Fuel Pump Sizing
To achieve proper fuel delivery, you must select the right fuel pump for your vehicle. In most cases, where the engine has been modified only with “bolt on” performance items, there is rarely need for a larger fuel pump or larger injectors. Vehicle manufacturers typically design a “safety factor” into the fuel pump to accommodate the deterioration of the fuel system over time. This safety factor is intended to compensate for a fuel filter that is nearing the end its life, or for deposits in the injector orifice. Our research has revealed that generally there is about a 15%-20% oversize in most factory fuel pumps.
If the engine is enhanced via forced induction or nitrous oxide, the stock fuel pump is inadequate. If the engine’s power is increased more than 15-20% fuel delivery must increase as a factor of the power gain.
The way to determine the proper-size fuel pump is based on the desired brake specific fuel consumption (BSFC) of the engine. This term refers to how much fuel in pounds per hour (pph) the engine consumes per horsepower and is a measure of the efficiency of the engine. It is a useful term in determining the total fuel requirement of the engine.
On vehicles equipped with forced induction or nitrous oxide, higher BSFC’s are required as an added measure of safety to prevent detonation or high combustion chamber temperatures. Below is a guide of BSFC’s with standard CR that AEM uses for various engines that run on gasoline:
• Naturally Aspirated engines have a BSFC of .48 to .50
• Forced Induction engines have a BSFC of .65 to .68
Methanol (alcohol) powered engines require twice the amount of fuel so the BSFC’s are doubled.
Calculating the total fuel requirement of an engine requires simple equations that we outline in the following section. You must know how much power the engine is anticipated to make and we recommend that you guess on the high end. The fuel requirement will be determined in pounds per hour of fuel flow. Since most pumps are rated in gallons/hour you must know the weight of your fuel/gallon. (The vast majority of gasoline based fuels run at 7.25 lbs./gallon.)
The equations to determine your fuel requirement is as follows:
• (Power x BSFC) x (1 + Safety Margin) = pounds/hour
• Pounds/hour / 7.25 = gallons/hour.
An example of this equation is:
• 500 hp gasoline engine using moderate boost with a 30% safety margin
• (500 x .625) x 1.30 = 406.25 lbs./hr.
• 406lbs/7.25 = 56 gallons/hour.
• If the pump that is being considered is rated in liters per hour, use the conversion factor of 3.785l/gallon. The pump described above would be rated at 56 gallons x 3.785 liters = 211.96 liters/hour.
In the fuel pump sizing, always use a safety margin greater than 20%.

Fuel Pump Location
The fuel pump should be located at a level that corresponds the lowest part of the fuel tank. This does NOT mean that the pump should be in a vulnerable position such as hanging below the tank. The pump should also be positioned so that it is protected from the road hazards (speed bumps, curbs, road debris etc.). In the event of an accident, the vehicle structure around the fuel pump should not deform to a point where the pump and its electrical connections are compromised.

