"Stand-alone ECU" is the jargon for an aftermarket ECU (Electronic/Engine Control Unit) which completely replaces the original ECU. Some examples would be AEM EMS (Infinity), VEMS, MegaSquirt, or ECUMaster Black. All "stand-alone" ECU are capable of being used as a Piggy-back ECU.

Some of the best retail-level stand-alone ECUs are available from Bosch, called the "MS" series -- modern iterations include the MS 3, MS 4, MS 5 and MS 6.x ECUs. These sell in the $3,500-$15,000 USD range. Mid-range options would be along the lines of ECUMaster Black which range between $1,100 and $2,400. Low-range options would include VEMS and MegaSquirt which tend to run between $750-$1,500.

When to Use

This depends on many, many factors. The main consideration would be the limitations of the OE ECU in comparison to your modifications and what they require to operate efficiently.

For example: if you want to use open ITB (Individual Throttle Bodies) without the use of an intake airflow sensor, your ECU will need to have the ability to estimate airflow from a combination of RPM and a variable TPS. This is known as Alpha-N tuning. Alternatively it could utilize a MAP (Manifold Air Pressure) sensor. This would then be known as Speed Density tuning. While many OE ECUs can be tuned for Alpha-N or even Speed Density, not all can, which may make using an aftermarket stand-alone ECU necessary for your needs.

Another example would be: if you want to add forced induction to your naturally aspirated engine, it's best practice to have an ECU with a MAP sensor. While not strictly necessary, depending on intake sensor and ECU used, a MAP sensor allows safer and finer tuning of positive pressure (forced induction) systems. It also offers you the ability to simplify your intake system by removing the intake sensor (MAF/VAM, etc). Many modern OE ECUs are equipped with at least one air pressure sensor, but not all, which may make using an aftermarket stand-alone ECU necessary if you want or need to use a MAP sensor for tuning.

A third example would be: if your OE ECU is not OBD-2 or does not otherwise have a standardized method of outputting data to a logger or digital dash. Being able to monitor the various operational parameters of your engine is massively helpful when tuning. Most every stand-alone ECU will allow for a PC connection for datalogging. Many older ECUs (most pre-OBD2) will not have the ability to log live data easily or without extensive modification. Therefore, this can be a major benefit when compared to OE ECUs that do not have this ability.

A fourth example would be: if you want to convert to a flex-fuel setup and the OE ECU does not support this feature. Many aftermarket stand-alone ECUs will offer fuel sensors that can automatically switch mapping to match various bio-fuels or standard petroleum. It is worth noting that many modern OE ECUs either have this ability built-in, or can be modified to use an aftermarket fuel sensor and switch mapping automatically.

However, by far the most common reason to use a stand-alone ECU is their ease of tuning in comparison to OE ECUs. While many OE ECUs can require extensive knowledge and experience to even get to the maps and settings, stand-alone ECUs often come with simplified software and everything defined for you. However, with simplified tuning comes simplified operation -- OE ECUs almost always have more granular, more advanced, and much better tuning capabilities than most stand-alone ECUs. OE ECUs can and do regularly out-perform many stand-alone ECUs in almost every way, including reliability of engine performance.

Differences

The difference between an aftermarket stand-alone ECU and your OE (Original Equipment) ECU depends entirely on which ECU your vehicle was equipped with. However, all ECU are basically the same thing. They contain a processor, memory, ignition driver(s), injector drivers, and sensor inputs/filters, among other things. They read various sensors, such as engine speed, intake rate and temperature, exhaust gas-oxygen ratio, and throttle pedal position, then output signals to things such as throttle valves, spark plugs, injectors, and emissions related components.

Both OE and stand-alone ECUs will contain very similar components, sometimes nearly identical. OE ECUs are generally designed to much tighter and higher standards, use higher-quality components, and will offer complete and immediate compatibility. Aftermarket ECUs will generally offer more straightforward tuning and the ability to remove or add various sensors, but are generally not as well designed in terms of hardware and software.

Benefits

The benefits of a stand-alone ECU depend entirely on which ECU you start with.

If you want to convert your naturally aspirated engine to forced induction, it's often recommended to use an ECU with a MAP sensor. If your OE ECU does not have this, using a stand-alone ECU with a MAP sensor will allow for safer and finer tuning.

On vehicles with a singular coil and distributor, using an aftermarket stand-alone ECU will allow the use on individual coils through a coil pack or coil on plug system.

Software

Tuning

Often, the tuning available on OE ECUs is far greater than that of stand-alone ECUs. Though this does vary from ECU to ECU.

Most tunable OE ECUs, such as Bosch Motronic and Siemens MS are just as tunable as stand-alone ECUs. In fact, many of them offer far superior tuning capabilities than some systems such as ECUMaster Black, MegaSquirt, and AEM Infinity. They offer superior internal software, filtering, closed-loop control, idle control, and just about everything in comparison to most stand-alone ECUs. For example, a Motronic 3.x ECU (402, 404, 413, 484, 506, etc.) are equal or superior in almost every aspect to many MegaSquirt ECUs, and can be tuned just as much or more than most MegaSquirt versions. They have variable TPS, and with extensive software and minor hardware modification, it is possible to add a MAP sensor input to these ECUs. The OBD-2 Siemens MS/MSS ECUs are far superior to almost all stand-alone ECUs, can be completely tuned in every sense, and are often based on the exact same hardware and software that 700+ Hp racecars are using. There is often little/no reason to change to a stand-alone ECU over these.

