When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
mountainears asked: Why is there no option to just disable the AFM / V4 mode on the cars.
Even if it reset and had to be selected on each start up, the driver should have the choice to turn it off completely.
Tadge answered: I am going to start by assuming this question pertains to automatic transmissions only since AFM is turned off in all modes except Eco on the manual. I also assume folks know that when the auto is operated as a manual (i.e. "M" mode on the shifter), AFM is not operational.
As with many things in automotive design, we are not free to simply engineer cars to what each customer might want. We have many other constraints. Government regulations play a huge role in how we execute vehicles. Automobiles are the most heavily regulated consumer product in the world with every major economy in the world governing their design and sale in various ways.
Although it may seem like a simple matter to have a " turn off switch" to allow customers to choose between maximum efficiency and full time V8 operation, it is not. We use AFM (Active Fuel Management, or 4 cylinder mode) to enhance fuel economy under light load conditions. It only takes 12 - 40 HP (depending on model and speed) to push a Corvette down a flat road at highway speeds. Producing that small amount of horsepower with all 8 cylinders firing and then practically closing the throttle is not as efficient as running on 4 cylinders with the throttle blade more open. There are very measureable real world economy benefits in addition to fuel economy label, federal fuel economy standards (CAFE) , and gas guzzler tax benefits. The EPA sets the fuel economy and greenhouse gas emissions test procedures for our cars. They are very sensitive to possible customer "work-arounds" that would let customers quickly and easily by-pass efficiency mechanisms. There are even special test protocols that keep manufacturers from claiming a fuel efficiency performance that few customers will actually see. The way this is done is to require testing in a variety of the vehicle modes that customers are likely to use. If there is any question, manufacturers are required to actually survey customers to find out which modes they are using. Test procedures have also gotten more conservative to lower label values in response to customer complaints that they can't achieve the fuel economy printed on the label. This has not been an issue on Corvette, but we have to follow the new procedures along with all other manufacturers. That is why you have seen fuel economy label reductions even though the car hasn't changed.
OK, so if customers demanded it, could we put in an "AFM off" switch, and just let the label values and gas guzzler taxes fall where they may? We could, but that is where CAFE (Corporate Average Fuel Economy) becomes an important consideration. General Motors must achieve fleet average fuel economy targets or face stiff financial penalties from the government. It is true that Corvette volumes are but a small part of GM's total fleet, but every tenth of a mile per gallon matters when trying to balance the entire fleet. Further complicating Corvette's contribution to GM's fleet average is the fact that fuel economy regulations are a function of a vehicle's size. The fuel economy target is set based on wheelbase multiplied by track width (if they are different front to rear, you use the average). In the eyes of the government, Corvette is a very small car. It has a compact wheelbase and even though it is fairly wide, the track width is narrow because it has wide tires and track is measured at the tire centerline.
I hear from many customers that they are happily surprised with Corvette's fuel economy, especially on long trips. The Corvette is often the most fuel efficient vehicle they own. So even though the Corvette gets outstanding fuel economy given its performance potential, that doesn't help us on corporate CAFE. As far as the regulation is concerned, based on the vehicle's size, the target fuel economy is about 40 mpg - and that is combined, meaning average city and highway. Because of this, Corvette does have an important effect on our fleet average. We have to do everything in our power to minimize the penalty. That is why we use AFM everywhere we reasonably can on the automatic transmission which is about 80% of Corvette sales.
Of course these trade-offs are nothing new. We have used skip-shift on manuals for decades despite some customer's preference against it. Most of our competitors now have start/stop. Start/stop is not so bad in a traditional car with a quiet idle, but for cars where the engine has a lot of character, it can be very disconcerting to have the engine stop every time you come to rest. Customers have expressed extreme distaste for that feature on a Corvette and so far we have avoided needing to implement it. So, like many of the questions I'm asked, it comes down to how we must balance tradeoffs. We know what customers want and do the best we can to minimize any negative implications arising from government requirements.
It's nice to see that he factors customer feedback so much when weighing his options. The 1-4 skip shift sucks, but a start/stop would be terrible. Glad he's paying attention to customers when having to balance tradeoffs.
Maybe there is an aftermarket answer to those who want to turn AFM on and off with a switch? Skip shift eliminators are an easy answer to the 1-4 CAGS Skip Shift, it would be cool if GM could make it easy for the aftermarket to produce a bypass. Maybe they did and we just don't know yet?