The wiring for the fuel pump MUST be rated for the amperage of the pump. As with all high current wiring, a fuse rated for the amperage of the pump should be used. It is always better to err on the large side for the wire size. The ground for the pump must be the same size as the power lead and be mounted to a location that is clean and clear of any undercoating or paint.
Fuel Injectors
The AEM PEMS requires the use of “saturated” or high-impedance fuel injectors. If “Peak and Hold” or low impedance injectors are to be used, an injector resistor must be used or you will damage the ECU. Resistors can be purchased from AEM. The PNP version of the AEM PEMS is configured for the stock injectors and no additional parts are required.
To determine the size of the injectors, the total engine power must be estimated or known. The fuel pump calculations and BSFC information mentioned in the previous section provides a good understanding of the fuel requirements for an engine. The following equation will allow you to determine the requirements of your injectors:
Using the same engine as above:
• ((Power x BSFC) x (1 + Safety Margin))/Number of Injectors = pounds/hour
•
An example of this equation is:
• 6 CYL. engine rated at 500 hp on gasoline using moderate boost with a 15% safety margin on the injector
• 500 x .625 = 313 lbs/6 = 52 lbs/hr/ injector. 52 x 1.15=60lbs/hr/ injector
If we take the flow of the injector (60 lbs/hr) and multiply it by the number of cylinders (6), we arrive at a total of 360 lbs/hr of flow. As you can see, the fuel pump described above has enough capacity to feed the engine with a little room to spare.
It is a good idea to know the maximum operating pressure of the fuel injectors. In some cases the fuel injector will not open if the fuel pressure exceeds the design limit of the injector. Also, at the higher pressures the injector fuel flow may become non-linear and cause inconsistent fuel delivery, usually creating a lean condition. Most injectors can withstand up to 70 psi. Many of the pintle style injectors can withstand higher pressure.
In the fuel injector sizing, always use a safety margin between 15-20%.
Fuel Hoses & Routing
Even with proper injector and fuel pump sizing, a fuel system will not flow adequately unless the hoses that deliver the fuel to the fuel rail are of sufficient size and are routed properly. On systems that use the PNP version of the AEM PEMS, there is no need to replace the fuel delivery hoses unless the engine is heavily modified.
NEVER route fuel hoses through the interior of a car. Put bluntly, this is a dangerous thing to do. Whenever possible, use a delivery tube to make the connection from the pump discharge to the filter in the front of the car. The lines should be rated to withstand at least twice the maximum pressure of the EFI system.
Using the above parameters of our sample engine with moderate boost, we expect to see pressures in the 65-70 psi range. This will require a line with at least 140-psi rating (most AN hoses exceed this by a large margin). When routing fuel lines, it is imperative that they are protected from road hazards and the exhaust system. The fuel line should NEVER be routed near battery cables. Use clamps to secure AN hose every 15 inches, or 24 inches if a rigid tube is used.

The following table will help you determine which hose size is correct for your application: These sizes are based on a nominal fuel pressure of 40 psi.
Fuel Delivery Hose Sizes
Gasoline Powered Engines
Up to 499 HP .344” hose -6AN
500 - 799 HP .437” hose -8 AN
900 – 1100 HP .562” hose -10 AN
Methanol Engines
Up to 499 HP .437” hose -8 AN
500 - 799 HP .562” hose -10 AN
900 – 1100 HP .687” hose -12 AN
The above table should be used for typical passenger car applications. However, for custom applications the hose run length will affect fuel delivery. If you have a long hose run, then the actual flow will have to be determined by running the fuel pump into a graduated cylinder, then measuring the flow vs. time and calculating the flow in gallons per hour (g/h). Also note that if fuel banjos are used in the system be sure they have adequate fuel flow capability.
The fuel return hoses should be one size smaller than the delivery hose. For the sample engine described above, we would use a .437” (-8) delivery hose and a .344” (-6) return hose.
Fuel Filter and Fuel Rail
Often overlooked in EFI installations, the fuel filter must have the capacity, filtering efficiency and burst strength to withstand the pressures of an EFI system. It must be able to flow the amount of fuel that matches the maximum fuel pump output. The filter is always located after the fuel pump, however it does not matter if it is positioned in the front or rear of the vehicle (we prefer to put it toward the front for easy serviceability). AEM carries fuel filters for high-powered engines, which use an easy to find, high volume, replaceable element.
It is imperative that a pre-filter be mounted to the fuel pick up in the tank. These filters are very high volume and create very little pressure drop. The use of a pre-filter ensures long fuel pump life and can eliminate low flow conditions caused by debris entering the pump inlet.
The final link in the fuel delivery system is the fuel rail. The fuel rail should be consistent with, or larger than, the hose size. The additional capacity of a large-diameter fuel rail helps to dampen the pulsations created by the fuel injectors and ensures even fuel delivery under all conditions.

