Introduction

What makes engine management tricky is that even the best theoretical models fail to accurately represent physical behavior in certain situations.  All engine controllers would use the same logic and procedures for running the engine 100% of the time if there was a perfect engine control strategy.  Instead, most engine management schemes incorporate several different modes of operation in which different sensors dictate fuel and timing requirements.  Also, engine controllers have specific logic dictating when to switch between different modes of operation based on different demands from the driver and different engine conditions.  Some of the most common mistakes made by people starting out (hell, even experienced tuners too) are changing some of the “main” functions in order to try to fix a problem that is being caused by a secondary table or the computer operating outside its normal mode(s).  Better understanding of the various modes of operation will help pinpoint what needs to be changed in a tune.

It would probably be a good idea for you to have read the other articles about Injectors, Speed-Density, Mass Air Flow, and Alpha-N before reading the rest of this.

Some basic vocabulary:

• ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
• RPM : Revolutions Per Minute – how fast the motor is spinning
• MAP : Manifold Absolute Pressure – (usually) the pressure of air entering the motor
• ECT : Engine Coolant Temperature sensor – sensor used to measure the temperature of coolant circulating through a motor.  Sometimes called different things by different manufacturers.  I will use ECT here
• IAT : Intake Air Temperature sensor – sensor used to measure the temperature of air entering the motor.  Sometimes called different things by different manufacturers – I will use IAT here.
• MAF : Usually used as a shorthand for Mass Air Flow Sensor / Meter
• MAP : Manifold Absolute Pressure Sensor – a sensor that measures the pressure of air in the intake manifold
• Idle Valve : A electromechanical valve controlled by the ECM that allows air into the engine in order to control engine speed.
• Displacement : the volume swept by a piston descending from the top to the bottom of the cylinder bore.  More here.
• AFR : Air Fuel Ratio – the ratio of air to fuel present in a combustible mixture.  Usually stated as a ratio, i.e. 14.7:1 for the stoichiometric AFR for gasoline.  Stoichiometric AFR varies from fuel to fuel.
• Lambda : similar to AFR, except usually expressed as a number where 1.0 represents a stoichiometric mixture for all fuels.  Lambda and AFR are the same concept expressed in different units.
• Stoichiometric : a mixture containing the precise amount of oxidants required for complete combustion of all fuel present.  See here or here for more information on chemistry involved.
• Injector : a special type of solenoid that allows fuel to flow through it when energized (more)
• Pulsewidth : the length of time the engine computer applies electricity to the injector, or how long the injector is commanded to be open
• Flow Rate : The amount of fuel an injector flows once open.  These values are typically given in units of cc/min or lbs/hr at a specified fuel pressure. (injector flow rate varies with the square root of fuel pressure.)
• Latency : the length of time after the injector is turned on before it achieves its linear flow rate.

Goals of Engine Management

Although the answer is somewhat obvious (“make the engine run as well as it can”) it is worth a closer look at what engine management systems try to achieve and why.  Operating optimally normally means one of several things:

1. Making the most power possible without engine damage happening
2. Consuming as little fuel as possible in order to make a specific power output (maximizing efficiency)
3. Minimizing emissions

Most of the time engine management systems aim for more than one of these at once, i.e. Fuel efficiency while minimizing emissions or power and efficiency.  Generally, you cannot have your cake and eat it too when it comes to engine management because the physical conditions required to achieve optimal fuel economy are vastly different than those required to achieve optimal power production.  Minimizing emissions frequently conflicts with BOTH power and economy!

So how do engine management systems deal with the conflicting requirements of economy, emissions and power?  The answer is the title of this section – engine management systems switch between different modes of operation based on input from the driver, measurements from sensors and how they are programmed from the factory.  Sometimes in the course of tuning it is necessary to change not only configuration parameters of an ECM but also how it switches from one mode of operation to another.

Common Modes of Operation

Different ECMs will have different modes of operation and different rules for switching among them.  Many modes of operation exist to service requirements common to all engines, leading to many modes of operation being shared between different engine management implementations:

• Cranking: This is the first task for an ECM – help an engine transition from being spun by the starter to spinning on its own propelled by combustion.  This might not sound like a very difficult task, but there is a LOT involved!  While cranking, cam and crank sensors need to be monitored so the ECM can determine how fast the motor is spinning and what angle the crankshaft is at in order to provide accurate ignition timing, injectors have to be fired in order to deliver enough fuel to get the engine moving, the ignition system has to deliver sparks at an opportune time to ignite the mixture, (in some cases) the idle valve needs to be opened to allow enough air into the motor to get it running on its own…  And more sometimes!  Combine this with (typically) the lowest operating voltages because the alternator is not providing electrical energy and you have a potentially tricky situation.
• Startup: Once the engine is spinning under its own power, the fun can start.  There are often special rules that change the behavior of the ECU immediately after the engine starts.  Idling higher to prevent stalling is a common task in startup mode.  Many engines add additional fuel and change timing in order to try to help the engine warm up to desired operating temperature faster.
• Open Loop: This is a critical mode for the overall operation of the engine.  Open-loop mode is the mode used most often for performance, but it is important all the time.  Open loop operation uses a control strategy like MAF, Speed-Density or Alpha-N to determine fueling and ignition parameters to use to run the engine.  If tuning parameters related to open loop are incorrect, the motor will never run optimally.
• Closed Loop: This is an important mode for fuel economy and emissions.  In Closed Loop mode, the fueling and ignition values from Open loop are adjusted using feedback from additional sensors (usually Oxygen sensors).  Small imperfections in a tune can be corrected in closed loop, letting the ECU maintain much closer control over operating conditions than is possible with open loop alone.  There are usually limits to how large changes can be made by closed loop, which can lead to diagnostic error codes.  (Too Lean / Too Rich / O2 sensor)
• Power Enrichment: (aka “PE” mode) This is a subset of Open Loop operation where engine conditions such as AFR and ignition timing are adjusted with the goal of maximizing power.  Frequently, TPS readings close to wide open throttle serve as a trigger for PE mode.
• Tip-in: Sudden changes are a problem for all control strategies.  When the TPS sensor indicates the throttle has changed quickly enough, the ECU can enter Tip-in mode where
• Decel Fuel Cut Off (DFCO): When you take your foot off the gas, many ECMs will shut off fuel injectors in order to decrease fuel consumption and help promote engine braking.
• Dashpot: Many ECUs implement some form of digital dashpot using the Idle Valve.  The idea here is to prevent stalling when the throttle plate closes suddenly by opening the idle valve enough to gradually bring the engine to idle.
• Idle: At idle, the ECU tries to maintain engine speed while little or no load is placed on the engine.  Idle is often one of the trickiest states to control well.  Usually a mixture of airflow control (via Idle Valve or Drive-by-Wire), spark control and fuel control is used.  Strategies for controlling idle vary immensely among manufacturers.
• Limiting/Protection: Engines have limits – how fast they can safely spin, how much boost they can handle, how fast the car can safely travel.  Part of the ECM’s job is to monitor engine conditions and take measures before damage occurs.  Frequently, spark or fuel will be cut off until engine conditions return below a pre-set limit.

More to come later on this topic…

Introduction

Before you read this, you should already have read the articles on Injector Theory and Speed-Density.  This article will not make much sense without the background information in those articles.

First, vocabulary:

• ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
• RPM : Revolutions Per Minute – how fast the motor is spinning
• MAP : Manifold Absolute Pressure – (usually) the pressure of air entering the motor
• ECT : Engine Coolant Temperature sensor – sensor used to measure the temperature of coolant circulating through a motor.  Sometimes called different things by different manufacturers.  I will use ECT here
• IAT : Intake Air Temperature sensor – sensor used to measure the temperature of air entering the motor.  Sometimes called different things by different manufacturers.  I will use IAT here.
• Displacement : the volume swept by a piston descending from the top to the bottom of the cylinder bore.  More here.
• AFR : Air Fuel Ratio – the ratio of air to fuel present in a combustible mixture.  Usually stated as a ratio, i.e. 14.7:1 for the stoichiometric AFR for gasoline.  Stoichiometric AFR varies from fuel to fuel.
• Lambda : similar to AFR, except usually expressed as a number where 1.0 represents a stoichiometric mixture for all fuels.  Lambda and AFR are the same concept expressed in different units.
• Stoichiometric : a mixture containing the precise amount of oxidants required for complete combustion of all fuel present.  See here or here for more information on chemistry involved.
• Ideal Gas Law : PV= nRT (Pressure times Volume equals moles of gas times ideal gas constant times temperature)  More to be read about this here.
• Moles : a measure of how many atoms are present.  See here.
• Induction stoke :  the part of a 4-stroke engine’s cycle in which air is drawn into the cylinder by the piston.  See here for more information if you are not familiar with a 4 stroke engine’s operation.

