Evidence, shmevidence. Just believe hard enough and you'll think it's true too. Facts no longer matter.

 

Never really for me was concerned with AFM causing engine damage.

Having an early 2020 C8 for me the AFM/V4 was an annoyance and could easily be heard. Bad on me for not taking to the dealer, but as we most probably feel going to a dealer with a new model C8 was usually not a great experience. So I just put up with it and doubt if AFM really saves or improves anything to do with better fuel mileage.

Tuned my C8 to E85 for performance only, plus we only have 91 Octane in AZ. Having AFM and Cold Start deleted forever was only icing on the cake.

Now for the ERay owners who have to deal with SOS this would rightfully create paranoia.

 

Do you think they would do something if you can hear the AFM kicking in which is most likely the exhaust valves closing/opening?

 

What? No pictures/diagrams? Shortest post EVER! Just couldn't resist.

 

They? You mean a GM warranty repair? They will if they can determine one of the exhaust flow control valves is defective. All 4 close (if you have all 4) when V4 kicks in according to the Service Manual description of how it works. I have at times heard the valves click, but its mostly sitting in the driveway and switching between Tour, which closes the tailpipe flow control valves at idle, and Track which opens those same valves at idle. I may have heard a click when V4 engaged while driving, but its rare and subtle. If there was a repeatable louder noise during transition it could be an indication of a defect. A stuck valve would cause a DTC. But.....some of us....not all... can hear or detect the V4 condition while in V4 mode. I can, at times. I not AFM averse and never complain about it, but I can hear it. Its sort of very low frequency rumble, vaguely similar to the sound and feel if the rpm is too low for the selected gear, but not attempting to accelerate. I'm not suggesting its the same as lugging, I am only trying to describe what I hear. Its subtle, not obtrusive, and many report hearing nothing. I notice it most on leisurely back-road cruises with the top down. It switches in and out of V4 frequently if engine/shift is not track and automatic shift mode is selected. I don't consider that a defect either. But I have started to use the Manual Shift Mode in those back road leisurely cruises. I prefer my own gear selection, and it avoids what I consider frequent transitions in and out of V4 mode. Personal preference - I make no comment about AFM reliability nor its utility.

 

I would be curious if whether hearing the valves actuate when going between V8 and V4 is a matter of GM has different suppliers supply the valves. One is quiet while the other supplier has them a bit louder( but still meets GM's laid out specs).

 

I do not know. Sometimes mine click, sometimes they do not. Could be temperature, or a bit of debris. The motion is very quick too. I have filmed it.

The attached is when the car starts in the default mode which is Tour Mode. The valves are defaulted to open while the car is off. And, during a crank event they remain open. But as soon as the car starts they revert, in this case, to closed at idle. So what you see is the valve is open, but you see it close immediately when the car starts.

 

The only indication that I have on activate or deactivate is the dash light. I can't hear or feel a thing.

 

Nope! The lifters don't make noise when collapsing or filling! Done very quickly and only while riding on the cam lobes. For those that hear V4 activate I'll show in Pics probably why (I didn't for 4 years in my 2020 C8 and now 11 months on my E-Ray when i use MY Mode!) I only know they are activated when I look at the dash symbol.

BUT for those that hear something, and some do, it's the V4 exhaust valve closing! Yep it closes inside the exhaust pipes so your V8 doesn't sound like it's missing on 4 cylinders (and to provide some back pressure.)

If you need proof I can post a long page of how they work but assume few would read here are some Pics:

Yep, Tenneco designs and builds the exhaust. They ship to BG as a fully welded assembly (one of their shops has ~150 of my MIG welding shielding gas - argon base- savers on their MIG robots!) They stated in a tech article they had a challenge compared to the C7 exhaust they also designed and built. The V4 valves that put a restriction in the exhaust pipes, are close to the Hot CATS, unlike the front engine C7. They had their Germany Division (they are an American Tier 1 automotive suppler etc) design the V4 valve system to keep heat from the electric motor that activates the valves. Note in V4 Mode all exhaust goes thru 2 small holes when the valve closes. It is only fully open or closed (the Z06 is different.) All the open area needed as you are ONLY in V4 when little hp is being used.
Note this is not a precession valve as for example the air intake throttle valve. Lots of exhaust heat, expansion and contraction. Some can hear a whooshing type sound. Might here a clink as the actuating levers and links operate in a hot environment.


The V4 valves are different than the NPP valves if so equipped. By the time the exhaust gets thru the muffler it is cooler so you can see the actuating electric motor is close coupled as it was in the C7!

Thought I could get by what IMO is the false paranoia re V4 valves failing with my 1st Post being NO! I have followed those comments from my 2014 and 2017 C7s. BUT looks like Pics are needed for some so false reasons for noise are not propagated.

