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Engine Management Systems

 - The manufacturer usually makes a lot of compromises when mapping the EMU.
 - Most production maps can be adjusted to give more power with better fuel efficiency.
 - The engine management system should be updated every time a change is made.
 - Even simple induction and exhaust changes should be accompanied by a fuel and ignition map change. 

The Engine Management Unit (EMU) controls all the engine systems in a modern car. The two primary systems being controlled are the fuel injection system and the ignition system. Almost all other electrical systems are also controlled directly or indirectly via smaller ECUs by the main EMU.

The EMU uses fuel and ignition maps to control the engine at all loads and RPM levels. The AFR and the ignition advance are controlled in this way.
Key elements in the successful installation and mapping of an EMU are a comprehensive understanding of the engines varying fuel delivery and ignition requirements over a broad range of operating conditions and study of the factors influencing the engine’s fuel and spark needs measured and interpreted by the control systems.
A thorough understanding of the electronic systems affected by the EMU and of the electronic systems onboard the EMU itself and a good knowledge of software development is also essential… these latter two areas are the most neglected or lacking in most tuning environments.

Air-Fuel Ratio.
The engine has basic demands for air and fuel. The air-fuel mix can be manipulated to improve driveability, increase economy and horsepower and control the emissions. The main function of the fuel delivery system is to mix the air and fuel in the correct ratios dictated by the EMU. Small variations in AFR can have a dramatic effect on power and economy. A rich mixture is one with a lower AFR. There is insufficient oxygen to support complete combustion of the fuel. Rich mixtures increase fuel consumption and emissions of hydrocarbons and carbon monoxide. They tend to reduce power, increase carbon deposits and foul spark plugs and dilute the engine oil.

A lean mixture is the opposite. A larger AFR with more air than is necessary for complete combustion. The fuel will burn more slowly and at a higher combustion temperature. Lean mixtures reduce power, elevate engine temperatures and increase emissions of oxides of nitrogen. It also causes driveability problems and can lead to engine destruction.

Stoichiometric Ratio.
This is the ratio of the air to the fuel. Different ratios are suitable for different engines, conditions and applications. For a modern engine the AFR must be controlled very carefully and accurately. The best mixture for the most complete combustion and the best compromise between lean and rich mixtures is often the wrong mixture to use on forced induced and tuned NA engines. The stoichiometric ratio can also be represented as lambda. One Lambda equals a ratio of 14.7:1 air to fuel.

Mixture Control for Power, Economy, Emissions.
Power is always a concern, but so too is the demand for good fuel economy and reduced emissions. On a perfectly mapped system, adjusting the system for maximum power will mean increasing fuel consumption. Minimizing fuel consumption means sacrificing power and driveability. Both max power and max fuel efficiency means increased emissions. A good mapper can deliver good power, quick starting and smooth trouble free driving without sacrificed fuel efficiency.

Most production maps can be adjusted to give more power with better fuel efficiency.

Ignition Timing.
In terms of all four major aspects of performance – power, fuel economy, emissions and driveability – ignition control is equally as important as the air-fuel mix. Combustion in the cylinders takes a certain amount of time. Simultaneous control of the fuel and ignition is essential to engine tuning.  For cold start, the best ignition timing is near TDC(top dead centre). More advanced timing allows the engine to fire while the piston is still rising. If the engine is hot, retarded ignition timing can prevent reverse torque and starter damage. It is desirable to retard ignition timing for low cranking speed conditions and more advanced ignition is permissible under less challenging warm starts.  After starting, the ignition timing can be advanced to improve the engine running and to reduce the requirement for enrichment. After this ignition retard can be used to quickly heat the lambda sensor and cat. Advancing the timing during warm-up under part throttle will improve power and driveability and fuel economy.

Opening the throttle very quickly places demands on the engine. The sudden transition from closed throttle to fully open throttle raises the manifold pressure and additional fuel is required, but this fuel is not instantly available. There is a lag between the ideal and actual delivery of fuel. For quick smooth throttle responses the basic air and fuel sensing is too slow. The fuel system must be able to compensate for rapid increases in manifold pressure. Acceleration enrichment is used to provide the required throttle response and the tuner must set up the parameters in the EMU to achieve this…. Some basic EMUs do this automatically. In most systems a separate acceleration enrichment table must be programmed to achieve this and a competition table will be very different to a standard road table.