As to the point about Start/Stop, I recently had a Mercedes AMG C63s - 503HP, twin turbo V8. It had Start/Stop, which can be bypassed by turning off a switch or changing driving modes, but the car started each and every time in Comfort (eco) mode, with Start/Stop activated - no matter what mode your last trip ended in. In my mind, this feature was maybe the biggest annoyance I have ever had with a sports car. I applaud GM for not going down this road with the Corvette. I despised the feature. While I get the fuel economy challenge, who wants their 500HP car to shut off and restart 25-30 times on their way to work in traffic? The Corvette gets 3-4mpg better than the AMG did, and does so without Start/Stop, Nice engineering in my mind, as the AFM is non-intrusive to the driver, IMO.
Maybe there is an aftermarket answer to those who want to turn AFM on and off with a switch? Skip shift eliminators are an easy answer to the 1-4 CAGS Skip Shift, it would be cool if GM could make it easy for the aftermarket to produce a bypass. Maybe they did and we just don't know yet?
There is the Range AFM disabler already out there, it's not exactly a switch to turn it on and off, but simply a way to keep it completely off all the time, which is what 99% of Corvette owners probably want, especially since that seems to be the source of the automatic transmission issues too.
I have a 17 GS "export specs" with A8. Since that we dont have CAFE or EPA here, my car goes V4 only in Eco mode. Doesn't do it in any other mode
The minor annoyance that we have is that it always defaults to Eco mode at startup, which is not the case with US spec cars
Much more informative than I had anticipated and there is significantly more regulation entwined in this than one would have guessed. Apart from the possible ramifications this V4 mode has on mechanical longevity or its contribution to the myriad of transmission issues some of us have I don't inherently object to the V4 mode.
LOL . . . yes well written, but many others, including myself on this forum, have many times explained CAFE and the many other regulations as the reason for things like AFM and CAGS only to hear cries of BS from the minions.
I'm in the aviation industry, and we are perhaps the only industry regulated more than surface vehicle manufacturers.
2016 Tahoe has V4 mode.. the vehicle is so heavy that it hardly ever sees V4 (not only can you see it in the DIC display but you can feel it when it happens...). in this case, an apparent waste of technology and a major annoyance with no apparent real-life benefit...
I'm sure there is a lot I don't understand but if they ran 93 octane when doing the mpg test it would save everyone a ton of money. Below is what I got running an average of 71 mph on 93 (with ethanol). When I run 91 (without ethanol) I get about 22 mpg. I just did Waterloo Iowa to Estes Park and back (2200 miles) with some pretty heavy mountain driving over 100 mph (highway 125 off of 34) and got 22.3 mpg. I would say it was 50/50 91 and 93, no 93 out there. The pic is from straight highway on 93. Sorry so blurry. 281 miles at 25.8 mpg at 71 mph. My car does better at 80 mph than 65. That gallon of gas GM runs for the test is $1300/gallon.
Maybe there is an aftermarket answer to those who want to turn AFM on and off with a switch? Skip shift eliminators are an easy answer to the 1-4 CAGS Skip Shift, it would be cool if GM could make it easy for the aftermarket to produce a bypass. Maybe they did and we just don't know yet?
See paragraph three of Tadge's original post:
The EPA sets the fuel economy and greenhouse gas emissions test procedures for our cars. They are very sensitive to possible customer "work-arounds" that would let customers quickly and easily by-pass efficiency mechanisms.
EPA tests emissions and mpg using 93 octane non-ethanol. They than reduce the measurement by 10% to compensate for normal drivers using 10% ethanol. If you want to see the actual documents from EPA, just Google EPA Octane.
I'm curious why the EPA ratings are basically identical for the manual and the auto, when the auto has AFM active far more frequently. I doubt the EPA would allow the manual to be tested only in Eco mode. The manual actually has 1mpg better in the city, the auto 1 better on the highway -- the latter almost certainly due to the 8th gear more than AFM.
This suggests, at the very least, that AFM isn't making much of a difference in ratings. Maybe it is literally a tenths of mpg thing, for other regulatory purposes?
I'm not opposed to AFM, it just scares me with the link to torque converter failure in the A8.
FWIW, the start/stop in the wife's BMW SUV is fine. Can't tell when it starts the minute your foot is off the brake. In fact it also has a variant of energy recovery. They energize the alternator only when costing or braking. Let's most charging occur with otherwise wasted stopping energy!
BOTTOM LINE:
Perhaps if folks want to hear the Vette when at idle (I have the sound system playing loud enough to drown out tire/road noise it's a none issue) then could always have a selectable background sound play! I might even use it if an option was a AA/Fuel dragster waiting for the green light on the line!
