Subaru Datalog Guide

Intro to Datalogging

Whether you own a stock daily driver or a dedicated track car, keeping tabs on how your car is running is vital to making sure your car will last without needing expensive repairs.  While monitoring gauges on your car can let you know what is happening with a few parameters at a given moment, it won't be able to give you a full scope of how the car is running. In order to gain perspective over a wider range of time and parameters, we run a datalog.


Here's a good reference on how you actually create a datalog
How To: Datalog


What is a Datalog?

A Datalog is a file taken by the Accessport where it records a user-specified set of sensor data and/or information that the vehicle's computer is using to run the engine. This can include things from simple data like coolant temperature or boost, to how often the computer wants the fuel injector to be open during a given engine cycle.


Why Can't I Datalog Everything?

While each generation of ECU has different limitations there is generally a limit to the number of parameters you can select.  In most cases, this is because the ECU needs to process the requests for data and output that information to whatever device is requesting it.  As a result, the more monitors you request, the more work the computer has to do in order to keep up.  On some vehicles, as you approach the limit you won't see any differences until it simply cannot log more items.  However, on other platforms, you will see the approach to the limit show itself as a slow down in the logging rate, so if you are logging a bunch of different items make sure to keep an eye on the amount of time in between each sample to keep things in perspective.


Things Every Good Datalog Needs

A Map with no reference points is never going to help you get from one place to another.  There are a few parameters that do wonders for helping to put other monitors into context, as well as showing your environment and driving style.

  • RPM
  •   This lets us see what speed the engine is moving at in Revolutions Per Minute.  Additionally, conditions of acceleration, deceleration, and maintaining a steady speed will all cause variations in the fueling, ignition timing, and boost.
  • Throttle Position
    • How far open the throttle is open changes airflow to the engine and when combined with the RPM helps to give an indication of the amount of load the vehicle is under.

  • Accelerator Pedal Position - APP (drive by wire cars only)
    • Measures the amount the accelerator pedal is pressed down by the driver.  It does not always equate to the same amount of throttle position and can give us additional information similar to what throttle position offers.

  • Barometric Pressure (when available)
    • A Measure of Barometric Pressure which is the air pressure at your location caused by multiple factors like temperature and elevation.  Different pressures have an impact on air density which in turn will impact the amount of fuel necessary as well as peak boost and the way in which the turbo spools.   At lower pressures (higher elevation) the turbocharger will have to work harder to achieve the same boost pressures.



Quick Reference Guide - All Subaru Platforms


Monitor NameWhat is it?What can it Tell Me?EJ Port Injected
(Normal Expected Behavior)
FA20F, FA24F Direct Injected
(Normal Expected Behavior)
IdleLight ThrottleHeavy ThrottleIdleLight ThrottleHeavy Throttle



AF Correction 1
(Short Term Fuel Trim)



A correction based on current fueling error

The values shown are a percentage correction applied to the injector pulse width.  Positive values mean that more fuel is being injected and negative values mean less.  These corrections are called trims.  Their purpose is to adjust fueling in order to help the engine run at the currently desired air/fuel ratio.  When you floor it or lift throttle and coast, you'll notice trimming stops (0% correction).  During throttle transitions, expect to see trims move around as a part of normal behavior.  Consistent corrections under similar conditions will be learned by the ECU and applied as AF Learning (See Below).  Excessive trimming at idle or cruise may indicate an intake tract leak, bad sensor, or using the incorrect intake for specific mapping



+/- 10%



+/- 20%



+/- 10%, 0% at full boost.



+/- 15%



+/- 20%



+/- 10%

AF Learning 1 (Long Term Fuel Trim)Learned Corrections based on AF Corrections needed in the past.Just like AF Corrections, these are percentages added or subtracted to fueling.  The values are learned slowly over time and are often referred to as long term trims.  AF learning and AF correction values are added together, and then applied as a total value to the cars final fueling.  Be mindful that you may experience long and short term trims which partially cancel each other out.  For example a short term trim of +12% and a long term trim of -15% would result in a -3% trim.  Generally, over time both trims will then get smaller as the ECU continues learning.  Excessive long term trimming may indicate a mechanical issue like those that applied to AF Correction.



+/- 10%



+/- 10%



+/- 10%, 0% at full boost.