The Basics of High Performance Mapping.

A whole book could be written on this topic alone, but as a general guide keep the following in mind…. Nearly all mods to the engine to increase power, revolve around improving the volumetric efficiency of the engine so that more air can flow into the cylinders on each stroke… most of the other links on this website relate to this in one way or another. The gains may be tailored to the middle or top end of the power band. Higher lift and longer duration cams, larger valves, ported heads, better intake manifolds and exhaust headers, free flowing exhausts and induction systems all do the same basic job.. improve air flow. And of course forced induction forces more oxygen into the cylinders.

The fuel system must compensate for this increase in air. The standard EMU and fuel and ignition setup is designed to be optimum at the standard air flow and when we increase the air flow we can quickly go outside of the stock EMUs most efficient range of operation and when we go to more extreme states of tune we can go completely outside of the stock EMUs range of operation. For every modification made to the engine, a remap of the air-fuel and ideally of the ignition is necessary to gain the full value of the changes. A few horsepower may be liberated without remapping, but to get any significant performance increase, a remap is required.

The Stoichiometric mixture setting (or the Lambda setting) on all production cars are not optimized for peak power, so a remap without changing anything else on the car will immediately liberate some power. Peak power is achieved with a slightly richer mixture in some areas of the power band and a leaner mixture in other places. There are some exceptions (such as the Subaru Impreza), but in general, we can achieve more power AND better fuel economy than the production EMU… this is especially true on hot hatches and almost all NA cars(the downside is slightly higher emissions, but still well within the NCT limits). Even cars with very aggressive full throttle tables can be significantly improved.

Conservative fuel control is usually accompanied by conservative ignition mapping… mainly because most manufacturers have to limit themselves to operating on 91octane fuel. The revised fuel control mentioned above can be further enhanced by better ignition mapping, tailored specifically for the fuel you are running on(normally 95 octane).

Trying to adjust the stock EMU without using a replacement or piggyback unit will at the very best produce a purely linear response in fuel and ignition change. You can tune one point in the power curve for best power, but the rest of the power curve will suffer. Resulting in a big drop in overall power. The net result  is that even the area you tuned will show less power because the surrounding load sites will be operating at reduced power and the added theoretical power of the site you tuned won’t have enough shove to overcome the reduced power of the points immediately before and after this point. We know every trick in the book when it comes to tweaking stock EMUs and the best that can be achieved is a perceived increase in power, but on inspection on the dyno, you end up with a slower car. These tweaks include upping or lowering the fuel pressure, altering the mass air flow sensor or the throttle sensor or the manifold absolute pressure sensor etc.. all these changes produce a LINEAR change in air-fuel mix that isn’t of real practical value. On the ignition side, you can rotate the crank angle sensor, or modify other sensors such as the ref or trigger sensors and in this way achieve a linear change in ignition advance or retard. The difference here is that you run the risk of detonation and pre-ignition which will lead to head gasket failure is you’re lucky and piston meltdown on a worst case scenario.

To repeat, the only way to properly remap is to use a fully mappable replacement EMU or to use a good piggyback EMU.

The exception to this is the boost pressure on turbo and blown cars. Upping the boost a little by altering the wastegate in some way (electronically or mechanically), will normally give a nice power increase across the entire charged power curve – without any change to the EMU. Obviously no gains are available in the off boost area. However, care must be taken to keep within safe limits. Once you go past a certain limit (different on each car) you can say goodbye to your head gasket or at worst suffer complete engine failure. A good example is the Celica GT4. The factory boost level is 0.7bar(depending on year), and on 95 octane the engine will happily run at 0.9bar, but much past this, without any other mods to exhaust or induction is not advisable. Competition engines can run at 1.3bar on 97 octane, but a revised head gasket is advised.

Note: just changing the induction and exhaust will usually up the boost without going near the wastegate or the EMU(e.g. Skyline GTR). Always monitor the boost levels after every change to the engine, even if the change isn’t intended to modify the boost level.  

Also note, just because the engine appears to run OK after a boost change doesn’t mean that you aren’t doing permanent long-term damage. Any change should be carried out on the dyno with all the diagnostic and knock – det sensors attached. Then engine should then be run at full power for a sustained period and all parameters carefully monitored.