The Art Of Calibration

Ray T. Bohacz Oct 25, 2012 0 Comment(s)

More than sixteen years ago GMHTP was founded to serve the enthusiasts of what was then defined as late model muscle cars. The qualifier for this title being the engine was equipped with electronic fuel injection and ignition control. As time progressed the EFI community has grown in both number and ability, but there still remains a good deal of mystery around the concept of calibrating an engine. This is rooted in two distinct but different domains: understanding what an engine wants and how to use the software of the specific engine controller you have.

The ability to navigate and intuitively understand the calibration tables in an ECU can be learned from reading the owner’s manual, through experience and attending any training program that the manufacturer may offer. Learning what an engine wants demands much more of a commitment and is a longer journey, but holds greater rewards. It is based on all of the synergies that are taking place inside an internal combustion engine that allow it to convert chemical to mechanical energy. Once you understand the dynamics of an engine, it makes little difference if it is a stock calibration you are tweaking or a 1,000+ horsepower monster—the basics are still the same.

As the author of this four-part series I come to you with many years of experience teaching this subject to the industry. From 1992 to 2005 I was responsible for the ACCEL/DFI training program for both its dealer network and end users. This was for the DFI Generation 6.0 and 7.0 systems. In the beginning of each class I would state (much to the chagrin of some of the students) that the best calibrators are engine guys that know how to tune a carburetor. These people simply have a better understanding of the function of an engine and how it responds to changes in the fuel delivery and the ignition curve. This does not mean that if you do not have carburetor experience that you cannot become an accomplished EFI tuner. But my best advice to you is the same that I have given all my students—obtain at least a cursory understanding of carburetor function and adjustments. This knowledge will go a long way in making the mystery of an electronic calibration, with no visible moving parts, much easier to grasp. For example, TPS based acceleration enrichment that evokes an asynchronous injector event is easier to embrace if you understand how an accelerator pump squirts fuel against the booster to break it apart and why. Or an advanced EFI logic to compensate for wetting of the intake wall with fuel (TAU) is visually seen with a poorly tuned (overly rich) carburetor.

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The other mantra of mine is: do not fool yourself into thinking you know how an engine works. Go back to basics and embrace the fundamentals. Quiz yourself on the four-stroke cycle, cylinder filling and emptying, ignition demand and the reason for spark advance along with cam profile and air flow theory. If you can explain it to yourself then you understand it. I do this all of the time and will for the rest of my life. When you grasp these areas then the way an engine responds during a calibration change becomes a wealth of information and will make your task quicker, easier and much more effective. Randomly punching keys on a laptop computer until something good happens in the engine is not tuning.

Understanding air/fuel ratio
For an engine to run there needs to be an exchange or conversion of energy from a chemical state to a mechanical one. The energy is contained in the gasoline, but other things need to happen for it to be released. The fuel must be mixed with air and ignited by the arcing of the spark plug. When the fuel ignites independent of the spark it is considered abnormal combustion in lieu of normal combustion. The entire process is rooted in chemistry that is nice to fully understand, but at this level and for the GMHTP audience it is not necessary. The important concept to take hold of is the gasoline needs to be blended with air and the ratio can be likened to a recipe.

The air/fuel ratio is the amount of air to a constant one part of fuel. Thus, a 13:1 ratio means there is 13 parts of air to one part of fuel. Since the fuel is constant, the lower the first number in the ratio is the richer the mixture. This means that there is less air diluting the fuel. Conversely, the higher the first number the leaner the mixture is considered. Within the engineering community there are other ways to measure the mixture strength such as Lambda, equivalence ratio and fuel/air ratio. The most common on our level is air/fuel ratio and that will be our reference.

For the majority of our audience their vehicles consume what is known as street gas. This is a fuel that meets all of the government standards for emissions and chemical components, readily available anywhere in the country, and is similar in specific gravity (weight). This is in contrast to race gasoline that is blended with a more defined need of a competition engine and is not concerned with daily use. If any enthusiast were queried about the difference between race and street gas the most common response would be octane but there is more to the story than that. Octane only defines one aspect of a fuel—its ability to resist auto (self) ignition through pressure and heat. It needs to wait for an electrical arc to ignite. Race gasoline has greater octane since most competition engines employ a higher compression ratio. The octane allows for a normal combustion event to occur under those conditions.