+/- 15%



+/- 15%



+/- 10%

BoostThis is a measure of intake manifold pressure/vacuum.This can help to diagnose failing turbos, boost creep, and help make sure boost levels are appropriate

Varies with map

Varies with map
Coolant TempTemperature of the engine coolant, taken at the engine block crossover pipe.A measure of engine temperature.  The car will typically alter timing and fueling at different engine temperatures.  Can also let you know if the car is overheating or not fully warming up which could indicate issues with the cooling system or a faulty sensor


<215F


<215F


<215F


<215F


<215F


<215F

DAMA Multiplier applied to dynamic ignition tables.Typically a good indicator of engine health.  If the car is at proper DAM levels the car is typically running relatively well.  Drops in DAM can be caused by a lack of maintenance, incorrect parts for the map, poor fuel quality, or boost/vacuum leaks.02-05 WRX: 16

06-14 WRX: 1.0
02-05 WRX: 16

06-14 WRX: 1.0
02-05 WRX: 16

06-14 WRX: 1.0


0.6 - 1.0


0.6 - 1.0


0.6 - 1.0

Feedback KnockCurrent Knock EventsThis is a measure of the ECU's response to knock at the current time.  This can be an indication of a few different things.  The knock sensors will sometimes pick up noise caused by drivetrain movements under hard driving or if you're having issues with your clutch or engine and transmission mounts.  Sometimes it will even pick up the air conditioning clutch engaging.  Alternatively, the knock may be real, so your car may need better fuel, maintenance, it may have the incorrect parts for the map being used, or you may have a loose bolt on the engine.  As you can see in the chart, the higher compression direct injected engines are prone to larger and more frequent corrections of this type during normal operation.


0


0 to -2.8


0 to -2.8


0


0 to -4


0 to -4

Fine KnockA response to perceived knock that persists each time the engine is operated under similar conditions.  The ECU will gradually lessen these corrections until the car is back to operating with no correction, or until the car knocks again, at which point it starts the process over again.Readings outside of the normal range indicate the same potential concerns as feedback knock.


0


0 to -2.8


0 to -2.8


0


0 to -4


0 to -4

GearCurrent transmission gear.It helps to double-check per-gear tuning items and to provide a context of how the vehicle is being driven during a datalog.





Intake Air TempThe temperature of the air charge going into the intake.As atmospheric conditions change, engine efficiency and operation are affected.  The ECU attempts to correct for temperature changes in various ways.  Knowing the temperature reading the ECU was given provides context for other readings such as boost.





RPMEngine revolutions per minuteThis is a measure of how fast your engine is spinning.  Inconsistencies at idle can indicate faulty sensors or vacuum/boost leaks.  When looking at a datalog or graph under acceleration, seeing erratic values can be an indication of misfiring due to spark plug, coil pack, grounding issues, a fuel supply issue, or even a clutch or transmission slip.





TD Boost ErrorMeasurement of boost pressure measured vs what is requested by the ECUTD Boost Error shows where your car is under or overboosting.  Keep in mind, you also won't always hit the requested boost pressure under every circumstance.  For example this value isn't very useful when you aren't driving at full throttle.  Some error is expected based on changes in driving and ambient conditions, variance in engine and turbo health etc.


+/- 1.5 psi after peak boost is achieved.




+/- 1.5 psi after peak boost is achieved.




FA20F Direct Injected

  • 14-18 FXT
  • 15+ WRX

FA24F Direct Injected

  • 19+ Ascent


Subaru's latest and greatest engine which utilizes Direct Injection (DI) rather than the port injection of the previous generation.  On a traditional port injected engine, the fuel injectors are positioned to fire down into the intake port of the cylinder head towards the back of the intake valve.  On the new engines equipped with Direct Injection, the injectors are positioned to actually fire directly into the combustion chamber.  Along with those changes Subaru made an effort to tighten things up a bit to improve things like emissions and fuel economy while avoiding the loss of performance.  We see these changes in the cars programming in the way it handles different changes and types of driving.  While still retaining many of the same systems and monitors from previous generations, some of the monitors are made a little bit more fluid than in previous iterations.  In previous generations, the Dynamic Advance Multiplier (DAM) would be expected to stay at one value the majority of the time unless the car has seen some sort of changes or an event that required it to be altered (long trips, poor fuel quality, etc.).  With the new DI cars, we typically expect the DAM to be a little bit more fluid than before, with the expected value to vary a little bit even when otherwise running normally.  Other changes we see involve the knock system.  When running relatively lean at times of part or no throttle cruising, knock events and changes to feedback knock are going to be much more commonplace than on the older vehicles.  This is completely normal and expected from the OEM logic and is not altered when using a COBB off-the-shelf map.  One other notable is that with other DIT platforms (i.e. VW, BMW, Mazda), the fuel pumps in Subaru DIT vehicles are not designed for ethanol use beyond 10-15%.




