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 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.
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
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
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
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.