Active Fuel Management control
CF Beginner
Joined: Feb 2008
Posts: 26
Likes: 0
RE: Active Fuel Management control
ORIGINAL: ZX1100F1
Cylinders 1-4-6-7 are the ones the are deactivated.
To put it simply;
One factor is:
When half of the cylinders are deactivated the engine has a much narrower operational range of efficiency due to the fact that (unlike a normal 4 cylinder motor) there is still a fair amount of resistance and pump loss to overcome from the dead cylinders, if the RPM's drop too low (>1,000) the engine will lug easily and either stall or stumble, keep in mind that you're still dealing with a V8 engine here not a 4 cylinder.
Another factor is that these AFM equipped motors are attached to much larger transmissions than a normal 4 cylinder vehicle is, these big tranny's absorb quite a bit of energy when Drive is selected at idle.
I realize that this is difficult for some to understand but the software program that operates the AFM system will enable 4 cylinder deactivation any time that it will improve fuel efficiency with almost no exceptions.
Cylinders 1-4-6-7 are the ones the are deactivated.
To put it simply;
One factor is:
When half of the cylinders are deactivated the engine has a much narrower operational range of efficiency due to the fact that (unlike a normal 4 cylinder motor) there is still a fair amount of resistance and pump loss to overcome from the dead cylinders, if the RPM's drop too low (>1,000) the engine will lug easily and either stall or stumble, keep in mind that you're still dealing with a V8 engine here not a 4 cylinder.
Another factor is that these AFM equipped motors are attached to much larger transmissions than a normal 4 cylinder vehicle is, these big tranny's absorb quite a bit of energy when Drive is selected at idle.
I realize that this is difficult for some to understand but the software program that operates the AFM system will enable 4 cylinder deactivation any time that it will improve fuel efficiency with almost no exceptions.
Administrator
Joined: Apr 2006
Posts: 11,337
Likes: 194
From: KC, MO area
RE: Active Fuel Management control
ORIGINAL: fredster
Actually, when you engage T/H at 45 you shift back to 3rd, out of OD. WIth T/H on, OD will not engage until about 56MPH. The TC may or may not have been locked up in OD at 45, but will lockup in T/H. To test this theory, put your shifter in "3" and drive 45 then engage T/H, and your RPM's will stay the same, maybe even decrease if your TC wasn'ta alreadylocked.
Actually, when you engage T/H at 45 you shift back to 3rd, out of OD. WIth T/H on, OD will not engage until about 56MPH. The TC may or may not have been locked up in OD at 45, but will lockup in T/H. To test this theory, put your shifter in "3" and drive 45 then engage T/H, and your RPM's will stay the same, maybe even decrease if your TC wasn'ta alreadylocked.
CF Active Member
Joined: Oct 2006
Posts: 208
Likes: 0
From:
RE: Active Fuel Management control
And opposite of logic, the firmer, harder shifts that occur when in T/H are actually better for the transmission. A long, smooth shift is actually much harder on the trans.
CF Beginner
Joined: Aug 2011
Posts: 1
Likes: 0
We've had no problem getting 19+ mpg hwy with 22 various GMT900 suv's in real world cross country tests.
One problem is that most people are coming out of several year old Tahoe's (very well broke-in) and going into 07 units and expecting a miracle right out of the gate, you have got to give this machine a chance.
A secondary reality is that now with gas averaging around $3 a gallon nationwide consumers are much more conscious about economy then they were when they purchased their last vehicle.
"16.7 mpg" is nothing to sneeze at from a vehicle this size (300lbs heavier than last year) that hauls 9 passengers and can tow a 7,700lb trailer, not all that many years ago I owned a K-5 that sat only five people, couldn't a tow tent trailer or get out of its own way and never got better than about 10mpg, for this matter a very good friend of mine drives a Toyota ext cab 4X4 Tacoma with a 4 cylinder and auto trans, would you care to guess what kind of fuel economy he gets on the highway?
FYI, these vehicles will continue to get improved mpg as the miles roll on, if yo don't have 30k miles on your Tahoe then it is not "fully broke-in".
One problem is that most people are coming out of several year old Tahoe's (very well broke-in) and going into 07 units and expecting a miracle right out of the gate, you have got to give this machine a chance.
A secondary reality is that now with gas averaging around $3 a gallon nationwide consumers are much more conscious about economy then they were when they purchased their last vehicle.
"16.7 mpg" is nothing to sneeze at from a vehicle this size (300lbs heavier than last year) that hauls 9 passengers and can tow a 7,700lb trailer, not all that many years ago I owned a K-5 that sat only five people, couldn't a tow tent trailer or get out of its own way and never got better than about 10mpg, for this matter a very good friend of mine drives a Toyota ext cab 4X4 Tacoma with a 4 cylinder and auto trans, would you care to guess what kind of fuel economy he gets on the highway?
