GM 5.3 vs. 6.0 mileage?

[GreatWhitehunter]

My wife's 07 tahoe has AMF and I don't feel the transition from 8 to 4. She's getting 17 average. I've seen it as high as 19. 30k so far and no issues.

Matt T.

 

[Rara Avis]

So are these AFM engines too new to have reliability records? How does it shut down the cylinders? GM has been down this road before with the Caddy with nobody too pleased, as I remember it.
Jim

That was pre-computer times...

 

[fgross335_2210]

I have AFM on my 2007 Tahoe with 5.3. It is 100% unobtrusive. Not sure how much it helps my MPG, when you are running 70-75 on the highway the only time you're in 4cyl mode is downhill. Seems to help most when running 55-60 on level ground.

I get 15.5-16 in mixed commuting, 18-18.5 on the highway, and 11-12 towing my 23ft enclosed snowmobile trailer.

 

[Eagleview]

Are these a pushrod engine ? What takes up the lash when the lifters are collapsed ? Can you hear any tappet rattle when it is activated ? How does it overcome the suction of the falling piston ? Thank's .

 

[jimmysisson]

And do the same 4 cylinders cut out each time? Also curious with Iron Horse about the 4-cyl mode, what happens with the valve train. We could all use better mileage, that's for sure these days.
Jim

 

[asylum575]

My last tank of mostly highway miles on my 6.0/4spd got just shy of 14MPG.
At $4.oo a gallon , it's pricey.

 

[Z-Michigan]

Are these a pushrod engine ? What takes up the lash when the lifters are collapsed ? Can you hear any tappet rattle when it is activated ? How does it overcome the suction of the falling piston ? Thank's .

These are pushrod engines, 2 valves/cylinder.

I don't know about lash.

I do not hear any tappet rattle or any other funny noises in V4 mode.

It is the same 4 cylinders each time. Either GM or Chrysler (can't remember which, and again their systems are almost the same) did wear testing showing no meaningful difference over 150k miles.

I think the valves are shut to leave air in each cylinder, which acts as a spring and minimizes any pumping losses (takes power to compress, releases that power on the downstroke; obviously there are minor heat losses as a result if you understand gas expansion/contraction physics).

From all I can tell the system has no drawbacks of significance. It is transparent. I can sometimes but not always hear the difference, but I'm really in tune with my vehicle. Most people don't hear it.

 

[KAB]

These are pushrod engines, 2 valves/cylinder.

I don't know about lash.

I do not hear any tappet rattle or any other funny noises in V4 mode.

It is the same 4 cylinders each time. Either GM or Chrysler (can't remember which, and again their systems are almost the same) did wear testing showing no meaningful difference over 150k miles.

I think the valves are shut to leave air in each cylinder, which acts as a spring and minimizes any pumping losses (takes power to compress, releases that power on the downstroke; obviously there are minor heat losses as a result if you understand gas expansion/contraction physics).

From all I can tell the system has no drawbacks of significance. It is transparent. I can sometimes but not always hear the difference, but I'm really in tune with my vehicle. Most people don't hear it.

Z.

Good explaination with a few minor corrections.
It uses oil pressure to initiate AFM. In the cyl's. that shut down there is a different lifter that is used. The oil pressure pushes in on 2 pins on this special lifter. The lifter basically adds a sleeve around a normal lifter. When Oil Pressure is not present it acts like a normal lifter. When Oil Pressure is increased (not normal engine oil pressure but pressure going to these lifters) it pushes these pins in and lets the outer sleeve slide up over the "normal" part of the lifter. The valves for that cylinder remain closed. Fuel is also shut off to the cyl but spark remains active. GM has done extensive testing to ensure that wear differences would be kept to a minimum. GM de-activates the same cyl all of the time and Chrysler rotates which cylinders de-activate. Chrysler was the first to market (Durango) followed closely by GM (Long Wheel base Envoy/Trailblazer), both for 2005.

If I heard correctly the lifters were created by Eaton Corp. It is easy to identify which trucks have it in by looking at the engine valley cover, if it is flat it does not have AFM, It it has raised runners (channels) across the top it has AFM.

I have not heard of any issue with either GM's or Chrysler's systems.


john_bud
You are correct the new HD's do nat have AFM but the do have variable valve timing, a Cast Block and 6 speed tranny. I have been hearing that the 6 speed tranny will be available in the 2009 1/2 tons but do not expect them with all engine choices. The 4 Speed Trannys will be around for a while.

Hope this helps

Kurt

 

[Z-Michigan]

Thanks, Kurt. More details than I knew. You refresh my memory about the system being used on the Trailblazer EXT; a friend of mine had one with it. If it's been used since MY 2005, I think that's long enough for problems to start showing up if there were any.

 

[john_bud]

Ahh technology!

Gone are the days when there was one belt going from the crank to wp to alt and that was it!

Then again, that car was a 1970 maveric with 200 CID inline 6 cyl with 3 speed manual trans. It got 30-35 mpg with a crazy kid driving it! Maybe I should look for one?

jb