How To Datalog: Subaru Vehicles

Datalogging On Subaru

Applicable Vehicles:

  • All Supported Subaru Vehicles

When And Why?:

When trying to diagnose driveability problems or assessing how well a new tune is working with the vehicle datalogging can be an invaluable tool.  By looking at what is going on with the vehicle under load or other conditions you're able to look at a picture in time and see if there were any issues or problems that would need to be addressed and sometimes where and why they occurred.  However a datalog is only as good as the monitors you record, much like how a map is only as useful as the information it has on it

How Should I Drive When I Datalog?:

Because most of the problems people have are those that show up under heavy load, we want to have some good boost characteristics over the datalog.  Therefore we recommend you find an empty stretch of road so that it's not only safe for other people, but safe for you.  When trying to diagnose a problem it's easy to have too much to look at, so when trying to diagnose something we recommend keeping it short and simple.  In these instances a datalog of a single run from 2500rpm to redline in 3rd gear (if running a 5-speed) or 4th gear (if you have a 6-speed) ensures you don't have to spend time sifting through extraneous data to find what you need.  If your problem has to do more with idle characteristics perform the 2500rpm run, but sometimes taking a look at what your car does at idle can be of some help.  Lastly if you're a road racer you may want to datalog for long streches of time which, when paired with track telemetry, can help you figure out when and where things were going well, and when things were going bad, this can help diagnose more intricate problems like fuel slosh or oil starvation.  However as datalogs get bigger sifting through the lenghty spreadsheets can get difficult and time consuming at that time you may want to invest in a datalog viewer program which converts your spreadsheets into simple graphs so you can go through a bunch of data all at once.

Things To Look Out For:

  • A/F Learning 1
    • In general, the values should not exceed a range of +/- 8, but the closer to zero, the better. If you see values outside of this range, the first thing to verify is that you are using the proper intake for the specific mapping. If you use a brand intake that is not listed on the mapping, you could create potential negative issues with the tune on the vehicle. It is also important to verify that the filter is clean and there are no other leaks in the intake tract. These are best tested for using a smoke tester. Other reasons these values might exceed this range are due to a dirty or bad MAF sensor, or a failing front O2 sensor.
  • Boost
    • The various maps have designated peak boost levels and are designed to taper towards redline. You can look up your mapping’s target boost by looking at the map description or by reviewing the map notes on our website. The peak boost value variance is generally +/- 1 psi. So, for example, if the target boost is 15.2 psi, a logged peak boost pressure anywhere between 14.2 psi to 16.2 psi would be acceptable. If the car is still unable to reach target boost, you should check for boost leaks, and then try using a High Wastegate (HWG) map. If the car exceeds target boost, you should try using a Low Wastegate (LWG) map.
  • DAM
    • The starting value will depend on the vehicle and tune, but it should always learn up to the maximum value. In general this value is determined by historical  detonation/knock as well as the default mapping value. For the 2.5L ECU DAM is stated as a decimal ranging from 0 to 1. For the 2.0L ECU DAM is stated as an integer between 0 and 16. DAM will tend to vary the most immediately following a reflash or ECU Reset procedure. If you see this value begin to drop under any other circumstance, you most likely have a severe knock situation. You should immediately analyze the tune and mechanical condition of your vehicle.
  • Fine Knock Learning
    •  This value represents minor learned corrections currently being applied by the ECU as a product of historical noise. These values are reset if the DAM value is changed; once DAM has stabilized, any learned corrections will be intermittently evaluated for sanity when certain  thresholds/conditions are met. Fine Knock Learning is learned (and thus applied) for specific ranges defined by load and RPM; once a correction value has been learned into a particular range, it will be applied whenever the engine is operated within that range.  Occasionally under low loads you might see an initial knock value followed by several values of that number decreasing in the incremental value (ex: -1.4, -1.05, -0.7, etc). This would mean that the ECU is already learning the correction away, and would be nothing to worry about. These values would also immediately go away after an ECU reset. If the ECU is making consistent and multiple knock corrections under load (such as full throttle and full boost) all in a row, you could have a potential knock situation.
  • Feedback Knock Correction
    •  This value represents a current realtime minor timing correction made by the ECU in response to a perceived noise. Feedback Knock Correction is the default correction used by the ECU. The ECU will “respond” with this monitor by immediately removing timing and then slowly decaying the value back to zero assuming no further noise is detected.  On occasion you might see a few initial correction values pop up under low load situations that do not respond to timing or fuel changes. Just like Fine Knock Learning, consistent and multiple corrections made under high load and not due to sudden throttle changes or shifts can be indications of knock. If you see corrections like this, you should immediately inspect the tune and mechanical condition of your vehicle.
  • Knock Sum
    • This is a somewhat arbitrary value that should only be analyzed under the specific conditions you wish to evaluate, such as during wide-open throttle (WOT) operation over a set RPM range. Some later ECUs can report this on a percylinder level, some only on a global basis. If a noise is perceived, this value will increment at all times, regardless of if the knock detection system is deemed to be accurate or not, such as idle and very low RPM.