SIDE BAR: Watched the Canadian GP yesterday. It was mentioned those 1.6 Liter engines, producing ~850 hp are matching that tracks lap times of larger cid, higher hp older F1 cars! And achieving 30% better gas mileage! That was attributed to the electric motor driven ~160 hp they can use for a few seconds coming out of a turn. Instant max torque at 0 rpm. The small motor/generator used is connected to the crank. Even this 74 year old gearhead would accept the small amount of weight for the ~6 seconds of extra power the F1 cars have. That's more than enough extra power to reduce the time for my 0 to 100 mph blasts!
FWIW, the start/stop in the wife's BMW SUV is fine. Can't tell when it starts the minute your foot is off the brake. In fact it also has a variant of energy recovery. They energize the alternator only when costing or braking. Let's most charging occur with otherwise wasted stopping energy!
BOTTOM LINE:
Perhaps if folks want to hear the Vette when at idle
GM has perfected start/stop extremely well. I have a 2017 Cadillac XT5 which starts within .3 seconds of lifting your foot from the brake and recharges the AGM battery when coasting (rather than costing) or braking to minimize power consumption of the alternator. My first exposure to start/stop was a 2015 Malibu rental while on vacation in California. I drove it for a couple of days before I noticed the autostop position on the tachometer. After seeing that, I watched each time I stopped and it was engaging nearly every time. The new 8-speed transmission was designed for start/stop to shift to neutral before the engine is stopped when the autostop engages. That minimizes the load on the engine bearings because the oil pump is driven by the engine. It takes too much power to drive electrically so the oil must be minimally semi-synthetic to maintain an oil film on the bearings for nearly instant load when restarting after autostop.
I think the biggest objection to autostop in the Corvette is the sound of the exhaust at idle and owners would object to a silent Corvette when stopped. That is why hybrid Corvettes will loose a lot of long time enthusiasts. Would you want a silent Corvette or silent Harley Davidson?
Like a lot of engineering "innovations", no one has studied or has any idea what the long-term effects will be of the stop/start feature. I'd be particularly concerned about shortening the life of both the starter and battery. And from what I've seen the gain in MPG, if any, is miniscule.
I have stop/start on my 2017 Jeep GC Summit and fortunately it has the capability to disable this function, which I always do. I considered buying the new Cadillac XT5 and decided not to because, among other things, it lacked the stop/start disabling capability.
There is no way I would buy a Corvette or any other performance vehicle that had stop/start that couldn't be disabled.
As to the point about Start/Stop, I recently had a Mercedes AMG C63s - 503HP, twin turbo V8. It had Start/Stop, which can be bypassed by turning off a switch or changing driving modes, but the car started each and every time in Comfort (eco) mode, with Start/Stop activated - no matter what mode your last trip ended in. In my mind, this feature was maybe the biggest annoyance I have ever had with a sports car. I applaud GM for not going down this road with the Corvette. I despised the feature. While I get the fuel economy challenge, who wants their 500HP car to shut off and restart 25-30 times on their way to work in traffic? The Corvette gets 3-4mpg better than the AMG did, and does so without Start/Stop, Nice engineering in my mind, as the AFM is non-intrusive to the driver, IMO.
I have to agree... I have the same vehicle but the coupe. I hate that start stop.. but you can turn it off in the Benz. Just go to the car configuration menu and select "individual" and configure the car not use it. I have mine set for open valves, stiff suspension and no ECO. Thing is you have to select it every time you start.
I would hate to see that on a Corvette... it is truly hideous...
We should have the option of turning AFM off... without having to invest more $$$ to turn it off.
There is the Range AFM disabler already out there, it's not exactly a switch to turn it on and off, but simply a way to keep it completely off all the time, which is what 99% of Corvette owners probably want, especially since that seems to be the source of the automatic transmission issues too.
Except it does not work well in the Corvette and causes CEL and codes and non-starts. If it worked properly, it would be a solution but unfortunately it has issues.
Auto stop/start is a waste for mpg improvements.
80% of vettes are automatics?? WTF.
Cylinder deactivation gives marginal gains at best. Also need to know the long term impacts of only having 4 cylinders running and causing more wear to only half of the top end (this is why Toyota skipped deactivation and is using Atkinson cycle)
Can't GM just produce more fuel sipping A to B cars and then have their way with the Corvette so as to meet the aggregate CAFE standards? Don't get me wrong the Corvette is a fabulous car just seems its being held back.
Thanks for the answers to the original question Tadge.
06-09-2017, 02:43 PM
[ANSWERED] Why can you not just disable the AFM?