All Other Subaru Vehicles - EJ Port Injected Engines

  • 02-14 Impreza WRX
  • 04-20 Impreza WRX STI
  • 09-10 Impreza 2.5 GT
  • 02-12 Legacy GT
  • 06-09 Legacy GT Spec B
  • 04-13 Forester XT


The venerable EJ engine has been rumbling into our hearts from its first introduction nearly 3 decades ago.  Through dozens of configurations and iterations from front-wheel-drive naturally aspirated to all-wheel-drive twin turbo, many add this flat-four to the list of best 4-cylinder engines of all time.  While it is a very well-known platform at this point, there are a few specific issues that crop up with some vehicles that are important to keep in mind. 

The 08-14 STI (as well as the current generation of 15-20) have a relatively well known problem known as a fuel stumble.  Due to the way the fuel flows through the stock fuel system at certain positions of fuel flow you'll see your fuel pressure unexpectedly drop, causing a stumble in your vehicle's acceleration.  While there are many different options to fix this, we've seen that some options will work for some vehicles, while others won't.  The most thorough way we've found to resolve this issue is to replace most of the fuel system in the engine bay while leaving a few of the fuel lines as rubber.  While not as durable (or good looking) as AN lines and fittings, the rubber lines have more flexibility and elasticity to their makeup.  This means that any odd surges in fuel pressure can be partially canceled out by the rubber hose.

The 15-20 STI are all great cars straight from the factory, and respond very well to modification. That being said, the fuel system is not nearly up to the task. We were unable to get the fuel levels into a safe territory when utilizing both a downpipe and an intake. So unless you step up to the necessary fuel system modifications, you won't be able to reach the air/fuel ratio required in order to safely run both an intake and downpipe.  However, the cure for the fuel stumble listed previously also happens to solve this problem, leaving your car with a fuel system that can handle E85 and some further modifications without trouble.




Special Notes:


Custom Tunes

With a custom tune, keep in mind that since they're made to specifications other than what we use, normal values may vary.  For troubleshooting on a custom map reach out to the Protuner responsible for your tune and they should be able to help you out!


Engine Swaps

With thousands of cars around the world utilizing these engines, oftentimes swapping a used motor into your car can be a cost-effective alternative to rebuilding a tired or broken engine.  However there are many important things to keep in mind when doing this as not all engines are created equal.  Oftentimes foreign engines can come from the exact same model but have completely different hook-ups, sensors, or manifolds attached to the engine itself.  Harder to detect are the engines that have different cam specs or a different compression ratio (a common occurrence between United States (USDM) and Japanese market (JDM) vehicles).  This means that while getting it to fit can be a simple matter of just swapping a few parts over, getting it to run healthily and correctly is a different matter.  Due to the fact that none of the engine's sensors will detect any of these changes, or the potential for a difference in sensor to cause skewed values, you will need to get a custom tune when swapping your engine out for anything other than the exact same engine your car had initially with stock specifications.


Built Engines

In the quest for ever more speed and power, eventually you come to the point of ordering up a built engine block or even having one custom built to your specifications.  This is an incredibly exciting day, as you'll no longer be held back by the limitations of the stock bottom end and can explore the upper limits of what your car (and in some cases you) are capable of.  Similar to swapping for another stock engine, you'll need to get a custom tune in order to keep your engine running safe and in optimal condition.  An added reason for this is that when using aftermarket parts that are forged or billet, there are differences in how much expansion the materials will undergo when they start to absorb the heat of the engine running.  As a result when attempting to run a stock tune with a built engine, you'll probably see a lot of knock numbers from piston noise.  It's important to tune the car appropriately for that so that these false knock readings don't mask the ones which are real, just like brakes can help you avoid an accident and keep your car safe, a tune can help avoid detonation or running lean, and keep your new engine safe.




How To: Update Accessport Firmware - Can provide assistance with getting your Accessport running the most up-to-date firmware.

How To: Change Datalog Monitors - This shows you how to adjust your datalog monitors so that you are viewing what you'd like to look at.

SUBARU CALIBRATION SUPPORT - A link to the map notes for all Subaru vehicles.  This way you can determine what map you should be on for the parts equipped on your vehicle.

Subaru Monitor List - A list of all data monitors used by the various Subaru platforms.


Customer Support

Email: support@cobbtuning.com

Phone support available 9am to 6pm Monday-Thursday. 9am to 4pm Friday (CST)

866.922.3059