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.

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