Fuel Pressure Regulator and Pulse Dampener The fuel pressure regulator maintains a constant pressure across the fuel injector. The inlet manifold pressure varies with throttle angle, and engine speed. Small throttle angles and high engine speed produce low manifold pressure (high vacuum). While high throttle angles and low rpm give high manifold pressure. In addition to these conditions, low manifold pressure is associated with idle and high manifold pressure is at full throttle. It is the fuel pressure regulators job to keep a constant fuel pressure across the injector(s) regardless of manifold pressure.
Currently, there are several types of fuel pressure regulators in use. Many late model cars use a return-less system where the fuel pressure regulator is mounted in the fuel tank adjacent to the fuel pump (and therefore requires no return line back to the fuel tank). In most naturally aspirated applications these types of systems are adequate. With forced induction or heavily modified engines, an adjustable fuel pressure regulator with manifold vacuum reference must be fitted.
The two common types of fuel pressure regulators used are non-adjustable and adjustable. As the name implies, a non-adjustable regulator is set at a fixed value and is manifold-vacuum referenced (whenever a regulator is said to be vacuum referenced, this means that the inlet manifold vacuum/pressure is ported into the chamber above the regulator diaphragm).

Spark Plugs
Spark plug selection affects engine performance. On forced induction engines, it is critical that the proper heat range and gap is used. Heat range refers to the ability of the spark plug to conduct heat away from the electrode to the engine. A plug that has high thermal conductivity has a short insulator that comes in contact with a large portion of the metallic plug shell. This large area allows the combustion heat to be carried through the plug shell to the cooling jacket of the cylinder head. In the case of a hot plug, the insulator is recessed deeply into the plug shell with minimal contact to the shell. The plug has low thermal conductivity due to the lack of contact with the shell. The nose of the insulator should operate at between 400 – 850 degrees C. Temperatures above 400 degrees C are desirable because at higher temperatures deposits from carbon, lead or soot are burnt off. Temperatures of 850 degrees C and over should not be exceeded because this is typically the point where detonation or auto ignition can occur. Lower heat range plugs have a higher resistance to auto ignition while higher heat range plugs have less tendency to foul.

The spark plug gap on forced induction engines should be reduced REGARDLESS of the type of ignition system. We have read many instruction manuals for aftermarket ignition systems that recommend that the plug gap be opened up for better flame propagation. Although this recommendation may have had some merit when vehicles had carburetors, it does not apply to modern engines with electronic engine management systems. The smaller gap on forced-induction engines requires less spark energy to arc across the ground and the electrode and has a lesser tendency to misfire under the extreme pressures of a racing engine combustion chamber. Also there are spark plugs made with exotic fine wire highly conductive center electrodes that require less energy to fire such as the Denso Iridium that are well suited to racing conditions. The following is a chart of gap sizes for various engines on gasoline:

Naturally Aspirated up to 11.0:1 CR 1.1mm (.044”)
Naturally Aspirated 11.0:1 to 14.0: 1.8mm (.032”)
Forced Induction to 20-PSI .7mm (.028”)
Forced Induction to 40-PSI .6mm (.022”)

Engine Knock (Detonation) and Preignition
It is important to understand the mechanisms that cause knocking and preignition to set up an ignition map that is suitable for the engine. Auto ignition, also known as knocking, pinging, or detonation, is generally caused by improper combustion in an engine. An internal combustion engine runs properly when the spark-initiated combustion wave expands rapidly but smoothly throughout the combustion chamber. Combustion knock is caused by spontaneous ignition in the hot unburned portion of the fuel mixture (typically referred to as end gas) in the combustion chamber. The remaining charge portion is compressed first by the upward piston movement and then by the moving flame front. Knocking is the almost instantaneous ignition of part of the remaining mixture. This mixture auto ignites because the rapidly rising pressure and temperature caused by the piston movement and the expanding gas from the flame front are sufficient to ignite the remaining gasses. To illustrate the loads imposed on the engine components by knocking, note that normal combustion speeds are about 12-25 m.s-1 while knocking combustion speeds may be as high as 250-300 m.s-1.