FYI, these vehicles will continue to get improved mpg as the miles roll on, if yo don't have 30k miles on your Tahoe then it is not "fully broke-in".
But for the fuel mileage, I cannot believe them. Maybe it's that $150 o2 sensor that's causing my -5mpg difference, but the guy has a point. It would be nice to have a way to adjust it, or for the dealer to reprogram the computer that controls this "AFM" to where it will kick into 4cyl mode maybe a little more in town. I think an ideal "AFM" would run all eight cylinders if my foot were more than 1/2 way down on the accelerator. I don't think I need to accelerate with all eight cylinders in first gear. I think four would be alright until I got to third gear.
What about my corvette...(from 1985!) with a 350, that I'd rather take on vacation because it gets 22mpg? 22mpg IN A V8 FROM 1985! Granted, yes there is quite a bit of weight, gear ratio and displacement difference. But it's still a V8 (a bigger one), 4 speed, and, I drive a bit more aggressively in it, than I do my truck, and I can usually average at least 20mpg if not better.
I'm realistic though, I didn't expect to get great gas mileage out of a full-size truck. But I will say, if the technology is there (AFM), why not get the most out if it?
Administrator
Joined: Apr 2008
Posts: 7,573
Likes: 47
From: Austin, Texas
Welcome to the forum, try driving for about a week at 50mpg or less your MPG will go up considerably... I think when GM does the ratings the do it under some of the best possible circumstances.....
Straight from the man....http://www.fueleconomy.gov/feg/why_d...detailed.shtml
Straight from the man....http://www.fueleconomy.gov/feg/why_d...detailed.shtml
Administrator
Joined: Apr 2006
Posts: 11,337
Likes: 194
From: KC, MO area
I think the ratings are computed from an EPA prescribed driving cycle where the vehicle is driven on a road dyno thru the cycle that lasts about seven minutes. The EPA actually modified the cycle a few years back to more closely represent actual usage and the window sticker MPG actually went down.
CF Junior Member
Joined: Mar 2011
Posts: 72
Likes: 1
From: Wisconsin
I could only read through 2 pages before I got bored 
I have a 2008 Suburban 4X4 with the 5.3L. I drive 15 miles to work, a mix of city and highway. I get 16.7 on the gauge. I also do the manual calculations and have seen as high as mid 18s and a low of high 15s
I have a 2008 Suburban 4X4 with the 5.3L. I drive 15 miles to work, a mix of city and highway. I get 16.7 on the gauge. I also do the manual calculations and have seen as high as mid 18s and a low of high 15s
CF Beginner
Joined: Oct 2011
Posts: 2
Likes: 0
Chevrolet needs to come up with a way of turning off the system, something maybe simular to the tow mode. If you don't want it - turn it off. Me personally, I would turn it off all together beacuase I hate mine. It's always going back and forth between 4 - 8 cyls, shuttering right before it goes back to 8 and acting like it doesn't have enough power. And when it does go to 8 cyls the RPM's jump 200-300 hundred and the truck speeds up and then slows down again just to have the process start all over again. Not to mention the way the 6 speed tranny jumps and bucks trying to figure out what gear it wants. I took it to the delaer twice and they say it normal for the truck to do this. ????? The last time in they gave me a print out telling me how normal it is and maybe I should try disconnecting something to lessen the problem.
I believe they did this anyway because since I've gotten it back it does switch cyls a little less noticable but it also has less power and the tranny is acting different now, slower.
I believe they did this anyway because since I've gotten it back it does switch cyls a little less noticable but it also has less power and the tranny is acting different now, slower.
Administrator
Joined: Apr 2008
Posts: 7,573
Likes: 47
From: Austin, Texas
Chevrolet needs to come up with a way of turning off the system, something maybe simular to the tow mode. If you don't want it - turn it off. Me personally, I would turn it off all together beacuase I hate mine. It's always going back and forth between 4 - 8 cyls, shuttering right before it goes back to 8 and acting like it doesn't have enough power. And when it does go to 8 cyls the RPM's jump 200-300 hundred and the truck speeds up and then slows down again just to have the process start all over again. Not to mention the way the 6 speed tranny jumps and bucks trying to figure out what gear it wants. I took it to the delaer twice and they say it normal for the truck to do this. ????? The last time in they gave me a print out telling me how normal it is and maybe I should try disconnecting something to lessen the problem.
I believe they did this anyway because since I've gotten it back it does switch cyls a little less noticable but it also has less power and the tranny is acting different now, slower.
I believe they did this anyway because since I've gotten it back it does switch cyls a little less noticable but it also has less power and the tranny is acting different now, slower.
I suspect that what your are feeling is not the AFM but the torque management....driving in 3 and not OD will will remove hunting you feel and I believe it will also disable the AFM or at least it seems that way when I have been in 3...