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Inlet and Exhaust - Fuel Delivery

 - The fuel delivery system goes from the fuel tank to the injectors.
 - The max capacity of the stock fuel system may be reached when modifying.

The fuel 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.

Anytime 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 capacity items.

Injector Duty Cycle.
 - The duty cycle shouldn’t exceed 75 to 85%.
 - Bigger injectors or increased fuel pressure will necessitate a remap.

The injector 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%.


If the 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.

Care must 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.

The correct 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 dyno readout. 

Raising Fuel Pressure.
 - Can be an alternative to buying new injectors.
 - Max of 4 bar should normally be used.

However, 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.

Lowering Fuel Pressure.
 - Never go below 2 bar.

The opposite 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.

Needless to 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.

Fuel Pressure Regulators.
 - Regulates the fuel pressure at the injectors.
 - Many configurations possible for NA and forced induction engines.

Naturally it 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.

There are five common options available:

1. Manifold pressure referenced with the fixed base pressure.
2. Manifold pressure referenced with adjustable base pressure.
3. Non-referenced fixed pressure.
4. Non-referenced adjustable pressure.
5. Adjustable rising rate manifold pressure referenced. 

The big 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.

The last 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.

Fuel Pumps.
 - Used to pump sufficient fuel to the fuel rail.
 - Must be big enough to pump more than the max possible fuel consumption.

High 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.

The fuel 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.

Multiple 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. 

Getting 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.

Fuel 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 1000bhp.

The fuel 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.

Fuel 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.

When a 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.