Choosing Parameters for Logging

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The following two parameters can be looked up from tables in the tune, however I tend to include them in my logs for the sake of convenience:

• Target Boost (psi relative sea level) - it’s useful to compare target boost and actual boost. Big differences are generally bad. Note that if you want faster logging, you can also omit this parameter, and look up the values from your target boost table, provided you do log the engine speed and throttle angle parameters.
• Primary Open Loop Map Enrichment (2-byte) - this is basically the desired AFR. This can be looked up from the tune, provided you log the engine speed and load parameters.

If you’re working on boost control specifically, you’ll want to log these:

• Throttle Plate Angle
• Engine Speed
• Initial Wastegate Duty Cycle

• Ignition total timing - You can get timing from the tables in the ROM, but this includes compensations (e.g. intake temperature, coolant temperature) which can be interesting

You want to dial in your boost control

If you’re working on boost control specifically, consider dropping a few parameters from the list above, and adding these:

• Turbo Dynamics Integral
• Turbo Dynamics Proportional

There’s something weird going on with your car

Weird things happen, and logging can sometimes help us find out what’s up. At minimum you will probably want to log Engine Speed (RPM) and Throttle Plate Angle (degrees) and Load (g/rev) or Mass Air Flow (g/s), so that you can tell roughly what was going on when the weirdness happened. Add whatever parameters you think might be interesting. But don’t get too carried away, as you might miss the weirdness if you select too many parameters and only get two log rows per second.

Log what you need, and only what you need

It may be tempting to select every parameter that sounds interesting. The problem is that it takes a significant amount of time for the ECU to transmit each parameter to your laptop. The more parameters you log, the more time it takes to get each row of log data. If you log a dozen parameters, you’ll get about seven rows of log information per second. If you log two dozen, you’ll get only 3–4 rows per second.

Why are you logging?

There are lots of good reasons to log, and you’ll want a different set of parameters depending on why you’re logging. The following sections list a few parameters that are useful for a few common logging scenarios.

1. You just want to know if the car is healthy
2. You want to dial in your MAF scaling in the “closed loop” range (below grams per second)
3. You want to make more power
4. Boost control: you want to fix spikes, or oscillation, or too much / too little taper
5. Your car is acting weird and you want to know why

You want to know if the car is knocking

Knock is the #1 cause of death for modified Subaru motors, so it’s always a good idea to keep an eye on it.

For some Subarus, there is a parameter called Knock Sum that will increment every time the ECU detects knock. This makes it very easy to monitor knock. If Knock Sum is supported for your car, you should probably always log it. It’s only one byte, and it’s always interesting.

However, Knock Sum is not available for all Subarus - if that’s the case for your car, you’ll need to log three separate parameters to know if the car is knocking:

• Ignition Advance Multiplier
• Feedback Knock Correction (also known as FBKC)
• Fine Learning Knock Correction (also known is FLKC)

For a complete description of these parameters, read this thread started by Merchgod. It’s complicated, but really should understand it - even if your car does support Knock Sum.

In addition to the parameter(s) above, you should also log Engine Speed and Load. The timing tables use Engine Speed on one axis and Load on the other axis, so if you see knock at a particular speed/load combination, that tells you which cell needs to be reduced to prevent knock in the future.

If your car does support Knock Sum, you generally do not need to log IAM, FBKC, and FLKC unless of course you are simply curious about how they work. If your car does support Knock Sum, it is advisable to log IAM, FBKC, and FLKC at all times, unless you are very confident that your car doesn’t knock.

You just want to know if the car is healthy

Not many people log for knock alone. Generally we’re also interested in the conditions that facilitate knock, particularly air/fuel ratio and boost. Following is a list of parameters that are useful to log in addition to the knock parameter(s) above, to assess the overall health of your car:

• Manifold Relative Sea Level Pressure - aka “Boost.” There are a few parameters for boost, so far this is my favorite, but my mind is not made up… The target boost table is also relative to sea level pressure, though.
• Target Boost (psi relative sea level) - it’s useful to compare target boost and actual boost. Big differences are generally bad.
• Primary Wastegate Duty Cycle - known this will help you tune your boost levels
• Mass Airflow - Helps with dialing in MAF scaling. Also, if you add boost and this doesn’t go up, adding boost didn’t get you any more power - it just made knock more likely
• Primary Open Loop Map Enrichment (2-byte) - this is basically the desired AFR
• Wideband O2 AFR - the actual AFR
• Throttle Plate Opening Angle - this plus engine speed tells you what cell you’re in on the target boost and wastegate duty cycle tables
• Ignition total timing - You can get timing from the tables in the ROM, but this includes compensations (e.g. intake temperature, coolant temperature) which can be interesting