View Full Version : Vag-Com Data Logging FAQ

October 30th, 2007, 11:12 AM
Courtesy of bhvrdr at Audizine.com
Hi all,
Just starting a write up on vag com data logging. I have a lot more channels to cover but would appreciate you all looking this over and posting suggestions and corrections as necessary. Most of the info comes from my memory and experience so I would very much appreciate corrections and additions. Hopefully this will be helpful to people. cheers! Mike

Note: In order to have a log that will be valid, you will need to see at least a 3rd gear wide open throttle run from about 2500rpm until redline. Any lifting of the throttle during the run will result in inaccurate A/F numbers for the purpose of our evaluation.

Channel 1: Lambda Correction

This is a fuel correction table used by the computer when it senses the car richening and leaning. The numbers will be displayed as a percentage starting from 0% indicating NO fuel correction is needed. This means the car is doing what it should be. No fuel is having to be added or removed to maintain harmony. The computer can correct for these rich and lean conditions up to 25% or so. Note that seeing numbers higher than 0 does not necessarily mean your car is running dangerously rich or lean just because of a little correction. Positive numbers displayed indicate fuel is having to be added to compensate for lean conditions. Negative numbers indicate fuel is having to be removed to compensate for a richer condition. The lower the numbers the better. Numbers upwards of 20 indicate some real problems that bear much closer attention. Remember that lean is your ultimate enemy.

Channel 2: g/s MAF Sensor

This channel measures the grams per second or airflow taken into account by the MAF sensor. Input I have collected from tuners suggest that this is not a direct measure and depends on other sensors, so programming may be able to affect the g/s independent of changes to actual airflow. Nevertheless, while this may not be a reading that can completely state the airflow your car is taking in, it can provide a good estimate. This is a great log to look at if you suspect a common failure, the dying MAF sensor. On a chipped car one would expect MAF numbers to increase linearly as the car approaches redline. Your highest numbers will be seen at or near redline and are likely to be in the neighborhood of 160-180 g/s. Low numbers at redline such as 120 g/s are a good indicator your MAF is on the way out. Codes may not be thrown at this point. Terribly low or NO readings means she is dead.

Channel 20: Timing Retard for Each Cylinder

This channel is very straight forward. You should see a field of 0s everywhere. The number 0 in each of the cylinder boxes indicates NO timing retard is taking place. This means no timing is having to be removed by the computer as it senses knock or leanness. Now, what if you see some random numbers like "1.5” and "3” every once in awhile? This should be fine. If you were a tweaker, ideally you would want to find that point where you are able to use the most timing without triggering problems. Since most people do not mess with timing adjustments, we want to see as close to zero as possible though. Timing retard of greater than "6” would have me worried and I would want some further investigation and adjustments made.



2500 0 0 0 0
2750 0 0 0 0
3000 0 0 0 0
3250 3 0 1.5 4
3500 3 3 4 4
3750 4 6 6 6
4000 3 0 3 4
4250 6 6 6 6
4500 6 6 6 6
4750 6 4 6 4
5000 8 6 8 4
5250 0 4 6 8
5500 6 6 6 6
5750 8 4 6 6
6000 6 6 6 4
6250 8 8 8 8
6500 8 6 6 6
6750 6 6 6 6


RPM CYL 1 CYL 2 CYL 3 CYL 4 Retard
2500 0 0 0 0
2750 0 0 0 0
3000 0 0 0 0
3250 0 0 0 0
3500 0 1.5 0 0
3750 3 0 0 1.5
4000 0 0 3 0
4250 1.5 1.5 3 3
4500 3 3 3 3
4750 1.5 3 1.5 1.5
5000 3 3 6 3
5250 0 1.5 3 0
5500 3 3 1.5 1.5
5750 3 4 3 1.5
6000 1.5 3 1.5 4
6250 3 3 3 3
6500 4 3 3 3
6750 1.5 3 1.5 3


RPM CYL 1 CYL 2 CYL 3 Cyl 4 Retard
2500 0 0 0 0
2750 0 0 0 0
3000 0 0 0 0
3250 0 0 0 0
3500 0 0 0 0
3750 0 0 0 1.5
4000 0 0 0 0
4250 0 0 0 0
4500 0 0 0 0
4750 1.5 0 0 0
5000 0 0 0 0
5250 0 1.5 0 0
5500 0 0 0 0
5750 0 0 0 0
6000 0 0 0 0
6250 0 0 0 0
6500 0 0 0 0
6750 0 0 0 0