Tuning granularity can be an issue. On some MegaSquirt 1 and 2 systems you are limited to a 12x12, 12x16, or at most a single 16x16 fuel and ignition map. Comparatively, most Motronic offer at least two 16x16 maps for both fuel and ignition, and Siemens offer 16x16 to 24x24 maps, sometimes even larger. Motronic 1.x systems often offer multiple 16x24 maps for both fuel and ignition. Compounding that, Motronic and Siemens ECUs almost always offer automatic map switching depending on fuel quality and running conditions, while other ECUs such as AEM Infinity, MegaSquirt, and many every other aftermarket stand-alone ECU offer only one singular operational range. ECUMaster Black offers 2 ignition tables and a singular fuel table.

However, tuning can be quite convoluted on many OE ECUs, with maps, settings, and control bits hidden behind large pay walls or not available at all. You can expect to pay tens-of-thousands for software and definitions from the OEM. While there are cheaper, tuner-created options available for some more popular tuning ECUs, there are a lot of OE ECUs without much or any support. OBD2 ECUs map many things in "requested torque" which adds complication and confusion, and can often lead to unintended results. Most stand-alone ECUs will use very simplified tuning based on speed density, alpha-N, or raw-intake rate. It's important to gauge how much time and effort you are willing to put into the completed project.

Data-logging

In comparison to older (pre-OBD) OE ECUs, a notable benefit is datalogging. Most every stand-alone ECU will allow for a PC connection for datalogging. Many older ECUs (pre-OBD1/2) will not have the ability to log live data. Therefore, this ability may be a major benefit for tuning.

Modern OBD-2 ECUs have a standardized format for data output. This is the OBD, or On-Board Diagnostics standard. These OE ECU will almost always offer many more parameters for logging than most stand-alone ECUs.

Components

Here we cover each important component within ECUs and how they may affect your choice.

Almost all aftermarket stand-alone ECU we've worked with are poorly designed, use sub-par components, and come with lower-quality ignition drivers than OE ECUs. A very common problem with stand-alone ECUs, specifically MegaSquirt, would be its often very poorly designed thermals. On the MS3PRO from DIYAT, it uses a single block of aluminum (a very poor heatsink material to begin with) to attempt to dissipate the heat of ALL drivers (ignition, injection, power, and other TO-220 components). Because it uses a single aluminum block for all components, it also ties the tab of all these components together. Not all tabs are common, therefore some components need to have insulation placed between the tab and the heatsink because it cannot make connection to the tab of another device. It also has signal traces routed under this very heatsink, which distorts many inputs and outputs as heat increases. This design is atrocious and should never be allowed. In many cases, the MS3PRO ECU is a downgrade compared to the stock unit.

Processors

Processing speed can be a benefit in some cases, but it often won't make a large different in full-throttle output. Modern OE ECUs will have processors in the range of 40-1200 MHz, with some exceptions, and they are also tasked with dealing with dozens more sensors and cameras than most motorsport applications will use. Nevertheless, it's rarely a bad thing to have a more powerful processor, even if it's not being completely utilized.

To compare a few more common ECUs:

AEM Infinity uses a 32-bit 200 MHz processor.
VEMS uses a 16-bit 168 MHz processor.
MegaSquirt uses a variety of processors depending on version, ranging from 24 MHz to 200 MHz, most 16-bit, some newer versions are 32-bit.
The higher quality Bosch MS 6.x series motorsport ECUs use a dual-core 32-bit 667 MHz processor.
OE Bosch Motronic 1.x and 2.x ECUs use up to three 8-bit processors between 20 MHz and 100 MHz, while older versions can be as low as 12 MHz.
- 2.x and older Motronic ECUs will use 1 processor per every 6 cylinders, meaning 4 and 6 cylinder engines use 1 processor while 8-12 cylinder engines use 2, even sometimes 3 8-bit 20 MHz processors.
- Newer Motronic ECUs are generally 16- or 32-bit and between 40 and 100 MHz.
ECUs used by the Mitsubishi Evolution and Subaru WRX/STi to this very day operate at 40 MHz, are 16- (pre-2004) and 32-bit (2004 and newer), and are often used in motorsport perfectly fine.
- In 2002 the WRX used a 16-bit 40 MHz processor, and in 2021 it still uses a 40 MHz processor just like the Evo X and Nissan GT-R35, however they are now 32-bit.
The 2021 Ferrari 458 uses a 66 MHz 32-bit ECU.

To put this all in some perspective, the Apollo 11 moon lander used an 8-bit 4 MHz processor. Yes, just 4.

