General consensus on AFM/DoD

[lucas287]

Hey y'all,

Your typical truck forum is filled with a bunch of hyperbolic opinions about AFM that really isn't rooted in any understanding of how it works or mechanically what could go wrong. Therefore, I don't really take their opinions seriously. I trust tuners. Y'all have a vast understanding of how it all works together from a engine management and mechanical standpoint.

What's your experience with AFM in the following categories:

1) Mechanical - are there actually reliability issues with the lifters themselves? Simply disabling AFM doesn't change the lifters are a weak point. Also, are there any other related issues like with torque converters? I understand that some slip is enabled in AFM mode.

2) Tuning perspective - is it just easier/more predictable to eliminate the variables? If you make adjustments to fueling/timing do those translate over to AFM mode? Can AFM be optimized in any way so that it stays in the mode longer? I notice that if I cruise in tow/haul mode it actually engages AFM much more often because it stays in a higher gear.

Basically, I notice that I get hands down better fuel economy with AFM enabled. Driveability is reduced though. But for me, when I want to play around, I just put it in M-whatever, tow/haul, and disable traction/stabilitrak. When I'm just cruising and don't plan on fooling around, I just let it do it's thing.

Thanks in advance for your opinions!

[Ben Charles]

Reliability is main thing.. Leaving it on you have a higher likelihood of engine problems(typically lifter issues on those cylinders)

[smokeshow]

1) Yes, there are reliability issues. The stereotype doesn't come from nowhere. While I appreciate the technology, I'm not a fan of hardware that has a fixed expiration date. A conventional valvetrain Gen 3 GM V8 maintained properly could surprise you as it crossed the half million mile mark. Can't say the same for an AFM engine. The torque converters don't really have an issue as they are designed to be able to slip.

2) There are a lot of little nuances that go into making AFM work smoothly. However, the Gen 5 LT-based engines have that figured out really well. On my L86 Yukon, it's butter smooth - so smooth that I wouldn't change a thing. If one were so inclined to make modifications and keep the AFM, things get nasty pretty quick. Simple exhaust is easy enough, but things like camshafts and superchargers that shift the torque curve or put parasitic load on the engine change the 4>8 and 8>4 transitions a lot. GM puts a lot of effort into removing torque bumps and sags which is inevitably what those sorts of mods cause. Calibrating those transient airflow events is not a trivial task.

For some background, the sole reason AFM exists is to reduce pumping loss. You might be inclined to think you're saving fuel by shutting down half of the cylinders, but realistically what it does is turn the engine into a 4 cylinder turd that is dragging around 4 dead cylinders. However, the benefits outweigh the drawbacks; when you close the valves on half of the cylinders, you have to double the manifold pressure plus some. The closer your manifold pressure gets to barometric pressure, the less energy you lose by sucking air out of a vacuum. So that's where you gain your efficiency back. You could potentially modify a vehicle to stay in AFM mode longer, but it would cost more than $0. And it would be a bear to calibrate.

[lucas287]

Reliability is main thing.. Leaving it on you have a higher likelihood of engine problems(typically lifter issues on those cylinders)

So essentially, the additional bypass spring (made that term up) that's used to absorb the motion from the cam lobe isn't called into play until oil pressure/load dictates AFM? And by disabling it, you never utilize that component of the lifter which is inherently weak?

[lucas287]

1) Yes, there are reliability issues. The stereotype doesn't come from nowhere. While I appreciate the technology, I'm not a fan of hardware that has a fixed expiration date. A conventional valvetrain Gen 3 GM V8 maintained properly could surprise you as it crossed the half million mile mark. Can't say the same for an AFM engine. The torque converters don't really have an issue as they are designed to be able to slip.

2) There are a lot of little nuances that go into making AFM work smoothly. However, the Gen 5 LT-based engines have that figured out really well. On my L86 Yukon, it's butter smooth - so smooth that I wouldn't change a thing. If one were so inclined to make modifications and keep the AFM, things get nasty pretty quick. Simple exhaust is easy enough, but things like camshafts and superchargers that shift the torque curve or put parasitic load on the engine change the 4>8 and 8>4 transitions a lot. GM puts a lot of effort into removing torque bumps and sags which is inevitably what those sorts of mods cause. Calibrating those transient airflow events is not a trivial task.

For some background, the sole reason AFM exists is to reduce pumping loss. You might be inclined to think you're saving fuel by shutting down half of the cylinders, but realistically what it does is turn the engine into a 4 cylinder turd that is dragging around 4 dead cylinders. However, the benefits outweigh the drawbacks; when you close the valves on half of the cylinders, you have to double the manifold pressure plus some. The closer your manifold pressure gets to barometric pressure, the less energy you lose by sucking air out of a vacuum. So that's where you gain your efficiency back. You could potentially modify a vehicle to stay in AFM mode longer, but it would cost more than $0. And it would be a bear to calibrate.

Thank you for that explanation! I agree with you on the Gen 3 engine. I have a 2000 Silverado also with the LR4 and it has 313,000 original miles. Heads have never come off, nor has the front cover. Just gaskets. Great little engine.

I, too, have struggled with the same concept that a now 3.1L V4 is dragging 4 cylinders worth of rotating assembly around. I notice that even on 93 octane, in AFM on the highway timing advance can get real low (less than 10*) and then it turns off and it's back to V8 mode. I also get some KR as the timing gets that low too. Was primarily curious if that's something that can be tuned/optimized.

