Standalone VS Piggyback Engine Control Units (ECU)
An ECU (engine control unit), interprets sensory data from a car's electronics and uses this information to monitor and control the engine. These computers are also known as engine control modules (ECM) and powertrain control modules (PCM). If you modify your vehicle in a manner that the stock ECU can not compensate for then you must turn to an aftermarket ECU. This most commonly occurs as a result of modifications like adding individual throttle bodies or converting a naturally aspirated engine to forced induction. Please note that vehicles from 1997 up with OBDII ECUs can be reflashed. If you've only mildly modified your car, the stock ECU can probably support your needs. A simple reflash will save you a lot of time and money. If you have a car from 1996 or below, or have made significant engine modifications, you may want to consider an aftermarket ECU or hardmodding the original ECU.
A standalone ECU is a unit which completely replaces some functions of a car's stock computer. This includes complete control of ignition, fuel, and cam angle maps. Many other functions such as traction control, launch control, and more are also available on most modern standalone ECUs. The primary advantage of a standalone ECU is that it will offer complete control over engine parameters and the ability to customize your tune to a specific engine build. Additional sensors can also be added to your engine to allow for much more in-depth and accurate tuning compared to the ability of the OEM ECU. The downside, of course, is that standalone ECUs are generally the most expensive option here. Standalones are really the no compromise approach to engine tuning.
A piggyback ECU is an electronic component which intercepts signals between a car's various sensors and its stock ECU. This means that the OEM computer remains in complete control of engine functions. The piggyback simply "tricks" the stock ECU into behaving in a specific manner. For example, if you want to make your engine run a leaner air-fuel ratio, the piggyback will convince the ECU that the engine is running rich, even though it is not. This forces the ECU to inject less fuel, therefore causing the air-fuel ratio to lean out. Other similar types of "tricks" are used to adjust boost levels and optimize engine performance. The primary advantage of a piggyback ECU is the ability to tune most mild engine builds for a relatively low cost. The disadvantage, however, is that piggyback ECUs do not offer the same type of customization as standalone units. Piggyback ECUs cannot extract the maximum amount of power out of highly tuned engines because they must perform within the limits of the stock ECU and its sensors. Piggyback ECUs, unlike standalone ECUs, also do not offer functions like traction control and launch control. Piggyback ECUs are great for a budget tune and they work well for the majority of modified street cars. Just keep in mind, it might not be worth spending the money on a standalone ECU if you aren't building a dedicated track car or extensively modifying your engine.
Which to Choose
The ultimate choice between a standalone or piggyback ECU depends on the specific build, but there are a few cases where one would make much more sense than the other. Here are a few of these clear cut cases:
- Adding forced induction to a naturally aspirated car: A standalone ECU makes the most sense in this situation. The stock ECU was not designed for dealing with boost and it will be very difficult to properly tune fuel and ignition maps to make the most out of adding a turbocharger or supercharger to your car. Although it is possible to tune for forced induction using rising fuel pressure regulators and piggyback ECUs, they will severely limit the amount of power your engine can safely produce.
- Turbo upgrades and adding boost to a factory equipped turbocharged car: A piggyback ECU makes the most sense in this situation. If you have a car which came equipped with a turbocharger from the factory, the ECU should be well equipped to deal with your modifications.