Many ECMs (particularly older ones) use extremely slow processors to run an engine, especially by today’s standards.  In addition to doing all the math required by Speed-Density to calculate airflow, the processor often has many other extremely timing or IO-intensive tasks, such as processing crank and cam sensor inputs, firing spark plugs and firing injectors.  Additionally, most of these processors lacked floating-point units (short explanation: pieces of a chip that understand what fractions and decimals are) limiting their ability to accurately represent a model that involved lots of numbers with a fractional component.   Bottom line: engineers had to come up with ways to simplify and speed up the math involved in speed density in order to get older, slower, cheap microcontrollers to be able to run an engine.

Obviously, different manufacturers implement things differently.  In the remainder of this article, we are going to explore briefly how Honda and GM simplified the ideal speed density system to make it more practical to implement on cheap hardware.

GM: Base Pulse Width (BPW)

Ideally, n = PV /RT and then injector pulse = n / injector flow constant

GM introduce the concept of “Base Pulse Width” or BPW to reduce the “V” and “R” terms.  Basically, the BPW is how long the injectors need to be open in order to fill cylinders at 100% volumetric efficiency at a standardized temperature.  The BPW is then multiplied by the Volumetric Efficiency table (which is no longer a VE table in the ideal sense of the word) to determine fueling at different load and RPM conditions.  This is then modified further by coolant and intake air correction tables to account for temperature.  This cuts the number of math operations more or less in half.  The idea behind Speed-Density is being applied in a way that is less math-intensive.

Honda: Required Fuel Value (ReqFuel)

Ideally, n = PV /RT and then injector pulse = n / injector flow constant

Honda took a different approach to the problem of simplifying Speed-Density.  Basically, the MAP sensor and RPM values measured by the ECU are used to index a LUT that contains (more or less) a desired fueling value.  Looking at the math above, Honda essentially pulls the final desired injector pulse (n / injector flow constant) out of a table.  This required fueling value is then scaled by various tables indexed by ECT and IAT which attempt to correct for variations in air temperature.  Honda reduces about half a dozen math operations to one table lookup and a couple of additional easy math operations.  Again, the principles of Speed-Density are being applied in a non-ideal way that attempts to capture what is going on in a way that is fast to implement on slow chips.

Introduction

Be prepared to do a lot of reading in the numerous side links on this page.  More information that is beyond the scope of this overview will be available.

“Mass Air Flow” (MAF, for short) is a method of measuring airflow into an engine in order to supply an appropriate amount of fuel and adequate spark timing. First, vocabulary:

• ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
• MAF : Usually used as a shorthand for Mass Air Flow Sensor / Meter
• Vane Air Flow Meter (VAFM, “Flapper” type meter) : An early type of air meter rarely used today that relies on air pressing against a metering plate (“flapper”) to provide an airflow signal
• Karman Vortex air meter : A type of air meter that not used very much anymore that creates and counts vortexes (air disturbances) in order to measure airflow.
• Hot-Wire MAF : A type of MAF Meter that uses a thin wire heated by an electric current to directly measure air mass.  The most common type of MAF today
• Hot Film MAF : A type of MAF Meter that uses a metal film heated by an electric current to directly measure air mass.  Another type of MAF that is found today.
• TPS : Throttle Position Sensor
• MAP : Manifold Absolute Pressure Sensor – a sensor that measures the pressure of air in the intake manifold
• Displacement : the volume swept by a piston descending from the top to the bottom of the cylinder bore. More here.
• AFR : Air Fuel Ratio – the ratio of air to fuel present in a combustible mixture. Usually stated as a ratio, i.e. 14.7:1 for the stoichiometric AFR for gasoline. Stoichiometric AFR varies from fuel to fuel.
• Lambda : similar to AFR, except usually expressed as a number where 1.0 represents a stoichiometric mixture for all fuels.  Lambda and AFR are the same concept expressed in different units.

Types of MAF Meters and General Operating Principles

Hot Wire MAFs and Hot Film MAFs are the dominant technology in use today.  Earlier style meters (Vane/Flapper, Karman) required an external temperature sensor in order to provide a meaningful airflow reading.  Hot Wire and Hot Film sensors are often found coupled with a dedicated air temperature sensor but they do not strictly require one because the method in which they generate a signal accounts for the temperature of the air they meter.  If you want to learn more about meters, read up here.

