ECU: What It Is And How To Tune It | Auto Mechanic 101

The Electronic Control Unit is one of the essential components of your car’s engine management system. But what exactly does it do? An ECU, also known as ECM, PCM or EMS, is the brain that makes your engine run in all possible conditions. Whether it is -40 ° or 50 ° outside, the ECU is there to ensure the optimal operation of your engine. Here’s a quick guide to help you understand how an ECU does his thing and what can be done with it when you start modifying your engine.

Internal components of an ECU

The processor

The processor is the unit used to make everything work. Like a traditional computer, it will be used to perform the calculations needed to operate the various engine components to produce optimal performance.

Input circuits

This small part of the electronic control unit is used to take the electrical signals sent by the different sensors located on the engine and turn them into binary data so that the processor can perform its calculations. Most sensors operate on 5 volts and need to be transformed to be understood by the ECU.

Output circuits

The output circuits operate the engine’s accessories. Injectors, solenoids (such as the one used for variable valve timing systems) the pressure relief valve, the purge valve, and even the air conditioning pulley clutch are all controlled using an output signal.


The ECU’s memory is the part containing the program dictating to the processor how it should perform its calculations. These calculations and parameters were developed by engineers and are specific to the engine for which they were created. These are the program and parameters that tuners modify to adapt to changes made to the engine. There are different types of memory. The oldest and most widely known is the EPROM. It’s from this type of memory that the expression “chips” is born. This only means to add or replace the original EPROM in the majority of cases. Nowadays, control units use FLASH memories instead. This type of memory is the same used in USB keys. Faster and reprogrammable, this type of memory allows reprogramming with a tool that plugs into a port provided especially for this purpose. It also allows dealers to make corrections if the manufacturer discovers defects in the original program after the vehicle has been put on the market.

Why is it important to have a properly programmed ECU?

Tuners often use the wrong ECU for their engines. How many LS-VTECs work with a computer designed to be used on a stock B16? Using an inappropriate control unit can cause permanent damage to your engine. Since each ECU has been previously programmed for a specific engine type, when changes are made or the engine is replaced by another model, the calculations and parameters contained in the ECU are not suitable anymore. Major differences in the amount of fuel delivered, when sparks are generated, can cause irreversible and expensive damage. Other parameters, such as weather-related adjustments, can affect the reliability of the new engine. Correctly tuning your ECU to fit your new mods will protect your investment and will also make your car go faster.

What should be changed in an ECU based on modifications made to the engine?

It depends on the type of modifications and the type of engine. For example, changing an intake manifold on a turbocharged engine could have more impact than on an atmospheric engine. More oxygen mixed with more fuel results in more power. But for this to be true, the ECU must be reprogrammed so that it sends the right amount of fuel depending on the amount of air that the new modification brings. Here are some classic modification examples and the tuning required for the control unit to be compatible with the new setup.

1. Supercharging

When installing a turbo or supercharger on a naturally-aspirated engine, it is not unusual to have to replace the original injectors for bigger ones. Larger injectors cause higher latency (the speed at which the injector opens its valve.) This greater latency has the effect of screwing up all the corrections originally programmed for the original injectors. The engine will have a hard time to run at idle, start when cold, and produce enough power in hot weather. It’s also required to change the opening time of the new injectors. As the original ECU was calibrated for smaller injectors, the amount of fuel sent by the new injectors will be too large for the same injection time. Nonetheless, it will be necessary to change the ignition time. The new air/fuel mixture will be denser and more heat will be produced by compression, the ignition time delay will also need to be adjusted to avoid early detonation.

2. Camshaft replacement

In most cases, it is possible to keep most sensors and OEM injectors for this type of modification. However, adjusting the injection time will be required. The ignition timing and atmospheric correction factors will not be affected by this particular modification as much as when supercharging an N/A vehicle. This is why it’s often possible to replace the camshaft with a slightly more aggressive model without reprogramming the ECU and get away with it.

3. Nitrous oxide

Some nitrous oxide kits come with the necessary parts to increase the amount of fuel sent to the engine without having to reprogram the ECU. However, if you want a very high output system, it is advisable to have the control unit reprogrammed, as the modifications to be made are quite similar to those of turbocharged systems.

4. High revs

You will need to program your ECU if you change your cylinder head to take advantage of more aggressive cams and want to push the rev limit of your ECU even further.

Other ECU tuning methods available

Standalone systems

Standalone systems are made to replace OEM ECUs when they can’t be reprogrammed. The advantage of such systems is that most can be used on any type of engine. They are usually delivered with several basic configurations for several types of engines to help you start tuning it. However, this kind of system is not cheap. It is also quite tedious to program when compared to an original control unit, especially if you want the same driving and engine behavior as the original one. It is very difficult to redo the engineering work made by car manufacturers. You will need a lot of time and testing in all weather conditions to achieve this goal. A standalone system is not recommended for daily driven cars. Keep these for the track.

Piggyback systems

Quite inexpensive and placed between the ECU and the engine, Piggybacks only alters the input and output signals to cause different reactions from control units. However, the possibilities will always be constrained to the actual limits of the original ECU. For example, it may not be possible to increase the rev limiter or to engage certain accessories if the ECU detects something abnormal in the engine’s operation.

Which system should you choose?

It depends on your budget and the extent of the changes needed to your injection system to make sure your modifications work. If possible, opt for a modified original ECU. If you can not reprogram your original ECU, go for a Piggyback system. If, on the other hand, you can not reuse the original ECU with or a Piggyback is simply not enough, you will have to get yourself a standalone control unit system.