[cmitchell17]

Its my opinion that the TC shudder problems are not related to AFM at all. I think people associate it with AFM since the TCM calls for a different/higher proportion of "controlled" slip while in AFM mode, so the shudder has the potential to be worse in AFM mode so people think by turning AFM off it will fix it. My 8L90 still shuddered/slipped with AFM disabled, but after I put the correct fluid in the latest Mobil 1 Synthetic LV with the blue label on the bottle, everything has been fine.

As for the AFM valvetrain, I think from a design perspective they designed the same durability/reliability requirements into the system as the old GEN III stuff, but when you have new manufacturing processes involved and new suppliers involved in creates more chances for things to go wrong. I think all the perceptions of the system being unreliable are not inherent from a design perspective.

[kevin87turbot]

Its my opinion that the TC shudder problems are not related to AFM at all. I think people associate it with AFM since the TCM calls for a different/higher proportion of "controlled" slip while in AFM mode, so the shudder has the potential to be worse in AFM mode so people think by turning AFM off it will fix it. My 8L90 still shuddered/slipped with AFM disabled, but after I put the correct fluid in the latest Mobil 1 Synthetic LV with the blue label on the bottle, everything has been fine.

As for the AFM valvetrain, I think from a design perspective they designed the same durability/reliability requirements into the system as the old GEN III stuff, but when you have new manufacturing processes involved and new suppliers involved in creates more chances for things to go wrong. I think all the perceptions of the system being unreliable are not inherent from a design perspective.

Typically, the 8L90E TCC shudder problems start between 50k-80k miles. I've found that if you disable the TCC slippage tables, that it extends the life of the converter clutch tremendously. The revised fluid definitely helps as well.

I agree that the AFM valvetrain wasn't purposefully designed to fail. However, it happens all too often. I have a small shop, and we do quite a few DOD Deletes. This year, we've done 17 GEN 4 DOD deletes and 5 GEN 5 DOD deletes. The GEN 5's that we've done have failed between 80k-120k miles. Four of them had collapsed lifters and bent pushrods. One of them had already had two new lifters replaced under warranty. It's alarming to me at the number of valvetrain failures that we see. That being said, I own a 16 Denali with an L86 and an 18 Escalade with the L86. I'm still a GM fan. I just keep the parts on standby...just in case we have a failure. I've got an L8T engine sitting out in the shop that I'd love to put in my Denali. It just hasn't given me a reason to do it yet.

[lucas287]

[QUOTE=kevin87turbot;618510]Typically, the 8L90E TCC shudder problems start between 50k-80k miles. I've found that if you disable the TCC slippage tables, that it extends the life of the converter clutch tremendously. The revised fluid definitely helps as well.

Thank you for your feedback on this! Nice to hear from someone who's on the front lines. With that said, if AFM is disabled, the TCC is fully locked more often thus leading to longevity - right?

And, from what I've read the lifter issues happen randomly when AFM is active, then it tries switching back to V8 mode - and bam, the lifter locks up. Once again, I guess that's the logic behind disabling AFM. If it's never allowed to actuate into it's AFM position, it won't ever lock up from trying to switch back to V8 mode.

I, too, have had a great experience with the new Mobil 1 fluid. Had the full TSB flush done under my certified warranty and it shifts a lot better. The tuning still blows though. Negative timing on shifts makes it real akward at low speeds especially on the 1-2 shift.

[Frost]

1) Yes, there are reliability issues. The stereotype doesn't come from nowhere. While I appreciate the technology, I'm not a fan of hardware that has a fixed expiration date. A conventional valvetrain Gen 3 GM V8 maintained properly could surprise you as it crossed the half million mile mark. Can't say the same for an AFM engine. The torque converters don't really have an issue as they are designed to be able to slip.

2) There are a lot of little nuances that go into making AFM work smoothly. However, the Gen 5 LT-based engines have that figured out really well. On my L86 Yukon, it's butter smooth - so smooth that I wouldn't change a thing. If one were so inclined to make modifications and keep the AFM, things get nasty pretty quick. Simple exhaust is easy enough, but things like camshafts and superchargers that shift the torque curve or put parasitic load on the engine change the 4>8 and 8>4 transitions a lot. GM puts a lot of effort into removing torque bumps and sags which is inevitably what those sorts of mods cause. Calibrating those transient airflow events is not a trivial task.

For some background, the sole reason AFM exists is to reduce pumping loss. You might be inclined to think you're saving fuel by shutting down half of the cylinders, but realistically what it does is turn the engine into a 4 cylinder turd that is dragging around 4 dead cylinders. However, the benefits outweigh the drawbacks; when you close the valves on half of the cylinders, you have to double the manifold pressure plus some. The closer your manifold pressure gets to barometric pressure, the less energy you lose by sucking air out of a vacuum. So that's where you gain your efficiency back. You could potentially modify a vehicle to stay in AFM mode longer, but it would cost more than $0. And it would be a bear to calibrate.

Thanks for your contributions here; I mean that. We never talk about carbon crediting here, but there are reasons like this that it would seem to truly exist.

[B52bombardier1]

I think if we changed the engine oil on a DOD motor a little more often, a lot of these problems would vanish. Yea, I know that GM says to follow the computer's recommendation on oil life and when to change it. I'm just not comfortable doing that.

Pay Mr Goodwrench now or pay him later but I'd like to not pay him at all, do my own oil changes on a schedule of my choosing and hopefully get an engine lifetime like the Gen 3 motors generally get.

Rick

[lucas287]

^Agreed on the oil change frequency! You're better off running a cheap Dexos LSPI approved oil (like Walmart Supertech full synthetic) and changing more often than extending the intervals!

Also, the cleaner the air going in - the cleaner the oil. One of the big reasons I always use high quality paper filters. Plus since I've been a GM guy for a while now, I've coated a few MAFs by overoiling K&N type filters.