If the gasoline-air mixture auto-ignites somewhere in the cylinder (other than at the spark plug) just after spark ignition, the auto-ignition combustion wave can collide with the spark-initiated combustion wave, causing the vibration we hear as a knock or ping. Depending on its intensity, knocking combustion may range from barley audible “pinging” to a rather violent thumping. The point at which the knocking becomes damaging to the engine is dependent on the components used in the engine. If sustained knocking occurs, then the pistons may be damaged. When knocking reaches a violent thump, engine operation should be ceased or at minimum the load and temperature reduced to prevent engine damage. Light knocking that happens during acceleration is less harmful and may not damage the engine. Knocking tendency is increased by the following design or operational characteristics:
• High Engine loads encountered while towing a vehicle.
• Using low octane gasoline in a high-compression engine.
• Too much timing advance for the type of fuel being used.
• Higher air density, (this can be caused by starting a calibration at high altitude and then traveling to a lower one, or the addition of forced induction).
• Increased temperatures and pressure in the combustion chamber due to inadequate engine cooling.
• Excessive inlet air temperature.
• Spark plugs with an improperly high heat range.
• A non-central spark plug location in the combustion chamber.
• An elongated combustion chamber design.
• Too lean of an air/fuel mixture.
The following tuning adjustments can be performed on an engine to reduce or eliminate knocking:
• Reduce ignition timing.
• Verify that the air/fuel mixture is adequate for your engine set up.
• Verify that the spark plugs are of proper heat range.
Preignition
Preignition is the ignition of the charge in the combustion chamber before the spark occurs. This type of ignition is caused by a very hot, or even incandescent surface in the combustion chamber. These “hot spots” can be an overheated spark plug, a glowing remnant of carbon in the chamber or even a hot exhaust valve edge. The preignition condition flame front rapidly expands while the piston is still on its way up the bore. Due to the very high pressure generated by the expanding flame front and the piston approaching TDC, the combustion chamber pressure rises rapidly causing audible knocking. Detonation and preignition typically have a cause and effect relationship; when detonation is prolonged and overheats the spark plug to the point where the tip glows, preignition occurs. Preventative measures can be taken to avoid preignition by using spark plugs with the correct heat range, avoiding detonation by using fuel with the correct octane rating for your application, and when building an engine, ensuring that there are no machined components with sharp corners in the combustion chamber. Also, the cooling system must be in good working condition to effectively cool the combustion chamber. Sustained operation of an engine in either of these conditions can result in severe engine damage.

Manifold Absolute Pressure Sensor (MAP)
Before any discussion is held of what a MAP sensor is, it is necessary to understand what manifold pressure is. The definition of pressure is the force per unit area, thus it is an intensive quantity formed as a ratio force and area. So if a 100-pound force is exerted on a piston that has a total area of 100 in2 the pressure acting on each square inch is 100lb/100 in2 or 1 PSI. If the same force were to be applied to a piston with an area of only 1 in2 the pressure exerted on the piston would be 100 lb/ 1 in2 or 100 PSI. Now consider if a 100-pound person stepped on a nail that has a tip that is only .010” diameter. This would yield a pressure of 10,000 pounds. (100 lb / .010 in2 = 10,000 pressure)
Realistically, there is no such thing as “manifold vacuum,” just low manifold pressure. The average air pressure exerted on Earth under standard conditions is 14.7 psi or 101.325 kilopascals (kpa). An engine ingests air by creating a differential of pressure across the engine via the movement of the pistons in their bores. When a piston moves down the bore, pressure in the bore is reduced. When the inlet valve opens, the awaiting relative higher-pressure air above the inlet valve enters the chamber and fills the void caused by the motion of the piston in the bore. On an engine without a means of forcing air into the engine, the most this pressure can be is whatever the atmospheric pressure of the day is. At sea level the average pressure is 14.7 psi.
It is common to refer to low manifold pressure as vacuum, which is how we will refer to low pressure for the purposes of this discussion. Pressure is measured in two ways: one is absolute pressure, and the other is gauge pressure. The difference between these is where the zero point of each scale starts. With absolute pressure, zero is a complete void of all pressure. With the more common gauge pressure, zero is at standard atmospheric pressure (14.7 psi). Anything below 14.7 psi is referred to as vacuum and anything above that is referred to as boost or positive pressure.
Closed or very small throttle angles are associated with low manifold pressure (a vacuum on the gauge measuring style), and large throttle angles or full throttle is considered high manifold pressure (0 on the gauge measuring style).
Typical boost or vacuum gauges used in automotive applications use the gauge type of readout. In automotive engineering, the absolute method of measuring pressure is used.
We find it easiest to work with the accepted standard of kilopascals (kpa) of absolute pressure. It is important to know the relationships of the various nomenclatures of pressure. The units of pressure in use today are;
• 1 bar (b) = 100 kilopascals (kpa) = 14.5 psi = 29.529” Hg.
• 1 atm. = 101.325 kpa = 14.7 psi = 29.92” Hg = 1.01325 b

