Abstract: The ETC Scalar value for Gen 3 PCMs for DBW systems is not clearly documented and discussions on forums are generally vague and contradict each other. This is by no means a definitive guide, but what I've figured out throughout this process. Meant to be a discussion, I might be wrong with some points.
My dilemma: Had a 416 CI LS3 built for my 2001 C5 Z06. Fast 102 Intake with a NW 102 DBW throttle body. The cam is relatively mild for the displacement at 236/244 0.615"/0.605" @ 114 LSA. AC system was replaced and evac'd and charged based on weight, and checked for leaks.
I kept the stock 00+ C5 PCM (411 PCM) to control my engine with ease of effort.
Since this is a completely different and all new setup for my car, I had no basemap/close starting point to work with. So I had to start with a C5Z stock tune, modify the VE table to loosely resemble a mild cammed engine's VE, and try to adjust the scalar to work with the 102 NW TB.
I bought stock 6.2L flex fuel truck fuel injectors (50# injectors) for my build. I was able to port the Gen 4 injector data over to the Gen 3 PCM.
The Problem: After getting the VE pretty closely dialed in, then dialing in the MAF tables, it was time to start working on idle and another annoying issue I've not been able to fix. My car drove perfectly if I had the AC off. STIT/LTITs, STFT's are all close to 0, car returns to idle pretty well. As soon as the AC gets turned on, hell breaks loose. While I adjusted the AC torque loss tables to bump up the AC airflow values to get the STIT/LTITs with the AC on at idle to be 0 as well. This worked and all, but for some reason as soon as I revved the engine or clutched in while shifting, the engine would immediately nose dive and stall. Never even tried to catch itself, just immediate stall.
While my STIT/LTITs with the AC on and off are near 0, my AC airflow was 7-8 g/s in order to achieve it, but my MAF only showed a 3 g/s difference between AC on and off... Something didn't add up...
Calculating the ETC Scalar??
Lots of debate on this one for the Gen 3 PCMs.
Stock 01+ C5 Z06 ETC Scalar: 0.0255 (78 mm)
Where the hell did this value come from??
It's the area per 1% of throttle area, comes from this equation:
100/([Area of throttle body] - [Area of shaft])
Total area = [(TBDiameter/2)^2 * pi] - [TBShaft * TBDiameter]
Let's do the math for the stock 78 mm TB and see if it checks out:
Total area = ((78/2)^2 * 3.14159) - (11 mm * 78 mm) = 3920 mm^2
ETC Scalar for 78 mm = 100/3920 = 0.0255
Doing the math for 102 mm = 0.0141 (assuming shaft is the same size).
There are others that claim to simply multiply it by the difference.
For example, 102/78 = 1.307
Then 0.0255 / 1.307 = 0.01951, not quite the same...
Diagnosing what's wrong
First and foremost, before getting to this point, it is very important that your Injector data, VE data, and preferably MAF data (this is not as critical if you are running speed density mode, but it is a good idea to have it, so you can ensure that you get an accurate reading of actual airflow into the engine)
Onto the first part, what PIDs do we need to figure out where this discrepancy is coming from?
For the 00+ C5 with a manual transmission, I make sure to log at least the following:
- LTIT Gear
- Idle Adapt (STIT)
- Idle AC Airflow
- Idle Desired Airflow
- Mass Airflow (g/s, not Hz)
- Dynamic Airflow
Screen Shot 2017-06-12 at 8.00.40 PM.png
I had been playing with the ETC scalar a bit already, and actually had changed it to 0.0165 to go somewhere in the middle of the two calculated values.
As a note, I set my tune to force open loop and SD only (MAF fail high point at 0 hz).
Trying to get the values closer
Screen Shot 2017-06-12 at 8.29.41 PM.png
AC off... Mass airflow and dynamic are close to each other (good sign, means VE and MAF line up, assuming AFR is correct), but Idle desired airflow is 2 g/s above the others STIT and LTIT equal out to net +0.5 g/s, which is pretty close. Could be a bit better, but good enough for testing ETCs.
Screen Shot 2017-06-12 at 8.35.09 PM.png
AC on... MAF and dynamic air line up, Idle desired is now ~5 g/s above the other two... STIT + LTIT are net +0.3 g/s. AC airflow is quite high... But when I rev the car, it nose dives and attempts to stall, even though it gracefully returns to idle with AC off...
When trying new ETC scalars, I would start the car up, take note of the new STIT changed value (base airflow numbers will change with ETC scalar change). Base airflow changes because when you change the ETC value, the computer has a different calculated g/s per % of throttle value. Some error is ok, because you can account for it in the base airflow table and other tables.
When I changed the scalar, I had to start it up, and do a quick pass at correcting the base airflow to get the STIT somewhat close for comparisons.
Next value I tried was 0.0333. After correcting the base airflow and AC airflow (since they both will need to be changed for different values), we get this:
Screen Shot 2017-06-12 at 8.46.44 PM.png
Looking at the idling AC on vs AC off, we now see the opposite. For one thing, I didn't pull back my AC airflow nearly enough, as it's -3 g/s when ac off was 0 g/s.
But we do see that the idle desired is now WAYYYY less than MAF and dynamic airflow (which still agree quite well).
Based off this, It's logical to assume that the correct value is somewhere in the middle...
After trying this a few times, I ended up with the ETC scalar of 0.0210 looking like this:
Screen Shot 2017-06-12 at 8.54.38 PM.png
Even though my STIT is off for both AC on and off by a bit much, the engine gracefully falls to idle with the AC on and off. Once I clean that up a bit, it should be even better. All 3 values are pretty close in both situations (the desired idle airflow does include any STIT and LTIT adjustments as well).
Unfortunately my clutch foot is broken, so I can't yet drive the car to see how it behaves, but this is a dramatic improvement over revving and holding RPM then releasing gas and watching the engine return to idle gracefully. Normally this would cause the engine to nose dive and stall.
Summary
To sum this up, the following was discovered with some trial and error. Note Gen 4 PCMs use a different type of scalar than Gen 3.
For Gen 3, smaller ETC values for bigger throttle bodies. For Gen 4, larger values are for bigger throttle bodies
Once your VE table and Maf are dialed in enough for your dynamic airflow and MAF readings to be correct, you can dial in the ETC value to get the PCM's expected idle airflow to correlate with what it actually is. When trying to dial in ETC values, the following 2 tips seem to be the way to go. The calculated values don't seem to be correct, but perhaps a decent starting point. There is likely some variation depending on throttle body/intake combo or something else...
- If Idle Desired Airflow is higher than MAF/Dynamic airflow, then ETC scalar needs to be increased (decreased for Gen 4 PCMs) - I.e. To drop idle desired amount
- If Idle Desired Airflow is less than MAF/Dynamic Airflow, then ETC scalar needs to be decreased (increased for Gen 4 PCMs) - I.e. To raise idle desired amount
Hopefully more to come once I am able to drive the car.
Edit:
Now that I've been able to drive the car, I can say that it is a day and night difference! With the correct ETC scalar, it was much easier to dial in my other tables, since the car get's as much air as it's expecting at idle, return to idle with AC on and off is much more elegant and nice!
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