Before we look at aftermarket ECU's lets see what it is and what it does.
The ECU (aka electronic or engine control unit) controls many aspects of the modern engine. It takes readings like the intake air temperature and mass, amount of oxygen in the exhaust, engine load, rpm and other factors to calculate the optimum duration and timing of the spark.
Manufacturers tweak the settings rather conservatively so the car will run reliably in adverse conditions. These adverse conditions vary from poor quality fuel to missed service intervals and general neglect. It is worth noting that two identical engines are rarely exactly that.
We have seen massive differences in power output and curve from what should essentially be the same block and this highlights the difficult job of a manufacturer who needs a one setting for all ECU.
For this reason alone there is a good argument for uprating and tightening up the timing. When you add additional tuning parts and other modifications into the mix it becomes essential to upgrade the ECU.
Some modern ECU's have a degree of self learning and will trim the fuelling and timing based on readings taken over a reasonable distance. This is better than a set of fixed parameters but it still needs improving, especially if you have done other modifications.
Sadly though some engines are just too old to have the ECU reprogrammed. Even some engine ECU's are protected or locked and cannot be altered.
One answer to this is adding an aftermarket ECU. This effectively replaces the cars standard ECU and allows the user to program a custom fuel and timing map. The benefits are even greater on turbocharged engines when this is mated to an electronically controlled boost controller.
Do not confuse an aftermarket ECU with a piggy back ECU or plug in box. We are discussing complete engine management systems suitable for adding to older Carb engines converted to fuel injection and also used to upgrade cars with non remappable ECU's (mostly cars made before 2001).
The benefits are many. A standard NASP engine can see power gains as high as another 10% and if you have added other modifications you can unleash the full potential of these. On turbo charged engines the gains are even greater at around 30% extra power and even more combined with other mods.
There are many types of aftermarket ECU, basic ones control fuelling and timing on 4 cylinder engines with more complex ones offering management of V8 and faster processing and a finer degree of control. There is a big case for getting the best you can afford and to set these up properly you really should be using a rolling road. This allows you to see what the engine is doing at any given time and allows easy diagnosis of lean or rich running and other potential problems.
If you get the ECU settings wrong it is easily possible to destroy your engine. A mis timed spark, overfueling or other issue can literally tear an engine apart.
Immobiliser and other settings (windows, air conditioning and even some auto gearboxes) in some cars are also controlled by the ECU so in some cases you may need to bypass these or find a way to keep the cars standard ECU happy whilst the aftermarket one gets on with the real job of engine management.
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