ECMs generally have a routine (usually called the “MAF transfer function” or something similar) that converts the raw sensor readings into an airflow value. Sometimes this is a real-world unit (such as g/s or lb/hr) and sometimes it is a purely arbitrary synthetic unit that merely defines the shape of the curve. MAF transfer functions for hotwire MAFs are usually an exponential curve. The shape of the curve is usually determined by the physical characteristics of the sensor. The metering range of the sensor is usually determined by the cross-sectional area of the housing it is in. This means that an easy way to increase the amount of air a given MAF can meter is to put it in a pipe with a larger cross-sectional area. The new MAF transfer function can be approximated (usually fairly closely) by multiplying the old transfer function by the difference in cross sectional area.

Example Question: a meter in a 2″ diameter round housing can meter 1000g/s. The same meter in a 4″ diameter round housing will measure how much air?

Answer: First, find cross-sectional area of 2″ diameter pipe.  Area of circle = pi * r^2.  Diameter = 2 * radius. Radius = 1″, area = 1 * pi.  Second, find cross-sectional area of 4″ diameter pipe.  Area = 4 * pi.  New area / Old area = 4 / 1 = 4.  Multiply original airflow (1000g/s) by ratio of area (4) to get maximum value of 4000g/s.  Note that each individual point in a MAF transfer function can be multiplied in this manner to rescale.

MAF Systems

From here on in this guide, “MAF” and “MAF Systems” will refer exclusively to systems using Hot Wire MAFs and Hot Film MAFs. The reason for this is pretty simple: these type of sensors (at least theoretically) are capable of measuring air mass without the need for significant compensation for air density (i.e. altitude changes, forced induction, changes in air temperature).  In practice, many control strategies use other sensors to try to increase the accuracy of the MAF by additional adjustments but it is not strictly necessary.  MAF sensors do not know what “boost” or “vacuum” are – they deal exclusively with airflow.  If you are trying to make the transition from tuning mostly Speed-Density systems to MAF Systems, be very cautious with timing values as the same trends and rules do not apply to both systems.

Fueling with a MAF system is about as simple as it can get.  It goes something like this:

1. The raw sensor output is converted to an airlow value
2. The next step after determining airflow is to figure out how much fuel is needed to achieve a “target” AFR (more on AFR targets later) which is usually achieved by multiplying by AFR expressed cleverly (see footnote)
3. Finally, the desired fuel value is achieved by multiplying/dividing by a value (injector constant, injector slope, async BPW, …) to account for injector size.  Also any battery compensation is added.  (See Theory: An Injector Model for more information)
4. Done!  At this point, we have an injector pulsewidth!  PulseOut = (MAF_Transfer(RawMAFSensor) * TargetLambda * injector size) + injector latency

There is no “standard” way of doing timing with a MAF system, but all variants basically calculate a value that represents how much air is entering the cylinder each time the motor turns over.  It goes something like this:

1. Start with the same airflow value from step one of fueling. (MAF signal -> MAF Transfer)  This tells us the amount of airflow per unit time.
2. Measure how fast the motor is spinning (RPM) and from this calculate how many revolutions happened during the same time frame as our MAF sample.
3. Multiply/divide airflow by engine revolutions to get airflow / rev.  Most engine management stops here (GM, Subaru, Mitsufeces, …) and spark tables are indexed in grams/rev.  This is a measure of engine load (with a lowercase “l” to denote that we are talking about something different than “Load”, explained next)
4. Ford (and others?) instead use a “Load” (with an uppercase “L” to denote that we are talking about something different than “load”) value that is calculated by multiply/dividing airflow/rev by engine displacement to get a measure of how full the cylinders are relative to their maximum capacity naturally aspirated at sea level with certain air conditions.  If you’re at all familiar with Speed-Density, this should sound somewhat familiar because it is a concept VERY similar to Volumetric Efficiency.
5. Timing tables are usually in the form RPM x calculated load.  MAF timing tables will display a very different characteristic shape than RPM x MAP tables common in Speed-density systems.

Now that we have some concept of cylinder filling (“Load” or “load”), we should return to a piece of how fueling happens in a MAF system: target lambda/AFR.  Usually the same measure used to determine appropriate ignition timing is used to determine an appropriate target AFR/lambda.  In these cases, there is a table that dictates target lambda/AFR indexed by RPM and load.  Sometimes, RPM and TPS is used to determing target AFR instead of calculated load.

Strengths of Mass Air Flow

1. Extremely accurate fueling and spark delivery across a diverse range of engine conditions (at least while in steady-states): the holy grail for engine management. A properly set-up MAF system can adapt to changes in weather and altitude with ease.
2. Minor changes to engine equipment (i.e. headers, minor camshaft changes, intakes that do not significantly alter the placement of the MAF) do not require recalibration of the ECM.

