fuel delivery system goes from the fuel tank to the injectors.
- The max capacity of the stock fuel system may be
reached when modifying.
delivery system is an area that needs careful consideration when upgrading a car
much beyond the factory state of tune. The system starts at the fuel tank and
and ends at the fuel injectors. There are several areas in between these two
points that need careful scrutiny when performing power upgrades.
forced induction is added, or the boost of an existing system is significantly
raised, you can be sure that one or more of the fuel delivery systems will also
have to receive attention and will probably have to be replaced with bigger
- The duty cycle shouldn’t exceed 75 to 85%.
- Bigger injectors or increased fuel pressure will
necessitate a remap.
duty cycle is the percentage of time that the injectors stay open. The more fuel
that is burned the higher the injector duty cycle will be. In general, the suty
cycle should never exceed 75-80%. If you start going beyond these figures due to
upping the power, then you need to replace your injectors with bigger items.
As a rule of
thumb, Bosch injector max is 80%. ND and Rochester injector max is 75% and Lucas
injector max is 85%.
injectors are opened for longer than, then they will begin to experience flow
problems and the closer they get to 100%(continuously open), the more erratic
the flow will become and the injectors risk being overheated and burned out.
also be taken not to go too low with injector duty cycles when running at max
power. A duty cycle of 40-50% at full power is too low and means that your
injectors are too big for your engine.
injectors should be matched to the tuned engine using the engine dyno to run at
max power while observing the duty cycle of the injector using an oscilloscope.
After you have recognised which injectors perform within an acceptable duty
cycle, then choose the injector that gives the max horsepower according to the
- Can be an alternative to buying new injectors.
- Max of 4 bar should normally be used.
buying bigger injectors can be an expensive option. If the existing injectors
are only slightly too small for the power in your engine, then sometimes the
extra fuel flow can be achieved by keeping the stock injectors and upping the
fuel pressure so that more fuel flows for the same duty cycle. In some systems
you can get good flow while upping the fuel pressure by as much as 1bar upto a
max of about 4bar. This is as much pressure as any stock injectors should be
pushed to. Once 4bar fuel pressure is exceeded, injector life is dramatically
cut, and in many cases there is no corresponding rise in fuel flow once the
pressure is pushed beyond 4bar.
- Never go below 2 bar.
is also true, if your injectors are too big (because
you are future proofing your engine for future power increases), then rather
than fitting smaller injectors, you can lower the fuel pressure, and the
injectors will spray less fuel for the same duty cycle. But never lower the
pressure below 2bar, otherwise fuel atomisation suffers. Larger fuel droplets
will be expelled from the injectors which hurts combustion.
say, anytime the injectors are changed or anytime fuel pressure is raised, a
full fuel remap is necessary to maintain correct mixtures and avoid possible
engine damage. But usually a change like this without remapping the engine will
just mean the car will run very rough and because you are dumping excessive fuel
loads into the engine, the risk of short term engine damage is low…. But why
bother unless you gain performance by remapping the engine.
- Regulates the fuel pressure at the injectors.
- Many configurations possible for NA and forced
is important to regulate the fuel pressure and keep it constant during all
engine conditions from low power to max power operations. Otherwise, at lower
than desired pressure the engine would run lean and vice versa. A good fuel
pressure regulator should be used to keep thing constant across the powerband.
five common options available:
pressure referenced with the fixed base pressure.
pressure referenced with adjustable base pressure.
Non-referenced fixed pressure.
Non-referenced adjustable pressure.
Adjustable rising rate manifold pressure referenced.
advantage with manifold referenced regulators with a fixed or adjustable base
pressure is that they increase fuel pressure as manifold pressure increases, so
that in a supercharged engine, smaller injectors can be used, because as the
boost increases, so does the fuel pressure and more fuel is flowed at max boost
using smaller injectors. When mapping the car, the tuner doesn’t have to overly
concern himself with this and just has to open and close the injectors at the
correct rate to maintain the required AFR.
In a NA
engine we can use a non-referenced regulator or an adjustable pressure
non-referenced regulator (more common). With the
latter we can quickly increase or decrease overall fuel into the engine without
remapping the EMU.
regulator listed, the adjustable rising rate manifold pressure referenced type,
is used in low budget and low boost applications. This type of regulator is
added to the fuel return line and overrides the stock injector when boost starts
to rise. This type of regulator means that a remap can be avoided, but the
regulator still needs to be setup on the dyno while carefully watching the AFR.
- Used to pump sufficient fuel to the fuel rail.
- Must be big enough to pump more than the max
possible fuel consumption.
pressure, roller type electric pumps are used to supply the fuel from the tank
to the injectors. The pump must be big enough to pump more that the maximum fuel
needed at max power, so that full fuel pressure is maintained at all times.
enters the pump from the tank. It exits the pump and enters a high-flow fuel
filter containing a paper filter element which traps any impurities that would
otherwise damage the injectors. After the filter, the fuel enters the fuel rail
and goes to the individual injectors.
Fuel Pumps. As an
alternative to one large (and expensive) fuel pump, we
can use two pumps in parallel… the disadvantage is weight. A Hobbs type pressure
switch can be used to bring the additional fuel regulator online when additional
flow and pressure are required.
Fuel to the Pump. Getting fuel
to the fuel pump is another area that needs consideration. In a competition car
with a fuel cell this isnt a problem, but in a road car with a steel tank that
is being used for competition use, the fuel level may fall below the fuel pickup
pipe due to strong G forces – hard cornering/braking/acceleration. When this
happens the pump is pumping air and the engine will run lean and cause damage. The easiest
way around this is to run a second fuel pump that pumps into a small-capacity
swirl pot that is big enough to supply the engine for one minute.
Lines and the Fuel Rail. The suction
line, from the tank to the pump and the pressure line from the pump to the fuel
rail must be of a large enough diameter to flow the required fuel. The bends
should be kept gradual as possible to not unduly restrict fuel flow. Common
sizes used are as follows: ¼ in tube for 170bhp, 3/8 in for 500bhp and ½ in for
rail must also be large enough to process the required fuel. The fuel is forced
to make a 90deg turn inside the fuel rail to enter the injectors so it must have
a bigger internal diameter than the fuel lines. A bigger fuel rail provides a
more generous flow path and stores a greater volume of pressurised fuel for flow
into the injectors.
Injector Positioning. In most road
engines, the injectors are mounted close to the cylinder head or in the head.
They spray fuel directly at the hot inlet valve. This serves to stop the fuel
dropping out of suspension and not burning properly at low rpm or when air speed
is low. It also improves low and mid range throttle response.
second set of injectors are used on forced induced cars they can be mounted
further away from the head. In cars with an individual throttle plate per
cylinder, this causes the fuel to be smashed against the plate and be further
broken down. The fuel will draw heat away from the head while its travelling
into the cylinders… thus cooling the charge density. Another advantage is that
the fuel and air has time to mix before combustion.
When a broad
powerband is required its best to mount one set on injectors in the stock
position and the other set further out. However, a max power engine may respond
better to having both sets of injectors mounted outboard of the throttle plate.
The primary injector slightly closer than the secondary injector.