters, the Frequency Switch – see AutoSpeed Shop) can be set so that when airflow reaches a point where the closed loop system can no longer maintain stoichiometric air/fuel ratios (ie the air/fuel is just starting to go leaner that 14.7:1), the switch-over to high fuel pressure occurs. That’s of course assuming that you’re happy running stoichiometric air/fuel up to the point where it starts to go leaner! Otherwise, set the switch to click over earlier.

Relays Because of the number of electrical systems needing to be switched simultaneously, it’s likely that you’ll need to use multiple relays to achieve the changeover. In the Prius, a Simple Voltage Switch (SVS) was used to switch out the two oxygen sensors, while a duplicate relay (wired in parallel with the SVS’s on-board relay) was used to power a heavy duty automotive relay that switched power between the solenoid and the fuel pump. Another heavy duty automotive relay was used to feed power to the pump and solenoid, allowing a direct connection to the battery for this high current supply.

Setting the Mixtures

There are four ways in which the high load mixtures can be tuned:

• Alter the value the high pressure reg is set to
• Run a manifold pressure sensed regulator (normally boost sensed on a turbo car)
• Change the degree of rising rate of the high pressure regulator (ie how much it increases fuel pressure for a given manifold pressure increase)
• Alter the fuel pressure switch-over point

Even playing with all these variables, you’re unlikely to get an air/fuel ratio that’s as accurately tuneable as with programmable management or an interceptor used on a car that’s not always in closed loop. (So why not just run the programmable management or interceptor? Aaagh, for the latter, go back to Part 1. For the former, running a standalone programmable management unit is becoming increasingly difficult – impossible in some cars – if you want full functionality of the other systems to be retained.)

However, switching-in the higher fuel pressure (and the oxy sensors out) on the basis of airflow meter signal provided an air fuel ratio on the test car that went from a closed loop stoichiometric to a rich air/fuel ratio that stayed within half a ratio of 12:1 over the high load range.

Series Conclusion

In a car that always holds stoichiometric mixtures (ie about 14.7:1) and remains in closed loop 100 per cent of the time, making changes to mixtures is very difficult. In fact, it’s likely that altering the airflow meter signal, altering the oxygen sensor signal, changing fuel pressure or changing injector size will all result in no long-term change to the air/fuel mixtures. In other words, the car will always learn its way back to standard.

It’s therefore important that the ECU always sees standard operating conditions. That is, in normal use, the ECU is not trying to learn around changes.

When changed mixtures are needed, the oxygen sensors need to be switched-out. If the fuel pressure is then altered, the air/fuel ratio will be able to be set at whatever is desired. In the most common application, an enrichment in mixtures will be wanted at high loads.

To achieve this, an additional fuel pressure regulator, solenoid and fuel pump can be used. (In cars with returnless fuel systems, additionally the internal tank regulator will need to be modified, an external regulator added, and a return line back to the tank installed.) In this way, higher fuel pressures can be electrically activated.

The switch-over to enriched mixtures is best done by using a voltage switch monitoring the airflow meter output. When airflow exceeds the trip point, the low pressure regulator is bypassed, the high pressure regulator comes into action, and the oxygen sensor(s) are switched out.

The mixture which occurs in high fuel pressure mode is determined by the fuel pressure used. In turbo cars, some changes with load can be achieved by using a rising rate fuel pressure reg.