Actual load versus calculated load

[YoDoug]

Hello all. I have a 2008 MS3. It was previously tuned with Cobb. I decided to switch to VT for a few reasons. My Cobb AP had started having issues with the plug. I tried a different cord but still had issues with disconnecting with the slightest of bump. I also want to add some more go fast parts and didn't want to be stuck waiting for a tuner each time. Lastly, being an EE by degree and a controls engineer by profession, the aspect of tuning is just way to intriguing to me to not do it myself.

That being said I was able to pull quite a few logs from the AP before I switched to VT. I have been able to match everything log wise between AP and VT logs. The car is running great. On WOT pulls everything matches from RPM, Ignition, boost, Lambda fuel, etc. The only difference is the Cobb "calculated load" is hitting 1.9-2.0 and the VT "absolute load" log is hitting 2.1-2.2. Is there a difference between those two log points? The only other thing is the load/boost seems to peak around 5000rpm and then drop a little, whereas with the old tune it stayed strong until 6000rpm. I suspect it is the low BAT load limit request backing it down. Going to do a little more testing today.

TIA

[CrazyCanadian]

The Load settings work the same... Unless you are comparing tunes on the same day with zero enviromental differences then you'll get different Load values... As an engineer enjoy looking up OBD2 regulated Absolute Load calculation.. This is how the load pid is calculated..

Also data logging rates play a factor in what you'll see.. Cobb Access port has a faster logging rate then Versatuner..

[mituc]

I never found the calculated load a reliable measure to work with because it has too many inconsistent variables (ambient temp, barometric pressure, and so on), while the absolute load is simply a measure of air flow, engine displacement, RPM point and volumetric efficiency in that rpm/load point.
However, this data is read from OBD as reported by the ECU. So these two PIDs, absolute and calculated load are read from the OBD. It's not a Cobb or a VT thing.
Cobb decided to log calculated load even though the calibration load tables of the ECU software use absolute load. VT decided to not even expose the calculated load PID because it can be misleading (I was logging both calculated and absolute load back in the days when I was using a Dashdaq XL and at some point I had to ditch the calculated load because it was varying in a way that was not intuitive or predictable at all).

[CrazyCanadian]

I never found the calculated load a reliable measure to work with because it has too many inconsistent variables (ambient temp, barometric pressure, and so on), while the absolute load is simply a measure of air flow, engine displacement, RPM point and volumetric efficiency in that rpm/load point.
However, this data is read from OBD as reported by the ECU. So these two PIDs, absolute and calculated load are read from the OBD. It's not a Cobb or a VT thing.
Cobb decided to log calculated load even though the calibration load tables of the ECU software use absolute load. VT decided to not even expose the calculated load PID because it can be misleading (I was logging both calculated and absolute load back in the days when I was using a Dashdaq XL and at some point I had to ditch the calculated load because it was varying in a way that was not intuitive or predictable at all).

You've got some information mixed up... Automotive OBD2 regulations is what defines Calculated Load and Absolute Load pids... Calculated Load ranges from 0 - 100% Naturally aspirated or boosted... It's a normalized calculation that reflects current airflow vs peak airflow at WOT...

Absolute Load will range from 0 - 100 on an NA application and 0 - 400 on a boosted application... Absolute load is the normalized calculation of air mass per intake stroke, displayed as a percentage...

Both calculated Load and Absolute load use barometric pressure, Intake air temp in their calculations... Both are reflecting whats potentially measured at sea level and a standard intake air temp...

When you look at Scan data, the program you are using reads the ECU and the values are spit out based on how the diagnostic program has been written.... Cobb chose to label Absolute Load as Calculated Load... While Versatuner didn't change that.. It's hard to completely trust the data you get from scan tools... Every type of scan tool I've used has made mistakes with pid definitions...

The actual Calculated Load and Absolute Load values are defined by the OBD regulation mandates... Manufactures can colour outside the box if they wish when it comes to OEM level scanning... those pids can be calculated differently... but generally speaking the SAE versions is what gets logged and Used..

Also, because both values are normalized to reflect "airmass"... they will never be the same unless baro, ambient air temp and intake air temps are all identical... 20lbs of boost at 20 deg C with intake air temp of 100 deg... is going to have a different "airmass" then 20lbs of boost at 5 deg C and 60 deg intake air temps..

[YoDoug]

Thanks for the reply. I guess I didn't word my question well, but you answered it anyways. I understand how the Actual load is calculated and logged in VT, I just wasn't sure if Cobb was looking at the same PID. Knowing they are the same now, the differences I am seeing is easily corresponds to the temp difference between the two days I pulled the logs.

Thanks again.

[mituc]

I understand how the Actual load is calculated and logged in VT, I just wasn't sure if Cobb was looking at the same PID.

Both calculated and absolute load are standard OBD2 PIDs, nothing is calculated by VT or the Cobb AP. It's only that Cobb decided to use the calculated load and VT the absolute load.

[YoDoug]

I get that it is the ECU dong the calculation and Cobb or VT is just reading the value from the PID. I was trying to figure out if the Cobb Calculated load and VT absolute load are the same PID or different.

[mituc]

I was trying to figure out if the Cobb Calculated load and VT absolute load are the same PID or different.

Absolute load and Calculated Load are different PIDs, regardless of what you read it with. They are both available via the OBD.

[YoDoug]

Doing a little more research on the SAE PID's for calc versus abs load I found the following; It was very helpful in understanding what those values are measuring.

https://autonerdz.com/yabbfiles/Attachm ... ations.pdf