The MAP sensor provides manifold pressure information to the ECU for calibration based on MAP vs. RPM. The MAP information is used in both the fuel and ignition Y-axis of their respective maps. On racing engines that use individual runner manifolds, a TPS based fuel map can be used while the ignition can be MAP based. This is desirable because the ignition should always be load based to provide knock free operation.
A MAP sensor reads in absolute pressure, just like the name implies. The amount of pressure indicated by the sensor depends on the amount of voltage feedback delivered to the ECU. As the throttle is opened and closed, or boost is built up in the manifold, the sensor reacts to the changing pressure and outputs a voltage signal to the ECU based on the given pressure.
The MAP sensor has three leads: 5V+ into the MAP sensor, a return to the ECU based upon the resistance of the MAP sensor, and a ground. A MAP sensor MUST have a hose routed to it from the inlet manifold in order to read manifold pressure and it MUST receive a constant pressure signal to it at all times. If the pressure signal fluctuates, the calibration will be adversely affected because the fuel and ignition values will cycle with the MAP signal.
To avoid varying MAP signals, the pressure line must be connected to the intake plenum. In the case of individual runner manifolds, TPS-based mapping is best.
If a MAP sensor is used for boost compensation or load sensitive ignition timing, an accumulator must be used. An accumulator is a common closed container that has a hose from each runner routed to it. An accumulator dampens the pulsing commonly found in this type of manifold set up, which will aid in the delivery of a steady MAP signal (A fuel pressure regulator can have its pressure source routed to an accumulator for the same reasons).
The AEM PEMS has a variety of MAP sensor ranges to suit everything from naturally aspirated engines, to forced induction engines up to whatever boost can be generated by the turbo or supercharger(s).
Mass Air Flow Sensor (MAF)
A Mass Air Flow Sensor (MAF) is used to measure the mass flow of inlet air into the engine using a form of direct measurement. A MAF can be in the form of a gate-style sensor with a door that reacts to airflow, a hot wire that uses a proportional amount of current to keep a filament at a predetermined temperature above ambient, or a vortex generator that uses microwaves to measure vortices shed from an aerodynamic probe in the air stream. All of these devices deliver a signal to the ECU that is proportionate to intake airflow volume. The most common type of MAF is a hot wire, or a variant of a hot wire system. The amount of current required to keep the filament 300 degrees above ambient is directly proportional to the air mass flowing into the engine. Humidity in the air helps cool the wire even further, and the sensor accounts for the effect of moisture. The only drawback to an MAF is that it presents a physical air path restriction in the inlet manifold. Fortunately, the AEM PEMS can be calibrated to accommodate larger mass MAF sensors and allows for the use of different sensors as needed. Like TPS and MAP based systems, an MAF system can be set up to perform the fuel calibration by defining the mass flow as the Y-axis of the fuel map.
O2 (Oxygen) Sensors
There are many types of O2 sensors that are employed by vehicle manufacturers, and it is well beyond the scope of this manual to describe all of them. An O2 sensor provides a reading of the air/fuel ratio (AFR) to the ECU so that it can make the necessary fuel calibration corrections to achieve a desired Air Fuel Ratio (AFR).
An O2 sensor works by sensing whether there is an abundance or lack of oxygen in the exhaust gases, depending on whether the gas mixture is too rich or too lean. If there is excess oxygen and the mixture is too lean, output voltage from the O2 sensor to the ECU will be high. The ECU may then compensate by adding fuel. The converse is true of rich mixtures.