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What is not recognized is that street gasoline is designed to offer enough octane to keep abnormal combustion at bay, but is meant to cold start easily at very low temperatures, withstand a high thermal load under summer driving and elevated under hood heat, and burn efficiently to around 3,500 engine RPM. A modified engine that is run at 7,000 rpm on street gas will have a higher amount of fuel left over that did not burn since the fuel is being consumed too slowly. In an instance such as this you will find that the calibration may need to be altered in the upper RPM since the fuel burns so slowly. Tuners that only look at exhaust gas temperature as the criteria may see high RPM readings that may not make much sense if looked at in one dimension. 

When Detroit designs an engine and does a calibration they study something called mass fraction burned. This is a qualifier of how much fuel is consumed relative to the piston movement. It is used to determine the ignition timing. The goal is to obtain 100% burn before the exhaust valve opens and the energy is sent out the tailpipe instead of working against the piston. The problem being that all of the fuel does not burn at an even rate in any engine regardless of the make or gasoline used. The burn speed across the bore is usually a bell curve that starts out slow, accelerates and then slows down again before it extinguishes.

When the gasoline is first ignited, an area identified as a reaction zone needs to be established. This is where the heat from the burned mixture is transferred into the unburned mixture. The reaction zone speeds up the burn and the flame’s expansion across the bore accelerates. As the fuel is consumed and the area of the cylinder increases, because the piston is going down in the bore, the flame slows down. This is due to Boyle’s Gas Law. Another factor in the slowing of the flame is the quenching (cooling) effect of the cylinder wall on the opposite side of the bore. A general rule of engineering is that the average flame speed across the bore is between 7 to 25 meters/second. A modern engine is more in the range of 18 to 25 m/s. This is what we would loosely qualify as a fast or quick burn combustion chamber.

Though the mass fraction burned is more of a tuning function of the ignition timing instead of the air/fuel ratio it needs to be recognized. A street gas will usually require a different mixture strength than a race gasoline to produce the same power at high RPM and may impact the mass fraction burned. This is due to the difference in fuel blends that an enthusiast may employ versus a fixed gasoline composition that an OE calibrator would work with. It is not unusual for many GMHTP readers to have a forced induction engine that runs one fuel and calibration on the street and another at the track. If you were to talk to an OE engineer they would look at the ignition lead only for the mass fraction burned, but they are using a control fuel and it is not changed.

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The other aspect of gasoline that is important is the heat energy that is not advertised. Energy is measured in British Thermal Units (BTU) and varies from brand to brand and load to load. By law it needs to be in a certain range, but that specification is very broad. Though there is much debate, a good rule is that a gallon of regular grade unleaded gasoline will have around 120,000 BTU while the same volume of premium or super will posses approximately 117,000 BTU. The loss in energy is usually the result of the chemical components that are added to the fuel to raise its octane. The energy density of the fuel will impact the power the engine makes. In normal driving where well below peak output is realized, the density will impact the fuel mileage. A high-density gasoline will provide better fuel economy than a low-density grade. The energy density of the fuel is a function of the crude oil used, the refinery where it was made and the chemical components added to the gasoline to meet market requirements.

Almost everyone that is involved with EFI tuning has heard the term stoichiometric, but few really understand its meaning. Stoichiometric defines the air/fuel ratio for the most complete energy conversion from the gasoline. When dealing with street gas for the most part it is formulated to have a stoichiometric ratio of 14.68:1 that is usually rounded to 14.7:1. But that has also changed. In many regions the gasoline is considered reformulated and includes ten percent ethanol (corn alcohol). This has skewed the stoichiometric value to around 14.3:1 (instead of 14.7:1 with pure gasoline) along with the energy content.

