Before we begin changing PCM parameters, let’s take a look at the key elements to monitor. Most tables and functions in the PCM are constructed with axes that represent a range of engine operating conditions. In order to know where to make changes to the PCM tables, it is necessary to know exact engine conditions at any time. This data may be available in real time from instruments or a computer link to the PCM, or recorded in a datalogger for later review.
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Many PCM programming tools (including almost all aftermarket PCMs) allow for real-time changes while the engine is running. This is the ideal method of calibration for most parameters since it allows the calibrator to see exactly where in the tables the engine is operating. If the PCM does not support real-time adjustment, the next best thing is real-time monitoring by way of some scan tool. Multiple OBD-II capable data readers are available, such as EFILive, AutoTap, SCT Raptor, Diablo Predator, and OEM tools such as the GM Tech2, Ford NGS, and WDS These tools also allow the user to record brief periods of time, which is extremely useful when measuring transients and WOT behavior where it is impossible to plot one cell at a time on the tables due to fast engine sweep rates.
At a minimum, the calibrator needs to see real-time displays of RPM, load, ECT, IAT, and lambda to calibrate most base tables. Load may be shown as MAF or MAP output or calculated load. It is easiest to view these in real time while working on a dynamometer to facilitate holding at a fixed speed and load point for calibration.
Engine coolant and air temperatures should always be monitored to ensure that we are working in the normal operating range. We can always go back and adjust for cold start or heat soak later. Not confirming where the engine is operating on a correction table begs for trouble.
Actual fuel delivery should be monitored at low load to check for proper fueling. Excessively large pulsewidths at idle may indicate faults somewhere else. This may point you to a problem with a sensor or correction curve that is nowhere near correct. Additionally, WOT duty cycle should be verified to ensure that the fuel injectors are not going static, forcing a lean condition. If this is the case, it becomes impossible to correct WOT mixture by anything other than a pressure or injector size increase.
Ignition timing should be monitored closely at idle and WOT. An unstable amount of ignition lead at idle often leads to unstable fuel mixes. Many hours have been wasted by tuners attempting to fix a timing problem with the fuel trims. It is not uncommon to see 10 degrees of swing in idle timing to correct for fluctuations in speed, but 20 to 30 degree changes usually indicate a load measurement or speed pickup accuracy issue. Actual ignition timing at WOT should be recorded for review after a dynamometer pull or drive. Since the engine sweeps rapidly through the RPM range at WOT, it is best to review this data after coming to a stop. This also gives the calibrator time to review any indicated knock sensor input or temperature related spark retard at the same time.
When tuning idle speed and mixture, it is important to monitor IAC motor position or duty cycle. This is the fine adjustment to actual idle speed, and the calibrator should ensure that it is actually being used. If the IAC displays as either all the way open or closed at all times, it’s time for a throttle stop adjustment. The more range the IAC motor has in both directions, the better chances it has of controlling a stable idle speed.
PCM fueling adjustments should also be closely checked at idle and part load. If the engine has entered closed loop operation, it uses the signal from the HEGO to trim fuel delivery back to the target (λ = 1). If the engine has not entered closed loop, the calibrator can safely assume that the base fuel tables are determining actual fuel delivery. In open loop, the base fuel calculations in the PCM can be adjusted by referencing actual l from a wideband sensor against the desired air/fuel ratio from the PCM tables. If a closed loop operation is functional, the PCM’s fuel trims can be used to determine the error in the base fuel calculations.
Know Your Load
Engine loading is not the same for all vehicles. The intended use of the vehicle dictates the required calibration. Vehicle loads change the actual engine load greatly depending upon how the vehicle is used. A drag racer taxes the engine differently from a top speed racer, boat, or road racer. The largest difference between the definitions of WOT usage is temperatures and stabilization. Durability and temperature control become serious concerns as vehicle loads and time spent at full load increases. The longer the engine is operated at high loads, the more heat is transferred from the combustion chamber to the valves, cylinder head, coolant, oil, under hood area, exhaust system, and catalyst.
Due to its short duration, drag racing is a remarkably forgiving environment as far as racing goes. Races are completed in a matter of seconds. The vehicle load at the start is only its own weight. This load increases steadily toward the end of the track as aerodynamic forces increase resistance on the car. Because heat transfer takes time and the races are short, a lower amount of total heat is imparted to engine components in a drag race compared to other forms of motorsports. Extended cool-down periods between races further reduce temperature issues for serious drag racers. This means that engines can be calibrated to operate at slightly more aggressive power levels, generating more heat during these brief tests without overheating.