Weaknesses of Mass Air Flow

MAF systems are known for having these issues:

1. MAF systems are extremely intolerant of vacuum leaks.  Any leaks between MAF sensor and engine generally cause all manner of odd problems, running lean in most cases due to un-metered air making it into the engine.
2. MAF sensors can be extremely sensitive to how they are “clocked” – merely rotating the sensor at a given spot in the intake tract can be sufficient to significantly change its output.
3. MAF sensors require laminar flow to read 100% accurately.  True laminar fluids do not exist so this introduces some degree of inaccuracy to MAF sensor readings.  Placing MAF sensors near bends, size transitions or obstructions where flow is less laminar greatly magnifies this issue.
4. A MAF sensor can be a flow restriction in cases where the MAF housing is the smallest portion of the intake system.
5. Hot-wire MAF elements are very fragile.  Debris can destroy delicate wires easily.  Dirt and oil deposits can build up on the sensor element, adversely affecting readings.
6. MAF systems have a relatively poor response to transient conditions, such as sudden throttle changes.  This is explained by the time it takes air to move from the MAF sensor where it is measured to the cylinder where it can be involved in combustion.
7. MAF sensors are not “one-way” sensors – reversion from a camshaft with large amounts of overlap can cause air to be metered on its way in to the engine and then again on its way out resulting in an artificially high MAF reading.  This can almost always be fixed by placing the MAF sensor sufficiently far from the throttle body, however doing so comes at the expense of making transient response even worse.

It may seem like there are a lot of weaknesses of MAF systems, but it is truly hard to emphasize just how amazing and important the strengths are.  It is no secret that the majority of OEMs today are implementing MAF systems as the primary control strategy.  There is a good reason for this, namely that engines can be controlled much more precisely (with the goal of meeting stricter and stricter emissions standards) with a MAF system than any other type of control strategy.

Read the rest of this entry »

Introduction

If you do not have a strong background in physics and chemistry, be prepared to do a lot of reading in the numerous side links on this page.  This isn’t intended to be a brutal presentation of the topic, but theoretical concepts are necessary in order to be able to understand what is going on behind the scenes.

Speed-Density is a method of estimating airflow into an engine in order to supply an appropriate amount of fuel and adequate spark timing.  First, vocabulary:

• ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
• RPM : Revolutions Per Minute – how fast the motor is spinning
• MAP : Manifold Absolute Pressure – (usually) the pressure of air entering the motor
• ECT : Engine Coolant Temperature sensor – sensor used to measure the temperature of coolant circulating through a motor.  Sometimes called different things by different manufacturers.  I will use ECT here
• IAT : Intake Air Temperature sensor – sensor used to measure the temperature of air entering the motor.  Sometimes called different things by different manufacturers.  I will use IAT here.
• Displacement : the volume swept by a piston descending from the top to the bottom of the cylinder bore.  More here.
• AFR : Air Fuel Ratio – the ratio of air to fuel present in a combustible mixture.  Usually stated as a ratio, i.e. 14.7:1 for the stoichiometric AFR for gasoline.  Stoichiometric AFR varies from fuel to fuel.
• Lambda : similar to AFR, except usually expressed as a number where 1.0 represents a stoichiometric mixture for all fuels.  Lambda and AFR are the same concept expressed in different units.
• Stoichiometric : a mixture containing the precise amount of oxidants required for complete combustion of all fuel present.  See here or here for more information on chemistry involved.
• Ideal Gas Law : PV= nRT (Pressure times Volume equals moles of gas times ideal gas constant times temperature)  More to be read about this here.
• Moles : a measure of how many atoms are present.  See here.
• Induction stoke :  the part of a 4-stroke engine’s cycle in which air is drawn into the cylinder by the piston.  See here for more information if you are not familiar with a 4 stroke engine’s operation.

Basic Goals and Method

The goal of Speed-Density is to accurately predict the amount of air ingested by an engine during the induction stroke. This information is then used to calculate how much fuel needs to be provided and may also be used for determining an appropriate amount of ignition advance.

The theoretical basis for this is the Ideal Gaw Law (more here.) rearranged to solve for “n” (the number of moles of gas present :

• n = PV / RT

In order to use n = PV / RT to calculate the amount of air a motor ingests during the induction stroke we would need:

• P is pressure in the cylinder immediately after the intake valves close.
• V is volume, which we know from engine displacement.
• R we know (it’s the Ideal Gas Constant see here for more)
• T is the temperature of the gas in the cylinder immediately after the intake valves close.

