Question 1: Does AFM operate in Z Zmode?
Question 2: Does AFM always shut down same cylinders?
Question 3: Do deactive cylinder valves stay open or closed?
Question 4: is is advisable to avoid AFM during engine break in?

 

1-no
2-yes
3-don't remember
4-no idea

 

Not correct. AFM will activate in Z Mode unless you have the engine settings on Track.

 

3. The valves stay closed and use the trapped combustion gas as a gas spring.

 

Q4: There should be no issue with letting AFM operate normally during the break-in period.

 

I don't like, need or want AFM so I drive 100% of the time in M

 

👍👍

 

Here’s a table from this thread which is also a good read.

 

1: Does AFM operate in Z Zmode?
Yes, it can. It depends on the setting of the engine/shift parameter. In Zmode you can change that. If you set engine shift to Track it will disable AFM. If you use any other engine/shift setting AFM will not be disabled.

You can also disable AFM by using Manual Shift Mode, regardless of the Drive Mode or engine/shift setting you are using.

2: Does AFM always shut down same cylinders? Yes. It deactivates cylinders 2,3,5, 8.

3: Do deactive cylinder valves stay open or closed? As noted already, the valves close, and timing of the closure captures a combustion charge.

4: is is advisable to avoid AFM during engine break in? Its not the advice I give. You can if you want. There is nothing in the break-in procedure to indicate any need to avoid it. There are no restrictions on drive modes or shift modes - just RPM and full throttle starts. And you should not drive at any constant speed for an extended period of time. That said...since there are no restrictions on drive mode, or the use of manual shift mode, you can avoid AFM during break in if you want to. I wanted to make sure AFM was engaging normally during break-in and I also made a point of using all 8 gears. My theory is that all of the mechanics should be exercised within the constraints of the break-in limitations.

 

I would personally avoid AFM during the first 500 miles or so. Combustion pressure aids in sealing rings to the bores. I'd prefer all the cylinders fire during break-in. Although I don't care for the AFM feature, I let it do its thing. 10K+ miles and so far so good (along with 30+ track days).

 

Answered in quote.

 

From my understanding, AFM (regardless of the vehicle) will always deactivate specific cylinders. DFM will cycle through which ones are deactivated. Which specific cylinders AFM turns off--might be 2358 on the LT2 and 1467 on a different engine (eg. LT1, iirc). But every time AFM kicks in, those are always set for that specific engine; no sharing the workload around.

I kind of wish we did have DFM on the LT2, as I suspect that it helps with long-term longevity to spread the workload around. With AFM, half of your cylinders are getting used less than the other half. Even swapping from 2358 to 1467 every-other V4 activation...would probably help immensely. But that's not how it was implemented.

 

A feature of DFM is that it is not limited to 4 deactivated cylinders at a time. I think, if I follow this correctly, it can deactivate as few as 1 per cycle (7 firing) all the way down having only two cylinders firing at a time. And yes, it varies the pattern so no cylinder is shut down every time. At the time this article was written though, there were some issues with DFM. While older version of the AFM also had issues, there have been (so far) few issues with the AFM in the C8.

https://drjlt.com/article/tech/398/gm-dfm-pros-cons

 

Just clarify the answer to question 2, in Z mode, disabling AFM is accomplished by setting the engine/shift parameter to Track. You can put all of the other parameters wherever you wish - so if you prefer the Tour setting for steering, brakes, suspension, and sound, you can use those settings. They do not affect AFM.