4 cyl. 8 cyl. deactivation question
 

How exactly does this work? GM had this on caddys back in the day and it was terrible! I'm pretty sure every car that had that is now in the junk yard. Or it simply does not work and is stuck on the 8cyl. mode. Is this something similar but more computerized? I hope its much better.
Couldn't one of the chip or module companies write a program to make use of it more then the reported "downhill or foot almost off the gas"? Better yet driver configureable? I would like it to activate 4 cyl at 65mph and kick into 8 if speed drops below 55 or if I want to speed up 8cyl would kick on if 40%+ throttle position is sensed.

Just an early morning thought.


Chris

RE: 4 cyl. 8 cyl. deactivation question
 

Chris:

It works well. So far I have 22K on my truck and haven't had any issues with this system. From what I gather it stays in 4 cylinder mode more often if you have 4:10 gears vs. 3:73. If you made it user configurable ppl that didn't know what they were doing would have it in 4 cylinder mode going up hills on the gas. You could see this wouldn't be good on the drivetrain. I'm sure there is still room for improvement, but in my opinion it works pretty darn good. I'm sure with evolution it will only get better.

Jason

RE: 4 cyl. 8 cyl. deactivation question
 

This is not the first year for the new generation AFM active fuel management (this is the "new" name). It was used in 06 and maybe 05 in several GM vehicles, so it is not brand new in our trucks.


Displacement On Demand (old name, new generation)

Some 5.3L and 6.0L engines are equipped with GM’s fuel-saving Displacement On Demand technology, which can temporarily shut down the combustion process of half the engine’s cylinders under certain driving conditions to save fuel. GM was the first manufacturer to offer this fuel-saving technology on trucks. On the new full-size SUVs, it works with other attributes, such as improved vehicle aerodynamics, to maintain GM’s fuel economy leadership in the segment.

A sophisticated engine controller determines when to deactivate cylinders, allowing the engine to maintain vehicle speed in lighter-load conditions such as highway cruising. The process is seamless and virtually imperceptible. When the cylinders are deactivated the engine effectively operates as a V-4, with alternate cylinders on each cylinder bank disabled. The engine returns to V-8 mode the instant the controller determines the vehicle speed or load requires additional power. The key to DOD’s efficiency and virtually imperceptible operation is a set of special two-stage hydraulic valve lifters, which allows the lifters of deactivated cylinders to operate without actuating the valves.

The valve lifters have inner and outer bodies, which normally operate as a single unit. When the engine controller determines cylinder deactivation conditions are optimal, the outer body moves independently of the inner body on the disabled cylinders’ lifters. The outer body moves in conjunction with camshaft actuation, but the inner body does not move, holding the pushrod in place. This prevents the pushrod from actuating the valve, thereby halting the combustion process. Solenoids in the engine lifter valley operate to deliver high-pressure oil to the switching lifters, activating a release pin to separate the inner and outer bodies. Oil circulation and pressure do not vary, regardless of the engine’s operational mode. Lifter design and pushrod length are the same for every cylinder, but camshaft lobe profiles differ for cylinders designated to be deactivated.
The engine’s electronic throttle control (ETC) also is used to increase manifold pressure in V-4 mode so that the engine can maintain a V-8 torque load.

RE: 4 cyl. 8 cyl. deactivation question
 

Fyr

Excellent explanation.

Does it always deactivate the same cylinders?

RE: 4 cyl. 8 cyl. deactivation question
 

Chris,
There are plenty of articles around the Internet that explain the technical differences of the old Cadillac V-8-6-4 system vs. the new Active Fuel Management system.
Cadillac engineers had a great idea unfortunately they did not have the technology available at that time that would allow the system to operate properly and reliably, technology is no longer the problem and our new AFM system works seamlessly.

The only complaint that most people have is that the 4 cylinder mode isn't in play often enough to suit them, the reason is simple; the AFM computer utilizes torque (load) sensing along with some other time-to-distance calculations and various input parameters to determine when to engage 4-cylinder mode, if the vehicle is driven in 4-cyclinder mode when the loads are too great the efficiency (economy) would be reduced and the engine may sustain damage.
These GMT-900 SUV’s are heavy vehicles that a 4-cylinder engine can not effectively power under most conditions.

If you wanted a 4 cylinder you should have got a CRV or RAV4, then you’d have an SUV that had about half the power, 5,200lbs less towing capability and gets only about 35% better fuel economy and seats 3-4 fewer people.
Heft has its price and the laws of physics can’t overcome that (yet).

RE: 4 cyl. 8 cyl. deactivation question
 

Someone correct me if I am wrong here, but I believe it "alternates" the "dead" cylinders during the cycle to balance / spread the wealth....er wear or lack of it

RE: 4 cyl. 8 cyl. deactivation question
 

It deactivates the same 4-cylinders every time.

RE: 4 cyl. 8 cyl. deactivation question
 

I am sure GM spent a gazillion dollars to know that was the best way to go......oh well, I thought my analogy made sence LOL!!!!

RE: 4 cyl. 8 cyl. deactivation question
 

Your analogy and logic would appear to be 100% correct.

But I’ll explain why it isn’t:
Most normal wear and tear on an engine is produced by the rotation of the components and the movement of the piston up and down within the cylinder, in studies GM’s engineers found that it didn’t matter whether a cylinder was dead or under power the amount of wear was virtually the same.
With this information in mind it made the AFM system easier and more simple to implement as now only 4 cylinders needed to have the 2-stage lifters and one controller to run them as opposed to every cylinder needing the lifters and 2 controllers to activate (rotate through) each bank separately.

RE: 4 cyl. 8 cyl. deactivation question
 

Thank you everyone for your input! Very informative..

Chris