Could someone kindly do an AFM writeup to help some of us understand the workings of AFM, Active Fuel Management.
Questions:
All LT4's?
A8 & M7s?
Pro and con's of disabling?
Best disabling method?
AFM noise?
More.....

Thanks in advance

 

Most of this you can find by searching existing posts, which is what I'd recommend you do. Then once you have a basic understanding of most of what you've asked, ask the "hard" questions you couldn't turn up easily by searching.

Unless someone is really eager to summarize it all for you I guess...

 

Yeah: AFM is crap and GM is in a river in Egypt.

 

ALL LT1 and LT4 Corvette engines have this "feature" enabled.

AFM is the current name for GM's DoD (displacement on demand). Under light load the lifters for 4 of the cylinders are collapsed so that these cylinders neither intake nor exhaust air and fuel is shut off to the cylinders. The basic idea is instead of the pumping losses associated with 8 cylinders running at a very low percentage of capable power instead you have 4 cylinders operating with reduced pumping loss.

Bad things include potential irregular wear over the life of the engine (the same 4 cylinders always shut off), additional vibration while operating in V4 mode, very likely negative impacts upon the A8 converter clutch from the attempt to reduce vibration in V4 mode. Other negatives are additional hardware and weight associated with the AFM system, particularly with some LT1 applications where a steel torque tube is used (further vibration damping) and additional exhaust sound control valves.

With a M7 don't put the car in eco mode and AFM won't happen. With an A8 AFM is active in all driving modes and other than using an external device (like the somewhat problematic Range module) or a warranty killing tune you have to leave the transmission in manual mode to avoid V4 operation. If you select eco while in manual mode you will still get V4 mode.

Perhaps sometime GM will develop a flawless drivetrain integrated with AFM. This isn't it.

I mostly drive in manual mode but the A8 does work very well in automatic mode when V4 is defeated. Today I went for a nice cruise and plugged the Range module in after starting and this prevents V4 operation and doesn't create the annoying CEL problem due to communications loss. The LT4 in a Z06 doesn't go into V4 mode nearly as often as the LT1 does in other models but I don't want it happening at all thus my Z06 will either be in manual mode or have the Range module plugged in. A "hybrid" option is V4 won't become active during hard acceleration and the A8 can outshift the driver in these situation so you can always bump the selector into auto temporarily when you want a maximum performance pass, merge, etc.

 

Pros, steady good feeling V-8
Cons, slightly less fuel economy

Averaged 27mpg in V-8 mode on last trip (1200 miles). I have a Range AFM disabler so no V-4 for me.

 

Simple solution if you have an automatic. Leave it in M, use the paddle shifters, and AFM is a non issue. I sometimes put it in D for a long trip although from what i can tell, the improvement in milage is very small.
Overall it is a real pain in the **** for no benefit to the driver. Some say that they feel the switch is unnoticeable but for me (A6) i could feel it. So that is why i went the way of paddle shifting. Another advantage of paddle shifting is that, like with the manual, you get to decide when you want to shift (auto will shift much sooner than i want it to).

 

NSC5 - Thanks for taking the time to summarize AFM. Excellent. Everything I needed to know.

 

You are welcome!

 

I believe I read that the cylinders that shut off in sequence depending on when they are deactivated and therefore actually would be different and alternate. Am I incorrect?\\

 

I was actually trying to find where I read this too. I am certain that two different cylinders are chosen per cylinder bank. I believe they also take into consideration the balance, so im pretty sure it’s a pattern that’s used to minimize balance and vibration issues. Maybe it’s not on GM but thought it was.

 

Only 4 cylinders are equipped with the special AFM lifter setup so these same cylinders are shut down every time. They are shut down and reactivated in firing order sequence but it is always the same 4 cylinders.

I believe in the LT1 it is cylinders 1 and 7 on the driver's side bank and 4 and 6 on the passenger side bank. Delphi has patented a system which randomly shuts down any number of cylinders to avoid the uneven wear issue (found by a couple of the Japanese manufacturers at high mileage) and to further improve fuel economy but I don't believe any manufacturer has implemented this system yet.

 

I wouldn't worry about uneven wear as it only kicks in at near-zero load anyway, so the rings are going to be barely active (they exert their seal from combustion pressure, not ring tension as many would assume). So when AFM is active all it's doing is avoiding some pumping losses and allowing a wider throttle angle for more efficiency in those cylinders which remain active, and those active cylinders, while wearing more, are barely doing much at all.

I think it'd have to be some seriously high mileage to ever show up!