Common O2 sensors include 3-, 4-, and 5-wire heated or wide-band sensors. Three- and 4-wire sensors are ideal for determining whether a vehicle’s AFR is at the optimum stoichiometric ratio. Stoichiometric ratio refers to the ideal mixture of fuel and air by mass to completely consume both reactants (gas and air) with nothing left over. Based on the properties of most pump gasoline used today this ratio is typically a 14.64:1 air/fuel ratio. Although this ratio provides the best combustion characteristics with the least emissions output and optimum catalytic converter performance, it is NOT the best AFR for maximum power at full throttle or under boost. This mixture is too lean and may cause engine damage.
The typical working voltage for most O2 sensors is 200mv to 850mv. The AEM PEMS has a menu of the most common O2 sensors used and it requires that you enter the type of O2 sensor used for your engine. Keep in mind that because 3- and 4-wire O2 sensors respond primarily to stoichiometric AFR, they are less accurate than UEGO sensors for calibration purposes. If this is the only sensor available to you, we recommend that the calibration be done using a lab-type AFR meter, such as the Horiba Mexa 700. Once the calibration is completed, a narrow band O2 sensor can be used to regulate the AFR at idle and part throttle.
There are wide-band, four-wire O2 sensors that have the capability of accurately measuring AFRs. The most common one is used on the Porsche Carrera 4 and is a Bosch unit (part number 0.258.104.002). This sensor is compatible with the AEM PEMS and can be used for calibration purposes.
The most desirable O2 sensor for calibration use is the NTK Universal Exhaust Gas Oxygen (UEGO) sensor. It is a very accurate, fast responding sensor that reads well beyond each side of the stoichiometric ratio. It is a 7-wire sensor that uses a resistor to calibrate its output and has 5 or 6 wires returning to the ECU. The additional wire in the case of a 6-wire unit is a redundant ground wire. The AEM PEMS is also compatible with this type of sensor and it is required for the “auto map” mode of AFR tuning. The Horiba Mexa 700 uses this type of sensor because of its wide response range.
The following is a chart of typical AFRs. Note that every engine is different and this may not be the optimum set up for your particular vehicle. The AFRs on this chart are very conservative to minimize the potential for engine damage. These AFRs depend on the condition that ignition timing and fuel octane are adequate enough to prevent knocking. The very low loads found in the lower right corner occur during deceleration, and because there is no “work” being done by the engine, AFRs during deceleration can be very lean. We have seen AFRs of 20.0:1 during deceleration runs.

The AFR is often referred to as a Lambda (λ) number. A Lambda of 1.00 is equal to the stoichiometric ratio for the reactants in a system. For our purposes, the stoichiometric ratio is the ratio for the amount of ANY fuel used in an internal combustion engine. With Lambda measurement, any number higher than 1.00 is considered lean (more air than necessary to react with the fuel) and any number lower than 1.00 is considered rich.

Although Lambda is the term most often used when working with O2 sensors, we will use AFR in this manual because it is the most common term used when referring to internal combustion engines. The following is a chart for converting Lambda values to AFRs.



