Engine Management 101 (as taught by Cosmo) - Subaru WRX Forum
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post #1 of 1 Old 05-10-2004, 08:54 AM Thread Starter
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Engine Management 101 (as taught by Cosmo)

Engine Management 101
as taught by Cosmo

After seeing many debates about this subject (and being involved in many myself) I figured it was time to set out on an effort to give you, the loyal WRXTuners member, a comprehensive guide to Engine Management and the WRX.


Your WRX uses an Engine Control Unit (ECU) to control how your car runs. It uses inputs from the Mass Air Flow sensor (MAF), Throttle Position Sensor (TPS), and many other sensors to determine how much fuel, how much ignition advance, and how much boost your car should run.

The stock ECU is the form of engine management many WRX owners use, and they are perfectly happy with it. For some, however, the better choice is aftermarket engine management. Aftermarket engine management lets you change how your car runs, and get the most out of other modifications to the engine. Engine management in itself will often provide a very noticeable increase in power.


Your WRX comes from the factory in pretty good shape, even if it does run a little rich. If you don’t plan on doing any modifications to your car’s engine, you don’t need any engine management. Even small modifications like a cat-back don’t require engine management, but the performance gains will be minimal.

If, however, you want to make your WRX more powerful, engine management is a must. The best way to get power out of a turbocharged engine (like the one in your WRX) is to make the turbo create more boost (the amount of compressed air the turbocharger puts into the engine). Engines must maintain a certain Air-Fuel Ratio (AFR) to run correctly. In a perfect world, engines run best at a 14.7:1 ratio of air to fuel (14.7 parts air to 1 part fuel). Turbocharged cars usually run better - and much safer - at a richer AFR.

Your stock ECU determines the AFR for your car after factoring in many different parameters. For a stock car this works great. However, if your car is making more boost than a stock car, it will need more fuel to maintain the optimal AFR as well as possibly retarding timing to prevent detonation or knock. This is where aftermarket engine management comes in.


There are 3 main types of aftermarket engine management available:

A. Reflashed ECU – Your ECU has a series of parameters that tell your car how much fuel, timing, and boost to run at certain RPM levels and throttle positions. This is often referred to as a “map”. When your ECU is reflashed, the programmer changes some or all of the numbers in your map to make your car run differently. Your car will then run according to the map that was “flashed” into your ECU.

Some of the companies that offer this service are Cobb Tuning and Vishnu Tuning.

B. Piggyback Unit – This unit mounts on top or near your ECU (hence the name) and modifies or overrides the signal that your ECU sends to the fuel system, ignition system, and boost control system. These usually have their own map or maps that tell your car how to run.

TurboXS, Greddy, Apexi, and other companies offer this kind of engine management.

C. Stand Alone System – This system completely replaces your ECU, and your car will run solely on the stand alone system’s maps and other parameters.

Some of the companies that offer this are AEM, Electromotive, and Microtek.


This is where much of the debate lies. Unfortunately, there is no definite answer. Different cars require different kinds of engine management. Let’s look at some of the pros and cons of the different kinds of engine management:

A. Stock ECU – Perfectly fine for a stock car, or a car with very little modification. Requires no user knowledge or skill, but does not offer an increase in performance.

B. Reflashed ECU – Provides modified fuel, ignition, and boost maps. When your ECU is reflashed, your parameters and maps are overwritten, so your car will only run according to the reflash. This requires virtually no automotive or tuning knowledge (aside from knowing how to remove a few screws and unplug a few plugs). It offers a noticeable increase in performance, and your ECU will still appear stock. It is the safest and most fool-proof form of engine management.

The drawbacks, however, are that you must use a base map provided by the company that does the reflash. You have no way to fine tune your car to get the most performance you can safely get. For many people this is not a big deal, as they are simply looking to use a reflash to get the most out of the other modifications they do to their car. Also, many dealers reflash an ECU back to its stock form as a precautionary measure when your car goes in for warranty work. If that happens, you lose the reflash you paid for. In addition, when you make additional modifications to your car, you will need to have the ECU reflashed again to take advantage of them.

There are now some companies that offer a user-based reflasher (Cobb Tuning’s AccessPort). This allows you to change the map on your reflashed ECU to one of a limited number of maps provided by Cobb. This can be done while your ECU is still in your car. A reflashed ECU is the cheapest form of aftermarket engine management. They usually start around $500.

C. Piggy-Back System – This system overrides or modifies your ECU’s stock maps for fuel, ignition, and boost, along with other parameters. It is a very powerful system, but also leaves room for user error. Those errors can cause serious damage to your engine. With a piggy-back system, you can change your fuel system, ignition system, and boost control system with a few keystrokes on your computer. This is both a blessing and a curse. In the right hands, this provides a very powerful tuning device, and can help your car run its absolute best. However, in inexperienced hands it can also destroy your engine.

This is not to say, however, that only an experienced tuner should consider a piggy-back system. Many of the companies that produce piggy-back systems provide base maps that you can load with very little computer skill. In fact, if you successfully made it to this web site to read this post, you have enough skill to load the base maps into your car. As you get more experienced, you can start to fine tune your car to run its best while still maintaining safe parameters. Anyone with minimal computer skills and the ability to read can tune an engine. However, if you don’t know what the effects of those tuning procedures are, I recommend having a professional do it for you.

As the name implies, a piggy-back system is an external system that often mounts on top of your ECU. It is immediately noticeable, and in some instances can void your warranty. It can be removed, however, and your car will revert back to stock engine management. Most piggy-back systems also provide a way to bypass the system to allow your car to run on the stock ECU while the piggy-back system is still installed.
The most popular piggy-back systems are the TurboXS UTEC and Vishnu Xede. Cobb Tuning is also preparing to release their AccessTuner which will function as a piggy-back system.

Many piggy back systems also offer data logging features, which can be an invaluable tuning aid. Piggy back systems generally cost around $1000-1200.

D. Stand Alone System – This system is the most complex and expensive, but most powerful system you can use. There are several options in stand alone systems, and all have about the same advantages and disadvantages. The main advantage of a stand alone system is that they offer the most control over your engine, and the most ability to find tune your engine. The disadvantages (although some don’t consider it a disadvantage) is that you have to program every aspect of your engine. Idle, cold start, and other mundane features of your engine are all dependent on you. While I strongly believe that a stand alone system is the best choice for a race car, I just as strongly believe that they are not well suited for a street car.


Well, to answer this question you need to take a look at your car, your needs, your ability, and your budget.

If you are going to be doing minimal modifications to your car (uppipe, downpipe, turbo back), or feel completely uncomfortable with the idea of tuning your car yourself, you should consider a reflashed ECU. Also, if you are on a budget, this is probably the best choice.

If you want to tweak your car to get the best performance possible, make heavy modifications (turbo, fuel system), and want to be able to tune your car to take advantage of current modifications and any future modifications, you should consider a piggy back system. They are the best choice for cars with upgraded fuel systems and/or turbos.

I won’t recommend a stand alone system because if anyone needs one of those, they know it already, and don’t need my recommendation.

Anyone with any comments/criticisms, please let me know. This is YOUR web site, not mine, so I want this guide to be something that benefits everyone!

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