I have been looking at new trucks, just dreaming, and noticed that dodge hemi, and now chevy are offering displacement on demand v-8's. Now only firing 4 clyinder of a 8 , is not a new idea, but you must get uneven engine wear? I know chevy is just releasing this, but has anyone see the specs on dodge's 4-8 cyl.hemi? fuel ecconomy that much better? Has anyone heard of ford coming out with this?

 

I don't think that the DOD would cause uneven wear. Correct me if I'm wrong, but wouldn't all the pistons be moving just as in a regular V8? It's just the fuel is turned off to four of the cylinders. So, under that situation, you would get exactly the same wear on all eight cylinders. Anyone know for sure if that's how these systems work? Also, it's probably not the same four cylinders that are turned off each time. So that would even out the wear as well.

Either way, this is one of the technologies that is needed to help efficiency in large vehicles.

 

my truck trend tested The dodge Hemi DOD compared to chevy hybred agains regular F150
The chevy hybred electric got 14.1MPG
The dodge reg hemi got 16 even
The Hemi DOD got 16.2
the 5.4 F150 got 15.5,mpg
DOD iF those are indeed true number is not worth the engineering plus they said only would cut 4 cylinders if on flat ground no headwind, Not real ideal driving for .2 MPG

 

For a start welcome to FTE.

In my personaly opinion - Displacement on Demand is nothing more than a marketing gimmick.
It provides very little (or no) advantage in regard to fuel consumption over a well calibrated normal V8 with an efficient compbustion chamber. As seventyseven250 mentioned all the cylinders keep moving, so frictional benefits are minimal.
All you are doing is turning off 4 injectors. And injecting more fuel into 4 other cylinders.

 

It might provide an advantage, if the engine were already getting good gas mileage. It gives you something like 15% more fuel mileage, which equates to less than 3 mpgs in a truck on the highway--and that's if and only if you are cruising. You could get better mileage by swapping your rear gears.

 

Theres two Impalas at the Chev dealer across the street.. an SS with DOD...window sticker rates it at 32mpg highway
and an LS rated at 40 MPG highway. Makes you wonder why...

 

I'm calling bull on that. The 3.5L is rated at 31 highway. The 3.9L is rated at 28 highway and the 5.3L is rated at 27 highway. If they're stickered for anything differently, I'd buy it then sue for false advertising.

http://www.fueleconomy.gov

 

The new Ram1500's with the HEMI are rated at 19mpg highway. I think the only issues with this whole MPG thing is that people drive so much differently than the way the EPA does its rating. On average people are driving at 70mph+ and results in poor fuel economy. I've seen many guys over at the Dodge forums talking about slowing down to 65 or 60mph and seeing big gains in mileage. HEMI guys are getting 17-19mpg on long trips. That test that was in truck trends is also a little misleading because I doubt any of those trucks were 100% broken in. We all know that on a new car or truck, before they are broken in they are using a lot more gas. I think in order for these new MDS or DOD systems to work as intended, is to change driving habits/styles and not stay in the gas as much.

Also-these systems don't just stop the fuel, they stop the spark also and continue lubrication so nothing detrimental happens. The engine doesn't just cut off 4 injectors and over compensate on the other 4 injectors (spray more fuel), it keeps the rate the same.

 

Does anyone know if these engines do something with the valving so that they aren't fighting compression in the four "turned off" cylinders? Something like keeping the exhaust valves open?

And regarding the impalas. In a smaller vehicle, you'll always be able to get better mileage with a smaller engine. It's just in situations where you need the extra power (trucks) or want it (sports cars) where you will see some gains over conventional large V8's.

 

If you were fighting compression the air within the cylinder would act like a spring. It doesnt require very much hp to maintain a reasonable highway speed. I would guess less than 50hp.

 

Imperial gallons here in the Dominion of Canada probably some strange conversion formula from litres/100km.

I really didnt look that closely, except to notice that the standard engine had better ratings

 

That is kind of interesting though- an Impala LTZ with the 3.9 V6 epa's for 17/28, the 5.3 V8 SS comes in at 18/27.

 

It's like how the 530i gets better mileage than a 525i. More power and economy...then why get the 525i?

 

Um, because its cheaper to begin with????
Not to mention things like tires, insurance etc...

 

and let's not forget the patience factor you learn when you mash the gas pedal to merge on the autobahn, and the thing takes at least 15 seconds to spool up before it gathers speed, while the lunatics are bearing down on you at triple-digit speeds. Some thrills, money just can't buy.