Many of the things required to calculate the amount of air the engine ingests using the ideal gas law are missing, unavailable or at least incomplete.  Some notable points where reality is less than ideal:

1. Our MAP sensor measures the pressure differential caused by the downward stroke of the piston in the intake manifold, not pressure in the cylinder as the intake valves initially close.
2. We are assuming that there is no residual exhaust left in the chamber to contribute to “poisoning” of the intake charge.
3. Camshaft overlap (i.e. when both intake an exhaust valve are open simultaneously – see here) makes fluid flow modeling considerably more complicated.
4. T that we need is the temperature of the gas in the cylinder.  This is not usually MEASURED – instead it is ESTIMATED from the temperature of air in the manifold (IAT), the temperature of the cylinder heads (ECT) and other factors.  “RT” is often referred to as the “density correction term” as it tries to account for how air density varies with temperature.  Density correction is arguably one of the biggest problems with speed-density. (more on this later)

Speed-Density introduces the concept of Volumetric Efficiency (VE) to account for the differences between what it can observe and what is really going on.  (mostly problems 1-3 above)  Roughly speaking, VE is the ratio between the amount of air actually present in the cylinder and the amount of air we predict would be in the cylinder using MANIFOLD pressure (MAP) instead of cylinder pressure for our “P” Pressure term, REVOLUTIONS Per Minute (RPM) times Displacement (Volume / REVOLUTION) for our “V” term and an air temperature value estimated from some combination of ECT and IAT for our “T” term.

A motor said to be operating with 100% VE has the same amount of air actually in the cylinder as predicted by n = PV / RT.  Most engines operate at considerably less than 100% VE in most operating conditions.  The difference between actual airflow and theoretical maximum airflow is termed “pumping loss.”  Some engines (most notably Honda engines ) can achieve slightly greater than 100% VE in certain conditions.  Most engines operating under forced induction can be thought of to have a VE greater than 100% in some conditions.

Speed Density ECMs generally have one or more VOLUMETRIC EFFICIENCY (VE) tables that are a critical item to be adjusted.  These tables allow predicted airflow values to be more closely adjusted to observed reality.

Strengths of Speed-Density

Speed-Density has many things going for it:

1. Pressure sensors do not pose any restriction to the flow of air into the engine, unlike a MAF sensor.
2. MAP sensors respond to changing conditions very quickly, enabling it to have fairly good transient response especially compared to Mass-Air-Flow
3. Compared to a carburetor, it allows much more control over the mixture at different engine loads
4. Simplicity: all the sensors required are extremely reliable.

Weaknesses of Speed-Density

Speed-Density is known for having several notable issues:

1. Density correction, density correction, density correction.  You might not think that temperature is that big of a deal, but trust me it is!  Seasonal changes can wreak havoc on speed-density systems.  Superchargers or turbochargers that compress air and raise its temperature from adiabatic heating cause significant changes in density that must be accounted for.  Altitude can also be really problematic.  Many systems incorporate Barometric Pressure sensors to try to address this, but it’s an imperfect correction.
2. Large camshafts with extremely low vacuum due to high overlap close to idle.  Camshafts that have low or pulsing vacuum close to idle present a challenge for Speed-Density.  MAP sensor averaging can help.  Alpha-N blending can help.  It is still very tricky to use speed density to predict airflow with a pressure sensor with camshafts that do not build an appreciable amount of vacuum.
3. Volumetric Efficiency tables can be very time consuming to tune.
4. Engine modifications generally produce volumetric efficiency changes requiring re-tuning.
5. Quite a lot of math is required to do Speed-Density “by the book.”  Because of this, most manufacturers implement something kind of like theoretical speed-density and cut corners or combine math operations in order to allow faster execution on puny computing hardware.  (Remember, most ECUs made prior to 2000 have a slower processor than the average inexpensive cellphone circa 2010)

Sanity Checking a Speed-Density Tune

There are a few rules that transcend particular manufacturer implementations:

1. Volumetric efficiency rarely changes suddenly.  VE tables should almost always have very gradual changes.
2. VE usually DECREASES as pressure DECREASES (i.e. more vacuum = less VE)
3. VE usually maxes out at an RPM close to peak engine torque at maximum observed load, which is usually where peak cylinder filling occurs.
4. Remember that VE tables are not the only thing that controls fueling.  Temperature correction tables (ECT, IAT) are often implemented as multiplier/divider tables.  Don’t forget about injector battery tables either! (see the separate article on Injector Tuning for more on this)

This is going to be rather brief because Alpha-N is rarely used as the primary strategy for engine management today.  You should at least know what it is because it is often used as a “sanity check” for primary control strategies.

