The Ross-Tech VCDS (formerly VAGCOM) diagnostic system offers a powerful tool for understanding your B8 S4’s performance. Through performance logging, you can gain valuable insights into critical engine parameters, especially beneficial for tuned vehicles. This guide provides a comprehensive walkthrough of how to perform a VCDS performance log on your B8 S4, focusing on key metrics for analysis.
Setting up VCDS for Performance Logging
Before diving into logging, ensure your VCDS software and cable are properly connected to your vehicle’s OBDII port and that your car is turned on.
Fault Code Check:
- Initiate an Auto-Scan in VCDS to detect any existing fault codes.
- Address and clear any faults before proceeding with logging. Save the scan results for reference.
Selecting the Engine Module:
- Navigate to “Select Control Module” and choose “Select.”
- Under the “Common” tab, select “01-Engine.”
- Within the Engine Controller module, click “Adv. Meas. Values” to access the logging setup.
Recording Ambient Air Pressure:
- Select and record the “Ambient Air Pressure” value. This is crucial for later boost calculations. Deselect it after recording.
Choosing Logging Parameters:
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Select the desired parameters for logging. For a comprehensive performance analysis, focus on:
- 002 – Accelerator Position (%): Tracks throttle input.
- 009 – Charge Air Pressure Specified Value (hPa): Represents requested boost pressure.
- 018 – Engine Speed (RPM): Monitors engine revolutions.
- 089 – Air Mass (kg/h): Measures intake air volume.
- 186 – Ignition Angle; Actual (degrees): Indicates actual ignition timing.
- 188 – Intake Air Temperature (degrees C): Measures intake air temperature.
- 190 – Intake Manifold Pressure; Absolute (hPa): Represents actual boost pressure.
- 418-423 – IGA_AD_KNK[0-5] (degrees): Shows timing retardation due to knock on individual cylinders (select at least two).
- 540 – RFP_AV (%): Indicates bypass valve opening percentage.
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Save this parameter selection for future use by clicking the VCDS icon and naming the file (e.g., “B8 S4 Performance Log”).
Optimizing Logging for Accuracy:
- Crucially, enable “Group UDS Requests” by clicking the “Turbo” button. This ensures all logged parameters share the same timestamp, vital for accurate analysis.
Performing the VCDS Performance Log
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With the car running, initiate logging by pressing “Log,” then “Start.”
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Conduct a full 3rd or 4th gear pull (ideally on a dyno or controlled environment) from approximately 3000 RPM to 7000 RPM. Ensure:
- A/C and radio are off.
- Windows are up.
- Car is in Dynamic mode (if equipped).
- Traction control (ASR) is fully off.
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Stop logging once the pull is complete. The log file (.CSV format) will be saved in the Ross-Tech/VCDS/Logs folder.
Analyzing Your B8 S4 Performance Log
Open the .CSV log file in a spreadsheet program like Excel for analysis. Key calculations and interpretations include:
Boost Calculations:
- Requested Boost (PSI):
((Charge Air Pressure Specified Value) - (Ambient Air Pressure * 10)) * 0.014503774 - Actual Boost (PSI):
((Intake Manifold Pressure; Absolute) - (Ambient Air Pressure * 10)) * 0.014503774
Analyzing Key Parameters:
- Accelerator Position: Verify WOT (Wide Open Throttle) is achieved (typically 93-95%).
- Requested vs. Actual Boost: Compare to identify potential boost leaks or performance limitations. Stock turbos typically peak around 10-11 PSI, while tuned vehicles can achieve higher pressures.
- Air Mass: Higher values indicate greater airflow, potentially influenced by intake modifications.
- Intake Air Temperature: Lower is better for performance. Monitor for heat soak issues.
- Ignition Timing: Analyze for timing pull (negative values in IGA_AD_KNK fields), which indicates knock and potential performance loss.
- Bypass Valve: Ideally should remain closed (0%) during a pull on a tuned vehicle, indicating maximum boost utilization.
By analyzing these and other parameters in your VCDS performance log, you can gain valuable insights into your B8 S4’s performance characteristics and identify areas for optimization. Remember that professional tuning and data analysis are recommended for making informed modifications.