The reason gasoline was originally designed to be at 14.7:1 is the catalytic converter has the greatest efficiency (engineers call it a conversion rate) when fed this mixture strength. Thus, a traditional oxygen sensor is designed to identify 14.7:1 as its toggle point to keep the catalytic converter happy and efficient. When the catalyst is fed a mixture richer or leaner than stoichiometric its conversion rate drops dramatically.

You may already know empirically that an engine will not make the most power at 14.7:1 and question this definition of stoichiometric. You may also challenge this because a cold engine will not start at that ratio and you would be correct in both instances. Engines do not like the same air/fuel ratio under all operating states and this needs to be explained.

For gasoline to burn in the engine it needs three things to occur. It must be broken down into small particles, which is called atomized. This is the job of the fuel injector or carburetor. The atomized fuel then needs to be mixed with air, which is called emulsified. Next it must change phase from minute liquid particles that are mixed with air to a gaseous state (a rarefied form). The phase change occurs through the theory of latent heat of vaporization.

At approximately 60 degrees F only 50 percent of the gasoline vaporizes. For this reason, along with the internal friction of a cold engine, a much richer mixture than the chemically correct stoichiometric value needs to be evoked during crank. Under extreme cold, cranking fuel may be as rich as 2:1 with a carburetor, slightly leaner with EFI. Once the engine fires there is heat in the combustion chamber and a reaction zone is established and the mixture needs to be much leaner to support combustion. If it were not it would wash out the arcing of the plug and there would not be enough oxygen molecules to allow combustion to continue. As the engine warms and the phase change is more efficient and internal engine friction reduced, the mixture can be made leaner and end up at stoichiometric.

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As an aside to this, that is why the phase or placement of the injector is so critical on an OE application. The injector is placed to spray on the intake valve stem, which helps to atomize the fuel, but also the valve will absorb heat as soon as the engine fires. This will help the phase change. Even though the incoming air may be very cold, the fuel is working in an environment that is much warmer than ambient. For this reason an engine can go to stoichiometric much sooner than the air and coolant temperature would normally allow. This is important since it will get the catalytic converter on line in short order. The EPA has a test that monitors the emissions output during the cold start and warm-up phase (it is known as the intermediate period) and that is the hardest to pass. The majority of carbon monoxide and hydrocarbons are emitted in the first two minutes of engine operation.

During driving an OE calibration is designed to be at stoichiometric under all operating states except full power and coast down. Let us discuss full power first. Inside every engine there are three areas of loss from the BTU potential of the gasoline. These are thermal, pumping and friction. Thermal loss is the energy that is sent into the water jacket and out the tailpipe. Pumping losses are the energy consumed to move the charge into the cylinder and then exhaust the spent gasses. Frictional loses are the energy used to operate the engine, valve train, crankshaft, oil and water pump, etc.

Due to these losses an engine requires a mixture strength richer than the chemically perfect ratio since the energy is going somewhere else instead of into the crankshaft. Most engines will perform the best with an air/fuel ratio of around 12:9:1 to 13.5:1 at peak torque and then lean out slightly at peak horsepower.

The cylinder is the most filled with charge (a mix of fuel and air) at peak torque. This is measured as volumetric efficiency and is a qualifier of how full the bore is. Most normally aspirated production engines will only see a VE of around 80 to 85 percent at peak torque and drop slightly from there at peak horsepower. At peak horsepower the VE is diminished since the engine is beyond its ability to fill the cylinders and thus, requires a slightly leaner air/fuel ratio. With forced induction VE can be increased to values way above 100 percent depending on the CFM flow of the power adder and the boost pressure realized.

During idle another set of problems occurs. The amount of fuel consumed is small so the heat energy losses are diminished, but reversion of the exhaust back into the bore is a concern. This reversion dilutes the incoming charge and requires it to be richer than stoichiometric for good idle quality. On a stock engine the calibrator is able to achieve a mixture strength of 14.7:1 with a silky smooth and slow idle since the amount of reversion is designed to be minimal if any at all.