Channel 31: Lambda Reading or A/F Ratio

This value is particularly important to be viewed and interpreted only when the car is under full throttle input as lifting up on the throttle will result in funky numbers. Take your log in third gear (or higher if your local authorities will allow) from 2500rpm or so until redline. The values you will see are: "1” = 14:1 ratio, ".85” = 12:1 ratio, ".75” = 10:1 ratio. A car that runs 14:1 (lambda value of about .95) all the way up to redline on increased boost is running a bit lean. Conversely, a car that is running 11:1 (lambda value of about .80) from idle to redline is running a bit rich. Remember that lean is your ultimate enemy. Running too lean for too long will spell disaster for the motor. It is ok for making power, but it is not ok for protecting the turbo and motor from failure. Ideally you would like to see the A/F pass linearly from the factory 14.7:1 at idle towards 13:1 in the mid rpms (3500rpm or so) to at least 12:1 at redline. This would show a car that is getting good fuel mileage under easy driving, but richens up nicely as you wind it out under full throttle to redline. This would make you feel at ease driving the car under high load conditions at high speeds (freeway cruising at 120mph) or using the car for frequent track days.

This would be nice:

RPM Lambda
2500 .99
2750 .95
3000 .95
3250 .95
3500 .90
3750 .90
4000 .90
4250 .85
4500 .85
4750 .85
5000 .85
5250 .85
5500 .85
5750 .85
6000 .80
6250 .80
6500 .80
6750 .80

Channel 34: EGT (Exhaust Gas Temperatures)

Pretty straight forward here and a great channel to use to give you the heads up that other things are going wrong. This monitors the exhaust gas temperatures of the car. You want to see what the limits are here and it will take some beating on the car to find it. When the car is still warming up, your readings may not accurately reflect just how high these temps can get. Take your car out for some spirited full boost runs, then start logging in the normal 3rd gear WOT manner. Exhaust gas temperatures at 900 degrees Celsius and below are common for our engines when heavily boosted. This sensor appears to be only accurate up to 950 degrees Celsius or so. If you see readings creeping up this high, you have a great indicator that something is not right on your car and your engine is not happy with you. Running too high of temps for too long will spell disaster.

Channel 115: Requested Boost and Actual Boost

This is a very helpful channel for diagnosing boost leaks, trying to figure out why your car went into limp mode, and seeing if the software or hardware (electronic or manual boost controllers) are doing what they should be. This channel displays the boost requested by the computer (requested boost) in the first column and the boost actually made by the turbo (actual boost) in the second column. The readings you will see here can be misleading. First, you should know that the numbers you will see are not yet corrected for atmospheric pressure (about 1040mbar at sea level). The atmospheric pressure seen at the boost sensor is tough to estimate with complete precision, but I have found that subtracting 1000mbar from the numbers gets you close enough to actual boost unless you are living at 20,000 feet above sea level. The next thing you have probably noticed about these numbers are that they are displayed in mbar instead of psi like we are all used to. Well, this won’t be a problem thanks to the metric system. 1000mbar = 1 bar and 1 bar = 14.5psi. There you have it.

So, can we all figure out what boost level in psi this car is requesting and making at 3000rpm?

RPM Requested Boost Actual Boost

The correct answer is…

The computer is requesting 1.2 bar of boost at 3000rpm. This can also be expressed as 17.4psi.

The turbo is boosting 1.25 bar of boost at 3000rpm. This can also be expressed as 18.1psi.

So why are the above numbers important to us other than acting as a boost guage to entertain us? Well, as you can imagine, if you had a boost leak you would have a car that is requesting the correct boost but you would see very little in the actual boost column. In the case that you had just installed your new little boost controller or N75 valve you could do some logging to find that your actual boost was far exceeding your computer’s requested boost numbers causing your car to go into limp mode due to it’s sensing and “overboost” condition. For those of us who dare to run a turbo that was not designed specifically for the software they are using, this is a great way to see why it is not working out for you. For example, the boost maps on a K03 will show the ECU requesting max boost at around 3000rpms (this is a small turbo that makes its boost low in the rpm range). Now if you were to throw on a Garrett gt28r or T28 turbo or even larger you would see that the computer will still request the max boost at 3000rpm, but the turbo is not capable of making it’s max boost until closer to 3800rpm leaving you with an “underboost” condition.

NOTE: The sensor will only measure up to 2540mbar including atmospheric pressure. This means that if you are running more than 1.5bar or 21.75psi of boost this sensor will not measure beyond it. Both values will show maxed out at 2540mbar.