Road racing is a different environment for an engine. Road racing requires the car to operate at changing loads, often very high, for extended time. Much of this racing happens at high speeds, so aerodynamic loads are significant for most of the operation time. During the course of a race, it can be expected that oil, coolant, and under-hood components all eventually come to a stabilized, elevated temperature. To aid durability, exhaust gas temperatures should be considered strongly when performing WOT calibration. It may be necessary to sacrifice some power to reduce temperatures by running a slightly rich air/fuel ratio. Spark advance should generally be kept close to optimal since retarded timing increases exhaust temperatures. This becomes even more critical on street-legal vehicles that use a catalytic converter. Excessive exhaust temperatures can quickly destroy the monolith of the catalyst, destroying emissions and reducing power as the exhaust gets plugged by the melted substrate. Radiators and cooling systems for these cars often need to be upgraded to prevent the ECU from prematurely pulling power to protect against excessive temperatures.
Marine and top speed racers share the most arduous engine conditions. These vehicles are rarely equipped with catalytic converters or restrictive exhaust systems. However, vehicle loads from aerodynamic drag at high speed or fluid drag on the hull force the engine to work extremely hard for extended periods of time without relent. Without proper cooling, these engines would self-destruct from constant heating of the combustion chambers. Primary cooling comes from the radiator or lake water feed to the cooling system, but piston and chamber temperatures must also be controlled by adjusting burn temperature. Running a rich mixture with a maximum of tolerable spark advance is usually required to keep temperatures safe.
Calibrators use dynamometers to recreate engine-loading conditions in a controlled environment. Not all dynamometers are created equal, and some have significant advantages to the calibrator.
Engine dynamometers have been a long-time standard for both tuners and OEMs. Engine dynamometers attach the engine to a variable electric or hydraulic load source. Speed and load can be independently adjusted, and a controlled sweep can also be done. They allow for terrific control over coolant, oil, and inlet air temperatures. This takes a lot of the guesswork out of calibrating the base maps and makes it easier to build temperature compensation tables later. The disadvantage is that the engine dynamometer rarely employs the same exact exhaust system as the vehicle. There can be as much as 20% difference in an engine’s volumetric efficiency between operation on an engine dynamometer with open headers and in a vehicle with catalysts, mufflers, and tailpipes. Speed density systems need to be calibrated with the exact hardware combination they use in the vehicle to prevent a mismatch.
Chassis dynamometers allow the entire vehicle to be tested by placing the wheels on rollers and measuring output. Two main types of chassis dynamometer exist: inertial and load bearing. Each has its own advantages.
The inertial chassis dynamometer is a simple design that uses a weighted drum with a speed pickup. With a known drum mass, the acceleration rate can be used to determine power input. Vehicle torque output is extrapolated based upon speed. While this method is very consistent from run to run, the loading on the vehicle remains constant regardless of speed. Inertial dynamometers are very useful for “A” to “B” comparisons to see if a change to a vehicle increased engine output or not. The low cost of the inertial chassis dynamometer makes it a popular choice for many performance shops looking to show customers power increases from their purchases.
The load bearing chassis dynamometer is more complex and costly. The same rollers are fitted with a load absorption unit that is controlled by a computer. Actual vehicle output torque can be directly measured rather than extrapolated. In the real world, vehicles experience increasing loads from aerodynamic drag as speed increases. Inertia alone cannot give a true representation of vehicle loading on the street. The load bearing dynamometer has the ability to increase load with speed to accurately simulate actual vehicle loads. The dynamometer can be programmed to give different aerodynamic drag loads to a pickup truck versus a sports car. Additionally, the load absorbers can be used to hold the vehicle at a fixed speed or loading point, easing calibration work. Turbocharged vehicles in particular benefit from calibration on a load bearing dynamometer since engine load dictates how quickly the turbocharger responds.
Both the inertial and load bearing chassis dynamometers share a common drawback. Since the vehicle is actually stationary while the wheels move, airflow across the vehicle (particularly the cooling components) is not the same as seen on the street or track. While most dynamometers are fitted with cooling fans, few can replicate the sheer volume of airflow encountered in the real world. Engine temperatures should always be closely monitored to allow for proper cooling. If the vehicle is equipped with any other heat exchanger (such as an intercooler) charge temperatures on the dynamometer will almost certainly vary compared to real world operation. The dynamometer should be used to develop the majority of the calibration, but there is no substitute for real world testing at the end of the project.
Written by Greg Banish and Posted with Permission of CarTechBooks