Yea, Dodge, does fancy stuff, but misses the easy stuff. Nearly every 
Dodge pickup I drive behind, I can see the front axle U-joints turning. 
while they drive down the highway. That's a lot of drag. With the front 
hubs locked on our 4WD vehicles, the mileage was down more than 25% 
compared to hubs and transfer case in neutral.

Herbert Graf wrote:
> On Fri, 2009-01-02 at 15:22 +0000, Howard Winter wrote:
>   
>> Having (say) a V8 running on 4 cylinders doesn't make economical sense, because you'll need the same energy from the 4 cylinders to achive the performance your 
>> right foot is demanding, and the 4 non-running cylinders will be wasting energy as air pumps - all the suck-squeeze-nobang-blow energy would be wasted.  Added 
>> to which you're pumping slightly warmed air into the exhaust system, which would upset the electronics trying to set the mixture, and may cool down the catalytic 
>> converters to a non-working temperature.  If you had a completely separate exhaust system for each half you could hold open all the exhaust valves on the 
>> non-running cylinders to minimise the pumping loss, but it's still not zero.  But I still thing the only time you'd be saving fuel would be while idling in traffic.
>>     
>
> FWIW, "shutting off half the cylinders" is done with some engines, and
> works. The Dodge Hemi (5.7L) comes in a version that can shut off half
> the cylinders when not needed. There are various names for this
> technology:
>
> http://en.wikipedia.org/wiki/Variable_displacement
> http://en.wikipedia.org/wiki/Multi-Displacement_System
> http://en.wikipedia.org/wiki/Variable_Cylinder_Management
> http://en.wikipedia.org/wiki/Active_Fuel_Management
>
> The technology DOES work. However, the fuel savings are no where near as
> good as some hope for. I've seen numbers between 5 and 20% savings.
> Doesn't sound like much, but considering we're talking about some
> engines that normally consume 14-16L/100km on the highway, this can be a
> substantial savings over a fleet.
>
> A friend of mine has a car with MDS. His usage on the highway goes from
> 16L/100km to about 13L/100km (MDS has the most effect on light load
> driving, i.e. cruising on the highway).  TTYL
>
>   
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