Alpha-N is also sometimes called “TPS maps” because the only sensor that is used for determination of fueling is the Throttle Position Sensor.  (And measured RPM, or how fast the motor is spinning)  Fuel and timing requirements for the engine are expressed as a function of RPM and TPS.

Alpha-N is used most of the time in tricky situations:

1. When the MAP sensor or MAF sensor has failed and the primary control strategy is deemed to be invalid.  Something-is-better-than-nothing is the idea.  (“Load with Failed MAF” is an example from Ford-land)
2. In conjunction with ITBs (Individual Throttle Bodies) due to the extremely low vacuum created by them (making Speed-Density tricky) and the desire to avoid needing to fit a potentially restrictive Mass Air Flow sensor (making MAF impossible).  Again, something-is-better-than-nothing is the idea.
3. In conjunction with ITBs and MAP as a load multiplier. (PowerFC D-Jetro for GTR Skyline, most notable example)  ITBs + Boost – Alpha-N output is multiplied by a MAP sensor to come up with a composite load index.
4. In conjuction with Speed-Density and some kind of blending algorithm.  This approach is often used with very large camshafts that pull little vacuum at idle.  Basically, TPS and MAP are allowed to contribute varying amounts to the overall load calculation.   Net result: more stable and meaningful load index close to idle when MAP sensor readings are unstable.  Found on the Electromotive TEC3 among others.

Alpha-N is very poor at dealing with hills (think about engine load going up and down hills at a constant throttle position), temperature variations and just about anything else that you’d care about except close to wide open throttle where it does fine.

Introduction

Understanding your fuel injectors is one of the most important things you can do to ensure that fueling is appropriate for your engine.  First, some vocabulary:

• ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
• AFR, Air – Fuel Ratio : the ratio between how much air and how much fuel an engine is receiving or how “lean” or “rich” it is running
• Solenoid : a solenoid is an electromagnetic electromechanical device.  It operates by using electricity moving through a coil to generate a magnetic field which moves a plunger. (more)
• Injector : a special type of solenoid that allows fuel to flow through it when energized (more)
• Pulsewidth : the length of time the engine computer applies electricity to the injector, or how long the injector is commanded to be open
• Flow Rate : The amount of fuel an injector flows once open.  These values are typically given in units of cc/min or lbs/hr at a specified fuel pressure. (injector flow rate varies with the square root of fuel pressure.)
• Latency : the length of time after the injector is turned on before it achieves its linear flow rate.

Everything you ever wanted to know about injectors but never knew to ask

Injectors are pretty simple devices: turn on the electricity, wait till the fuel starts flowing.  Right?

Not quite…

Injectors are mechanical devices – once electricity is applied, the injector needs to move from its resting position in which no fuel flows to its open position where fuel is flowing at its published flow rate.  The problem is that this transition from “closed” to “open” is far from instant – some larger injectors can take several milliseconds to open fully.  During this time, injectors do not flow at their linear flow rate.  How long injectors take to open varies from injector to injector largely due to mechanical reasons.  Fuel pressure can also affect injector latency because of the force applied by fuel on injector internals.  And most importantly, the amount of electricity you supply to the injector controls how much magnetic force coils inside the injector can create.  Bottom line: when your battery voltage decreases (such as when cranking) your injectors take longer to open and fuel injector latency increases.

Tuning for Injector latency

Most engine computers have some kind of table to compensate for injector latency.  They can be called many things – “Injector Battery tables” or “Injector battery offset” or “battery tables” but they frequently look very similar: a table of how long to open the injector before it achieves linear flow (“latency”) versus measured battery voltage.  The idea here is that the ECM opens the injectors for a period of time (from the battery tables) to compensate for variations in injector opening time versus battery voltage.  If you change injectors, you probably need to update your battery tables, too.  If you vary fuel pressure, you may want to try changing the battery tables as well as other tables to account for changes in latency.