Frequently Asked Question:

Q: What type of logs should I take to monitor for detonation/knock?

A: The default logging list on the AccessPORT will include the relevant monitors, so that is a great place to start. In general, detonation and knock will be most dangerous when cylinder pressures are highest, which will usually occur during full power (WOT) operation. In general, a log over a singular gear through the full RPM range (from 2000 RPM to redline) is sufficient for a street car; for a race car you should consider logging longer sessions that includes gear shifts. Making sure to keep your logs short (IE, start and end the logging session when appropriate) will help prevent from having to filter through large
quantities of irrelevant data.

Q: What is detonation or knock? How is it detected by the ECU?

A: “When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of
fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits.”i From a more succinct technical perspective, “Knock is the explosive spontaneous ignition of fuel/air mixture ahead of the normal propagating flame
and the subsequent cylinder pressure oscillations.”ii Audible detonation is often referred to as “pinging” due to the sound created; knock takes its namesake from the audible sound created by the event (much like a fist knocking on a door) and is sometimes also known is pre-ignition. To facilitate detonation
detection, a Piezoelectric microphone sensor is hard-mounted to the engine block and relays detected noises to the ECU, where they are further filtered to hone in on the specific frequencies generated by detonation in an attempt to accurately determine their source and relevance. The timing of these events
relative to the measured crank shaft angle is used to estimate the source cylinder(s) by more advanced recent implementations.

Q: What causes detonation and/or knock?

A: In short, any number of conditions or factors can induce detonation. The sources are highly variable but in general, increased cylinder pressures and/or temperatures or reductions in effective octane level are the common “causes”. The factors that lead into those are again varied, but from a tuning perspective, detonation and knock occur:
• When the engine speed is low and the manifold absolute pressure (MAP) is high
• When combustion duration is long
• When temperatures are high (ambient, coolant, combustion chamber surface)
• When the charge dilution is low
• With high carbon deposits
• When the spark advance (or injection timing) is high
As can be seen, the “big three” of boost, ignition timing and air-fuel can all play an integral part in causing or contributing to detonation/knock.

Useful Monitors In Every Situation:


  • AAT – Ambient Air Temperature
  • CCT – Catalytic Converter Temperature
  • ECT – Engine Coolant Temperature 
  • EOT – Engine Oil Temperature
  • EVT – Exhaust Valve Temperature
  • MBT – Minimum Timing for Best Torque
  • AFR – Air to Fuel Ratio (Lambda)
  • DC – Duty Cycle (used with various solenoids)
  • ECU – Engine Control Unit (also known as PCM)
  • MAP – Manifold Absolute Pressure (Post-Throttle)
  • OAR – Octane Adjust Ratio
  • FRP – Fuel Rail Pressure
  • LTT – Load To Torque
  • OTS – Off The Shelf
  • OB – Overboost
  • LSPI – Low Speed Pre-Ignition
  • HDFX – High Degree of Freedom Executive
  • CAT – Charge Air Temperature
  • COT – Compressor Outlet Temperature
  • EFT – Exhaust Flange Temperature
  • TOT – Transmission Oil Temperature
  • IAT – Intake Air Temperature
  • BL – Borderline (for ignition timing tables)
  • CL/OL – Closed Loop/Open Loop (for fueling tables) 
  • DTC – Diagnostic Trouble Code
  • O2 – Oxygen Sensor 
  • TIP – Throttle Inlet Pressure (Pre-Throttle) 
  • VCT – Variable Camshaft Timing
  • VSS – Vehicle Speed Signal (MPH/KPH)
  • TTL – Torque To Load
  • FBO – Full Bolt Ons
  • UB – Underboost
  • PD – Power Demand
  • VE – Volumetric Efficiency