Acceleration/Deceleration Modifiers for Engine Fueling
When the throttle is rapidly opened or closed the demand for fuel increases or decreases. If at a low-throttle-angle, steady-state running condition the throttle is opened rapidly, manifold pressure increases. In this situation, the sudden demand for air (hence power) requires a temporary enrichment of the mixture to maintain a reasonable AFR. Because the rapid opening of the throttle is consistent with the need for high-power AFR during acceleration, it is equivalent to the value needed for full power.
The amount of enrichment required is largely dependent on the design of the inlet tract and placement of the injectors. Enrichment for systems where the injectors are placed far from the inlet valves will have to be higher than if the injectors are placed near the inlet valves. This is because when the injectors are far from the inlet valves, such as on throttle body systems, there is considerable manifold wall wetting.
At low manifold pressures (commonly high manifold vacuum), fuel tends to stay in the air stream in a vapor-like state and has relatively low wetting characteristics. The reason the wetting is lower at high vacuum is because the pressure in the inlet manifold is closer to the vapor pressure of the fuel, allowing the fuel to evaporate more readily (This is the same phenomenon that makes water boil at a lower temperature at higher altitudes than at sea level).
As the throttle opens, manifold pressure increases (vacuum decreases), which increases the pressure on the fuel vapor driving it to a more liquid state. This causes droplets of fuel to deposit on the manifold walls and come out of the air stream. When the air speed in the inlet manifold increases to a point where the liquid fuel on the manifold walls is reintroduced into the air stream, there is no need for additional fueling and acceleration fuel is shut off.
With most modern road cars the injectors are placed near the inlet valves so that manifold wall wetting is virtually eliminated. With the elimination of wetting comes the drastic reduction of acceleration fuel requirement. This configuration of fuel injector needs short duration and a small amount of fuel for acceleration enrichment.
The prime input for acceleration data for the ECU is the throttle position sensor (TPS). A secondary input for acceleration data is the MAP sensor. The TPS indicates the rate of change of the throttle plate to the ECU so that it can calculate the amount of fuel in both volume (additional pulse width) and time (duration of additional pulse width). Very rapid throttle movements usually require a short duration of a large amount of fuel, while slow throttle changes use minimal amounts of additional fuel over a longer period.
On cars that employ forced induction, a MAP-based acceleration fuel scheme is recommended. This is usually done with heavy vehicles or vehicles that experience very high loads. The rate of MAP change and MAP value can be used to provide additional fuel if necessary. An example of this type of acceleration fueling is a turbocharged engine that is used to pull a heavy load. A small opening of the throttle will cause a significant boost increase if the turbocharger is small. The TPS, because of the small throttle angle increase, does not provide accurate airflow information to the ECU. If there is a MAP-based acceleration parameter in the calibration, then the additional air supplied by the turbo can be accommodated.