A basic method for tuning injector latency requires a wideband and a multimeter (or better yet, datalogging battery voltage from the ECU).  Follow this procedure:

1. Start by hooking up you multimeter or starting datalogging battery voltage.  If you are using a multimeter, use a voltage source close to the ECM if possible.
2. Fire up the car and hold it at a few thousand RPM.  Observe battery voltage – it should be fairly high. (13.8 – 14.5 volts, depending on the vehicle)
3. Gradually, let the car return to idle while keeping an eye on battery voltage.  Many vehicles will run anywhere from 0.75 to 0.1 volts lower at idle compared to cruising RPMs.
4. Problems with battery tables can contribute to hunting or unstable idle.  Once the car is idling, do everything you can to put an electrical load on the car – turn on headlights, turn on the stereo, turn on the fan for the climate control inside the car.  As you do so, keep and eye on battery voltage and observed air fuel ratio.
5. If you see the car run progressively leaner when you turn on electrical accessories and voltage drops, start increasing injector latency at the battery voltage you observe until you minimize changes in air fuel ratio when changing electrical load.  This will result in a curve with a steeper slope.
6. If you see the car run progressively richer when you turn on electrical accessories and voltage drops, start decreasing injector latency at the battery voltage you observe until you minimize changes in air fuel ratio when changing electrical load.  This will result in a curve with a flatter slope.
7. If you feel really adventurous, you can disconnect the large cable between the alternator and the + side of the battery (or sometimes a wiring distribution block) while the car is running.  When you do this, the battery will stop charging.  Voltage you observe at the ECU will decrease as the car consumes the battery’s charge.  You can generally tune a much wider range of the battery table by doing this but it is much more of a pain to do and will eventually drain your battery to the point the car will not run.
8. Note: these injector battery tuning methods assume the car is reasonably well tuned close to idle and will idle at a reasonably steady AFR.  Doesn’t need to be perfect, but you may do more harm than good messing with injector battery tables when the tune is jacked.

Another sign that your battery tables may be off is when the car runs poorly at small throttle angles compared to large throttle angles.  Sometimes changing latency is a quick way to fix a car running too rich / too lean that runs well close to wide open throttle.  Latency changes will have a large effect at low pulsewidths (i.e. closed throttle) but will have comparatively little effect at high pulsewidths (i.e. open throttle.)

You shouldn’t be afraid to adjust injector latency as part of tuning but always remember that it is a BROAD SWEEPING CHANGE THAT WILL AFFECT HOW THE ENGINE RUNS EVERYWHERE.  If you have a problem in a specific load condition, chances are your problem is elsewhere.  When you start seeing PATTERNS of problems (i.e. closed throttle too lean, close to idle where battery voltage too lean, hard starts/cranking when battery voltage lowest, etc.) then it is worth looking into whether a latency adjustment can solve your tuning issue.

You can always sanity check your injector battery tables visually.  Injector latency always increases as battery voltage drops.  If you look at a 2D graph of battery voltage versus latency, it should always be relatively smooth.  As voltage increases, injector latency should level out and change much more slowly than at lower voltages.  This is not a Ford thing or a Honda thing – this is a universal thing that all cars that use fuel injectors will follow.

Tuning for Injector Flow

We haven’t said that much about injector flow up to this point, but it is equally important to having your engine run correctly.  Injector flow is the “obvious” thing that most people change when installing different injectors.  Most older systems account for injector flow with a “fuel constant” (it is called many different things in different systems such as… ) – when you change the size of injectors, you multiply the fuel constant by the difference in flow between your old injectors and your new injectors.  For example:

1. Fuel constant = 16.4
2. You have 24lb/hr stock injectors
3. You install 32lb/hr stock injectors
4. 24 (old) / 32 (new) = .75
5. New fuel constant = old fuel constant * change in injector size = 16.4 * .75 = 12.3

Keep in mind, this is just a guideline to get you close.  You can use the injector size / injector constant to make sweeping, global changes to fueling if your tune is off everywhere.  You *should* be able to get a tune very close to where it was before an injector change by changing nothing more than battery tables and an injector size / injector constant.

Some systems (Ford, GM LSx, newer Dodge / DCX Hemi, others) use a dynamic flow model of injector behavior rather than a single “injector constant.” These systems try to more precisely account for the flow of injectors by modeling how injector flow changes as a function of how long they are open.  Most ~87-2009(ish) Ford uses the concept of injector slopes.  There is a “low slope” and an “high slope”, along with a threshold to change from one to the other and often a minimum pulsewidth.  The injector slopes can be thought of as TWO injector flow constants and the ECM changes from one to the other as the injector opens.  When changing injectors on Fords or other manufacturers that use dynamic flow models, a good starting point is to scale both slopes (or all members of a dynamic flow table) uniformly by the predicted difference in injector flow rate.  An even better approach is to copy values from another OEM calibration that uses the injectors you have installed.  Some injector suppliers (but not many – Injector Dynamics is the one that comes to mind) do dynamic flow testing and can supply you with data precise enough to plug in.