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This now introduces another topic—the created versus delivered air/fuel ratio. As its name implies, the created air/fuel ratio is the one that is mathematically derived by the mass of the fuel discharged from the injector along with its specific gravity and the mass of the air. That is the mixture strength that would be delivered to the cylinder in a perfect world. But there are many obstacles along the way. Reversion is a concern since it dilutes the mixture strength with inert exhaust gas. Another problem is the wetting of the intake passage with fuel along with the absorption of the mixture into any carbon deposits that are on the backside of the intake valve, combustion chamber and piston crown.

Carbon is a real issue with an engine being calibrated that has been in use. The OE tuner does not need to be concerned with that since the engine is new and is carbon free. The problem with carbon is two fold. First it absorbs the fuel on its way to the cylinder, and then once it becomes saturated, it desorbs (gives up) and puts the hydrocarbons back into the charge stream. When it is absorbing fuel, the delivered mixture strength will be leaner than the created air/fuel ratio. Once desorbtion begins the delivered mixture will be richer than the created air/fuel ratio and you will be wondering where this fuel came from. The carbon is giving up the gasoline to the high velocity air stream since it cannot hold any more. In Part 4 of this series we will discuss this in depth.

The largest offender for slow engine speed reversion is the overlap of the camshaft. As overlap (the time in rotational degrees both valves are open) increase so does reversion. For this reason if you change the cam in your engine it may very well require a mixture richer than stoichiometric for a good idle quality. Modern valvetrain design has minimized this, but the fact remains that you need to add more fuel to combat reversion. It is just a matter of how much. That is why variable cam timing has been such a bonus to a modern engine. The overlap can be nearly eliminated at idle. As the engine climbs in RPM, overlap is increased to aid breathing. Do not be surprised if an engine with an aggressive camshaft will not idle nicely unless it is fed a mixture of around 13.5:1 to 14:1.

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The next aspect of air/fuel ratio is the fuel cut-off or deceleration fueling during coast down. Since the engine is basically being driven through inertia, an OE calibrator will let the mixture go very lean—way above 14.7:1 to almost no fuel at all. This allows for excellent fuel mileage and emission control. Once the throttle is evoked the mixture will then revert back to stoichiometric. Much of the fuel economy gains in new cars, that are realized though the calibration, are from very aggressive fuel shut-off strategy. The real trick here is not shutting off the fuel but integrating the other systems such as the torque converter clutch, so that everything is seamless when turned back on.

If your EFI engine uses race gasoline then the specification sheet on the fuel will state the stoichiometric rating and this can be used as an aid in tuning. Often this is moot since most aftermarket calibrators use a dyno and allow the engine to tell it what mixture strength it wants regardless of what the fuel company identifies as stoichiometric.

The major offender to mixture strength when doing a calibration is the cam profile, but other than that the protocols that an OE tuner will use still apply. During transient throttle opening the mixture is still at stoichiometric, but there is often more fuel added to achieve that result. As the throttle is opened, additional airflow is introduced and more fuel needs to be present. A common mistake of a novice calibrator is to over fuel the engine during throttle transient and then let it clear itself out. The car may drive well that way, but it is a dirty tune that is constantly relying on the oxygen sensor to correct back. That is a feed-back logic in lieu of a feed-forward approach. You never want to use the feed back circuit as a band-aid for a sloppy calibration. In Part 4 of this series we will discuss the theory for proper transient fueling regardless of how powerful the engine may be.

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You should now have a better understanding of air/fuel ratio and why it is important for proper engine performance. The job of the EFI tuner is to create a recipe that will allow for the greatest energy release from the potential of the fuel. Though other things help or detract from this, the first step is understanding air/fuel ratio.

Next issue: Understanding the theory of ignition advance and its impact on driveability, performance, and engine longevity.

RaceKrafters Automotive Machine


RaceKrafters Automotive Machine
Lancaster, PA 17601


How To
Understanding Air/Fuel Ratio
Ray T. Bohacz Oct 25, 2012


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