Electronic Fuel Pressure Increase

This article was first published in 2005.

If your engine is running lean right at the top-end of the power band, there are a few solutions. You can fit larger injectors (expensive and the whole fuel curve then needs recalibration), run your standard injectors for longer duty cycles (but what if the injectors are already flat out?), or increase fuel pressure. Most often rising rate fuel pressure regs are used to do this – they’re the ones that increase fuel pressure at a faster rate than manifold pressure is rising. But most rising rate fuel pressure regs are designed for use on turbo or supercharged cars - what if you have a naturally aspirated car? Or what if a rising rate reg is out of your budget?

One solution is to use an electronically controlled increase in fuel pressure, where above a certain engine load, the fuel pressure jumps to a higher level. The amount that the fuel pressure increases by is easily adjustable, the point at which the fuel pressure increase occurs can easily be tweaked, and the whole thing can cost less than AUD$100 – less by using secondhand parts. However, this approach also has some disadvantages – how big they are depends on your particular situation.

How it’s Done

The way in which the fuel pressure is increased is simple. A 12V solenoid valve is installed in the return line to the tank from the fuel pressure regulator. Plumbed in parallel with the solenoid is a ball valve – a valve that is easily adjusted in its opening.

When the fuel pressure increase is needed, the solenoid shuts, forcing the return line flow to pass through the ball-valve. If the ball-valve is fully open, no change in fuel pressure will occur. However, if the ball valve is partly closed, it will act as a restriction on the return flow, so increasing the pressure ahead of it. By adjusting the opening of the ball-valve, the increase in fuel pressure can be accurately set.

The triggering of the solenoid can be done with an electronic voltage switch, using the signal output of the airflow meter. Alternatively, an electronic frequency switch can be used on the output of frequency-type airflow meters or even on the basis of engine revs.

The Components

The first component you’ll need is a 12V solenoid valve. It needs to be able to handle the pressure and it needs to be compatible with fuel, but both criteria are easily met if you buy a fuel shut-off valve used in LPG conversions of carby cars. A fuel shut-off valve will cost about AUD$50 from an LPG conversion specialist. Make sure you get with it appropriate barbed hose fittings for the hose diameter you’ll be using.

You’ll next need a ball valve – these are available from hydraulic and pneumatics suppliers. Again, you’ll need appropriate sized barbed hose fittings and these are easily obtained from the same source. Cost? Well under AUD$20.

Finally, in the plumbing side of things, you’ll need some appropriate sized fuel hose, two T-pieces (brass is best) and a bunch of clamps.

So that you can see what you’re doing, a fuel pressure gauge is a useful thing to have around. Note that you don’t need a pressure gauge dedicated to measuring fuel – any pressure gauge that goes as high as about 90 psi will be fine. These can often be picked up at secondhand machinery dealers and on eBay – you generally don’t want to buy new, as these gauges can cost heaps.

If your car runs an airflow meter with a voltage output, the best way of switching the solenoid in and out is with the Simple Voltage Switch module – see The Simple Voltage Switch.  This allows the increase in fuel pressure to occur when a certain engine load is reached, which is much better than using (say) a boost pressure switch or a throttle switch.

If the car uses a frequency outputting airflow meter, the Frequency Switch can be used instead. Either approach is cheap and gives very good control over when the fuel pressure increases, and also when the fuel pressure subsequently decreases.

Doing It

So how do you install the system? De-pressurise the fuel system by pulling the fuel pump relay or fuse and running the engine until it stops, then plumb the fuel pressure modification system into place in the fuel pressure regulator return line. If you have a fuel pressure gauge, monitor fuel pressure by using a T-piece to plumb the gauge in ahead of the factory regulator or if it’s easier, ahead of the injector rail.

Make sure that the solenoid and ball valve are both open. Start the car and measure the fuel pressure – it should be standard. Now disconnect power from the solenoid. Use a remote switch to operate the solenoid – you don’t want any sparks near the fuel system, especially as there’s likely to be a little spilt petrol around.

With power removed from the solenoid but the ball valve fully open, fuel pressure should remain standard. Now slowly close the ball valve. Fuel pressure should rise and unless the car has really quick closed-loop learning, the engine should start to stagger as the mixtures are richened. Switch on the solenoid and the fuel pressure and mixtures should immediately return to standard. Leave the ball valve set so that there’s an increase in fuel pressure when the solenoid is switched off.

How Much Extra Pressure? How much extra fuel pressure you use depends on how lean the car is running at full load. However, you shouldn’t exceed 70 psi absolute max without fitting a high performance pump and upgraded lines. Standard fuel pressure in most cars is around 40 psi, so 70 psi represents a major pressure increase...

The next step is to set up the electronic voltage (or frequency) switch. At this stage don’t worry about connecting it to the solenoid – simply get it working so that it clicks over cleanly at the load or revs at which you want the increased fuel pressure. (At the trigger point the LED on the board will light and the relay will click.)

When the electronic switch is activating at the right time, wire the solenoid to the on-board relay so that as the switch clicks over, the solenoid closes. (Full wiring instructions for switching things is covered in the kit instructions.)

You can now go for a drive. Preferably this should be with an assistant who can monitor the fuel pressure gauge and an air/fuel ratio indication (eg a professional air/fuel ratio meter, the Smart Mixture Meter - see Smart Mixture Meter, Part 1 - or even a multimeter monitoring oxygen sensor output voltage). When the switch click-over point is reached, the mixtures should richen. If they’re then too rich, open the ball valve a little. If they’re still too lean, close the ball valve a little.

And that’s basically it!

But what about those disadvantages mentioned earlier? Well, there are two main ones. The first is that the mixtures are enriched in one step – there is a sudden change, rather than a progressive alteration. Secondly, because the fuel pressure increase is caused by a restriction in flow, rather than a second fuel pressure regulator, there will be a change in fuel pressure as load (and so the injectors’ demand for fuel) changes. In practice, this variation in fuel pressure can cause only a small change in air/fuel ratio (eg less than 1 ratio) but how much change there is depends on how much fuel the pump can flow versus how much the injectors take. Either way, by watching the fuel pressure gauge after the solenoid click-over occurs, you’ll be able to see exactly what’s happening.

Conclusion

This approach to enriching top-end mixtures is great if you have a budget car. It’s easily set up and even if it doesn’t have the absolute accuracy of more expensive approaches, it’s still a helluva lot better than running so lean you destroy the engine.

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