A 40 MHz ECU will generally be able to provide up to somewhere around 1,000-4,000 new ignition and injection values per-second. Though this is very complicated and depends on far too many factors to delve into here. For individual spark and injection control, a 6-cylinder engine at 8,000 RPM could utilize 800 updates per second; an 8-cylinder engine at 8,000 RPM could utilize 1,067 updates per second. Of course, using banked ignition and injection can cut these numbers in half a few times over. In any case, an ECU will still drive the injector/coil even if no new value is available, reusing previous values.

Generally it seems 10 MHz per-cylinder bank with 16- or 32-bit ECUs is perfectly acceptable in modern terms.

Bits

You will read a lot about the "bits" of an ECU. 8-bit, 16-bit, 32-bit are the standards for most ECUs. A few newer ECUs are 64-bit, but this is not yet common. So what does this term "bits" mean? It refers to the number of bits a CPU can process at a time (known as a "instruction sets") and the size of the virtual memory addresses available to the CPU (however not all CPU can register all bits in an address space). An 8-bit ECU can work with 8-bits per cycle/instruction, a 16-bit ECU can work with 16-bits per instruction, and so forth.

For 8-bit OE ECUs, a primary benefit to a stand-alone ECU is finer granularity of tuning. An 8-bit ECU has a maximum of 256 values to work with in any one particular data point. Therefore nearly every change represents a 1/256th jump. Timing, fueling, RPM values, load values, etc. Of course, some ECUs can work with multiple sets of 8-bit values and perform math on those values which means it can produce finer results than a standard 8-bit integer would allow, but it takes longer to do calculations of multiple byte variables when compared to simply reading a singular 16- or 32-bit variable, so it is sparsely used on older, slower ECUs. This means using a 16- or 32-bit stand-alone ECU can offer much finer tuning and quicker computation even at the same processor speed.

A 16-bit ECU has a much larger maximum of 65536 values to work with. Many of the cheaper, sub $1500 USD stand-alone ECUs are still 16-bit. So when compared to OE 16-bit ECUs, there aren't any granularity benefits. If the stand-alone ECU is 32-bit, there is a fair increase in how finely some values can be adjusted, though many (almost all) parameters still use 16-bit values to this very day. Again, a 16-bit ECU can still work with larger integers by using additional cycles for the math.

Most modern OE ECUs are 32-bit, nullifying any granularity benefit of a 32-bit stand-alone ECU. In fact, if you install a 16-bit stand-alone ECU, you may be performing a downgrade.

Ignition

Depending on the OE ECU you're running, you may be limited to wasted spark, distributed single-coil, or otherwise lack-luster systems. Using a stand-alone ECU with individual coil control can be an upgrade if your OE ECU does not have this ability.

None of the lower- and mid-range stand-alone ECUs come close to the design quality and thermals of OE ECUs. Sometimes the same ignition drivers may be used, but often the OE ECUs will use superior quality drivers. For example, the MS3PRO ECUs we've worked on use drivers with no fly-back circuit protection and no avalanche mitigation. This is considered unacceptable in most cases and an OE ECU would never have the same design flaws.

The granularity of ignition timing adjustments may be finer on stand-alone ECUs depending on the OE ECU you start with. A 32-bit map is better than a 16-bit map, which is better than an 8-bit map. However, 8-bit maps are often acceptable for many applications. There is little reason to have an ignition map beyond 16-bit, and most ECU (aftermarket or OE) use 16-bit ignition maps.

Injection

Depending on the OE ECU you're running, you may be limited to banked injection, where two or more injectors are controlled by one driver. This can be quite fine on engines that have multiple cylinders in the intake phase at the same time, such as some 8+ cylinder engines. Whereas engines with fewer than 8 cylinders can often benefit from individual injection control (known as sequential injection).

Sequential injection allows you to make per-cylinder injection time corrections to account for intake manifold design or inner-cylinder dynamics.

The granularity of fueling adjustments may be finer on stand-alone ECUs depending on the OE ECU you start with. A 32-bit map is better than a 16-bit map, which is better than an 8-bit map. However, 8-bit maps are often acceptable for many applications. There is little reason to have an injection map beyond 16-bit, and most ECU (aftermarket or OE) use 16-bit fuel injection maps.

ECU injection maps are simply modifiers to the base air/fuel conversion factor. These maps correct for engine dynamics and volumetric efficiency. Therefore 8- or 16-bit ECUs are perfectly capable of handling most of these maps with ease.

I/O

I/O, or Input/Output, relates to the sensor inputs and programmable outputs of a system.

Input granularity is an interesting thing to consider. ECUs such as ECUMaster, MegaSquirt, and VEMS use only 10-bit inputs, meaning every sensor will read from 0-4095, whereas many OBD-2 ECUs will have 10-, 12-, and even 16-bit inputs, offering vastly superior sensor accuracy. Older 8-bit ECUs will almost always use 8-bit inputs, although many 8-bit Bosch ECUs use I/O extenders and 10- or 12-bit ADCs to increase granularity of inputs, as well as exponential maths to increase the granularity of calculated values beyond the base 8-bit values used in the rest of the ECU.