 

My 2016 Escalade would beg to differ. It has been a while, but a stuck lifter wiped out the cam and half the valve train. I was lucky it was barely in powertrain warranty when it happened. I bought a Range device immediately after. There is debate on whether that will prevent the issue, but it is the best shot of avoiding another month long service visit and a $6,000 bill.

This is the thing that scares me the most about the C8, but I have not heard of common issues. It is very common with the trucks.

 

Note I said no evidence of AFM/DFM causing the bearing issue with these engines.

Until there is evidence, it is just from peoples hatred of the system and want to point to any failure as being caused by AFM/DFM. Spark plug failed? AFM fault!

I never said a collapsed/stuck lifter can't lead to other issues. Did you ever find out if that lifter was an AFM or normal lifter? My parents 2016 Suburban had a lifter collapse on the #2 cylinder. That is a normal lifter. Bent the pushrod because they didn't hear the tick in time.

 

https://www.corvetteactioncenter.com...2021-1506.html

 

Interesting first post. Does it have a point?

 

Yes new trucks have afm. I have 2026 High Country and new c8 . Do the two use the same afm? THANKS

 

Yep Both V8s use the same collapse 8 lifter, 2 each on specific 4 cylinders that turn into V4. BUT more reported incidence of falters reported in truck engines. Could be because of annual volume (850,000 x 1/2 for those sold with V8's versus 35,000 C8 sold per year. Therefore would expect 424,000/35,000 12 times the number of reports! Also and logical, some of the problem is attributed to "dirtily engine oil' clogging the small passages that activate the lifters to collapse. Vette owner's are more likely to change oil per GM's recommendations.

BTW, the new small block will reportedly have a new AFM method of operation. The was a none GM video that said it was more robust. A recent GM Authority (again not GM) showed a GM patent to using rocker are attachment NOT lifters to deactivate whatever valves desired.

 

JerryU
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My understanding is that Active Fuel Management (AFM), as used in the Corvette, is a fixed cylinder deactivation system that disables four of the eight cylinders under light-load conditions. In contrast, Dynamic Fuel Management (DFM) in newer GM V8 truck applications is a continuously variable cylinder deactivation system capable of operating on 1–7 cylinders through dynamic firing patterns, adjusting in real time based on torque demand, load, and driving conditions.

 

That is AMAZING mileage - I had no idea it could be that high.

The LC500 guys report 27-30 mpg with their 471hp V8 which doesn't have an AFM system.

It would be helpful if you could provide the mileages you're referring to.

 

On my 2022 SR Z51, I can clearly hear when V4 mode engages and disengages. It is not a clicking, but a "Swooosh" sort of sound. I don't need to look at the DIC to tell, as it's very obvious to me.

Note that I am a musician, with a well trained ear, although I've always used isolating in-ear monitors when playing on live stages. Thus I do not suffer from hearing loss at 51 years of age. I mention this to say I may be more sensitive to the noise it makes .... I also hear EVERY SINGLE rattle/rubbing sound, no matter how small it is. The first thing I tend to do with any new-to-me vehicle is tear out interior panels and address anything which can create a rattle. It was no different on the C8, as I've already pulled both door panels and addressed the clicking/ticking harness clips, and also preemptively addressed other areas I saw which could potentially cause a problem in the future.

Now all of that said ... my theory on what I am hearing is related to what Andy stated above. When the cylinders are shut down, the timing is very intentional, and designed to capture the exhaust gases in the cylinder and keep it under pressure. See attached PDFs for the broader details on this. But I wonder if the "Swooosh" sound is related to pressure change when AFM is enabling and/or disabling?

 

CARBYTE

Carbyte

• AFM/DFM Disabler: In Full-Time V8 Mode, Carbyte disables the AFM/DFM system and keeps the engine firing on all cylinders, all the time.
• Start/Stop Disabler: Eliminate the annoying Start/Stop feature, keeping your engine running 100% of the time.
• Diagnostic Scan Tool: Paired with an iPhone, iPad, or Android phone, Carbyte can transform into a full-featured diagnostic scan tool.
• Plug-and-Play: Stand-alone functionality allows Carbyte to work by simply plugging in the device. No app is needed for AFM/DFM and Start/Stop functionality!
• Optional Smartphone App Included: Control the AFM/DFM and Auto Start/Stop functions from your phone for the ability to revert to factory conditions on the fly.
• Easy Installation: Plug Carbyte directly into your OBDII port! No risk of damaging critical components from installing device into your engine

Works great on my 2022

 

With my stock performance Z51 exhaust I could faintly tell when it would go to V4. Now with the Borla ATAK exhaust I hear it constantly to the point it’s kind of irritating. Seems to want to hover in V4 now more than ever before. Slight pedal adjustment wakes up the Borla system with ease. Stock this was a non issue and didn’t seem to hover in V4 mode as much.

No cels in 450 miles, 2 months and it never had one I relearned the valves before I started the car day 1 with the Borla system. Car is in hibernation mode right now it’s Siberia cold down here in MS right now.