When the throttle is closed rapidly the need for fuel is reduced sharply. Just as the TPS and MAP sensors provide information on increasing TPS or MAP values, they also provide information on decreasing values. Under deceleration manifold pressure is very low (high vacuum). Any fuel that was on the manifold walls, port walls, or valve head is re-introduced into the air stream due to the rapid decrease in manifold pressure resulting in a temporarily rich mixture. The main fuel MAP values are usually very low when experiencing low manifold pressure, so minimal fuel is being injected into the engine. However, the mixture will still be rich due to the re vaporization of the wetted fuel.
The AEM PEMS can be programmed to turn off or nearly turn off the fuel injectors during periods of deceleration. This eliminates after burning in the exhaust manifold, and reduces hydrocarbon emissions. There is a range of RPM and manifold pressure that must be defined by the programmer to turn off the fuel. The RPM defined usually has a lower limit of idle speed plus 300 RPM. The manifold pressure is usually idle pressure minus 15 kpa. There is the possibility that a load value of idle – 15 kpa can be achieved during sustained running, but the throttle angle decrease must be sensed by the ECU to activate deceleration fuel cut. (Click here to see accel and decel fuel parameters set).
Fuel Injector Timing
The activation of the fuel injector should coincide with the flow of air into the cylinder. The advantages to injecting the fuel during this phase of the combustion cycle include a reduction in hydrocarbon (Hc) emissions, better atomization of the fuel, less fuel consumption, and higher power for the fuel consumed (lower BSFC). If the injector sprays fuel while the valve is closed, there is a higher incidence of fuel wetting the back of the inlet valve, which causes poor atomization, and higher Hc emissions.
In most automotive engines the inlet valve opens (IO) slightly before TDC (BTDC) and closes (IC) after BDC (ABDC). The exhaust valve opens (EO) before BDC (BBDC) and closes (EC) after TDC (ATDC). It is a good idea to have information on the valve train operation of your engine. If this information is not available, keep in mind that most engines have an IO in the range of 25-5 degrees and an IC of 40-60 degrees ABDC. The typical exhaust timing would be EC at 5-25 degrees ATDC and EO of 60-40 BBDC. For our discussion on injector timing we will use an inlet valve-opening event of 10 degrees BTDC and an exhaust closing event of 10 ATDC.

During the initial inlet valve opening, the exhaust valve remains slightly open while the exhaust cycle is completed. This valve sequence is referred to as “valve overlap.” During the overlap period the inlet valve remains at high pressure until the exhaust valve closes, creating a momentary backflow into the inlet port. Backflow is more prevalent under low manifold pressure (vacuum) conditions because of the significantly lower pressure in the manifold, compared to the positive pressure created by the end of the exhaust stroke. As throttle is opened, backflow decreases because the differential of pressure is reduced and the higher-velocity inlet air mitigates the backflow to a large extent.

Once the exhaust valve closes, airflow at the inlet port reverses direction and increases in velocity and flow as the inlet valve opens and the piston travels down the bore. It is at this point that fuel should be injected into the combustion chamber.
The AEM PEMS calculates the injector timing relative to the TDC SPARK EVENT, which is 180 degrees + timing prior to the TDC overlap event. Because of this relationship, you MUST ADD 180 to the desired injection firing time.
Lets use our example cam timing from above:
1. EC is at 10° ATDC, which leaves 170° of crank angle to BDC
2. Add 180° for the injector timing event relative to TDC SPARK to arrive at an injector timing number of 350°
In equation format this is:
• (180-EC) + 180 = injection timing.
From the example:
• (180-10)+180=350.
We must emphasize that there are many variables that influence backflow at the inlet valve. The method of calculating the injector firing event described here is an excellent starting point, but we recommend that performance testing is performed for optimal injector timing. The parameters to observe are: Hc count, power, and BSFC. Of course, the lowest Hc and BSFC with the highest power is the most desirable combination.
As engine speed increases, there is less time to inject the fuel into the engine. This is especially true for high throttle angles, because of the respective high injector duty cycles that accompanies full throttle operation.
High injector duty cycle referrers to the amount of time required to inject sufficient fuel into the engine relative to engine speed. Consider that at 6000 RPM an engine has 10 milliseconds (Ms) to complete a revolution (360° rotation) and 20 Ms to complete one cycle (2 complete revolutions). This means that the injector cannot be open more than 20 Ms at 6000 RPM. If the injector needs to be open 20 Ms to provide adequate fuel to the engine, then the duty cycle in this example would be 100%. This condition is known as having the injector go static, which means that it remains fully open with no closing time between injections. At this point there is no appreciable fuel control via the ECU, and the amount of fuel quantity delivered is controlled by the fuel pressure and static flow of the injector.

Effect of Mixture (λ) on Ignition Timing
Generally speaking, air/fuel mixtures that are lower than stoichiometric (&#955;<1) require less ignition timing due to their higher burning speed and, consequently, shorter ignition delay time. The converse is true of leaner mixtures.