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How to Datalog

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For a brief overview of how to take a datalog and what to look for, check out our Subaru Datalogging Technical Article (text below) or watch Episode 1 of AskCOBB below!


Introduction
If you've ever watched the movie High Fidelity, you know there's nothing better than making a top 5 list: top 5 cars to own, top 5 modifications, top 5 roads, or top 5 driving songs. On the Subaru ECU, the Cobb AccessPORT can view over 100 different parameters, everything from battery voltage to wastegate duty cycle. When viewed on a giant spreadsheet, these parameters can become pretty overwhelming and the varying numbers and units don't make deciphering the data any easier. Here's our top 5 list of Subaru data log parameters with descriptions of the monitor, what the numbers mean, and things to look at for while taking vehicle logs.

Data Log Recording
The first step to taking a good data log is collecting the proper information. If the data log is too short, there won't be enough to data to analyze, and if the data log is too long, there will be too much information to sort through. The best log is safely taken in 3rd gear for a 5-speed transmission and 4th gear for a 6-speed transmission. Start the data log at about 2500 RPM and drive to redline at full throttle. This kind of pull will have the most ideal load on the car in order to provide proper boost characteristics. Once the log is complete, transfer it to the computer using AP Manager and then open it using any spreadsheet program such as Microsoft Excel or OpenOffice Calc.

Top 5 Monitors
Our top 5 monitor list consists of; A/F Learning 1, Boost, DAM, Fine Knock Learning, and Feedback Knock Correction. There are many other parameters to log though, and with experience they can be useful to help diagnose tuning and mechanical issues on the vehicle. But with a quick glance and little knowledge, these 5 monitors can give you a good idea of your vehicle's health and performance.

Monitor: A/F Learning 1
What it does: A/F Learning 1 measures input from the front O2 sensor and applies fueling corrections.
What the numbers mean: The value in the log is a percentage correction of the injector pulse width base. A positive value means fuel is being added and a negative value means fuel is being removed.
Why you monitor it: This monitor can clue you in on potential intake tract leaks, bad sensors, or using the incorrect intake for specific mapping.
What to look for: 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.

Monitor: Boost
What it does: Boost is the measurement of current Manifold Absolute Pressure minus current Barometric Pressure.
What the numbers mean: Boost on the AP is measured in psi. Positive numbers mean pressure is increased in the manifold and negative numbers mean the manifold is under vacuum.
Why you monitor it: It is important to verify the car is boosting properly to make the best power.
What to look for: 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.

Monitor: DAM
What it does: DAM, or Dynamic Advance Multiplier, is a learned correction that adjusts overall timing.
What the numbers mean: For the 02-05 WRX, this number ranges from 0 to 16. For all other turbo model Subarus, this number ranges from 0 to 1 in tenths of a decimal (ex: 0.8). The number advances upwards when no knock is detected and decreases for extreme knock situations. You always want this number to be at it’s maximum value.
Why you monitor it: If you ever see this number decreasing, you have a potential severe knock situation.
What to look for: The starting value will depend on the vehicle and tune, but it should always learn up to the maximum value. This value will reset to it’s starting point after a map Reflash, an ECU reset, or a battery disconnect. 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.

Monitor: Fine Knock Learning
What is does: Fine Knock Learning is a learned correction that makes small corrections to timing once the DAM has settled.
What the numbers mean: The values are degrees of timing being added or subtracted. An initial correction is typically -1.4, but is learned away in increments of 0.35.
Why you monitor it: When Fine Knock Learning hits extreme values, the ECU may start to adjust the DAM.
What to look for: 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.

Monitor: Feedback Knock Correction
What it does: Feedback Knock Correction is an instant correction that the ECU applies to timing based on input from the knock sensor and in conditions where the learned corrections of DAM and Fine Knock Learning to not apply.
What the numbers mean: The values are degrees of timing being added or subtracted. An initial correction is typically -1.4 or -2.0 depending on the ECU.
Why you monitor it: Monitoring this can clue you in on instant knock correction.
What to look for: 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.

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