If this is the MDS, then it sure would be nice to have a button to enable/disable the MDS functionality when desirable...besides the Tow/Haul mode. Also it seems the cruise control programming could be adjusted to not trip up the MDS.

So the MDS "raises" the RPMs when firing on 4 cylinders?

 

Rearend gear turning the same speed, tranny turning the same speed if TC locked engine speed should be the same, 4 or 8cylinder mode. What are the rpm with the 2nd overdrive(5th) disengaged? I haven't noticed on mine, but it does the same rpm hunting. Funny thing is I have 3.92 rearend and have the same rpm hunt 1650 to 1900, think it would be higher with the lower rearend gear.

 

I also have the 3.92 gears. It seems that the scenario might be...cruising along in 4-cyl mode and TC is locked up (all is good)...then engine sees need for more power so 8-cyl mode is engaged...and when this happens the TC unlocks...which would explain both the increased rpms and the "surge" that I can feel. Now my only problem here is that I am on level ground with no apparent need for more power...why can't the computer deal with this steady state condition???

 

For what it is worth,
The Fuel Economy Calculator software from PerformanceTrends.com
predicts that
a 2008 Ram 4x4 Quad Cab
with 5.7 Hemi MDS, 3.55 diff, 545 auto, 31.6 in dia tires
Cd=0.465, frontal area =35.1 sq ft, weight =5800 lbs

will reach 8 cylinder 50% 'Reserve Torque'
which is 100% 4 cylinder MDS limit where the MDS must kick off
on a 85 degree F air temperature
windless level concrete highway
at 65 mph

with a 7 mph tail wind this increase to
73 mph

with 0 wind and a hardly noticeable 0.5% downhill grade
71 mph

on level blacktop pavement with 0 wind 0 grade
63 mph

on level blacktop, with 7 mph side wind
57 mph

on blacktop, with 7 mph side wind, and slight 0.5% uphill grade
48 mph

on level blacktop, with 7 mph side wind and 1800 lbs rated max load in bed
49 mph

from 2002 to 2008 the aerodynamics of the 3rd gen Ram improved a bit each year.
The 2002 Ram 4x4 had as bad an official Chrysler Media website listed Cd as 0.528
so your year Ram with MDS will vary from the above numbers due to aero,
and if you have changed the tires to wider, taller, mudder style tread that is deep
this will also load the engine down and make the MPH where MDS has to kick out lower.

MDS will not start again until the 8 cylinder Reserve Torque number is about 60%
meaning that 4 cylinder mode will start out at 80% .... (100-60) times 2

 

No, not really. I've never experienced RPM hunting with cruise on, I recently drove from Orlando to North Carolina, and with the cruise set at 72 on I-95 the only time it downshifted was on the bridge crossing the Savannah River. I've kinda regretted buying the 4.7, because in a quad cab, the engine is underpowered for that large of a vehicle and the fuel economy is even with or worse than a non-mds hemi. However, I was suprised with it on long trips, because it does have enough torque at cruising speeds to just cruise, without the constant downshifting on the slightest grade, which my Chevy Blazer used to do. I suspect it has to do a lot with the gearing in the sense that at X engine speed producing Y amount of torque, Y is less than the torque required to maintain speed. I think MDS was a brilliant idea for the Charger and the 300C, but on something as big as the Ram, 4 cylinders just aren't enough.

 

The FEC program predicts that for a 5200 lb
2002 Ram with 4.7 V8,
3.55 diff behind the 45RFE in 0.75 ratio gear
with 29.6 inch diameter tires
at 72 mph
on a level concrete highway
Reserve Torque is 37%

MPG is estimated at 16.7 if there is no wind

At a grade of about 2% at 72 mph
Reserve Torque declines to 15%
and the 45RFE trans will probably downshift.
MPG in the 0.75 gear on the 2% hill just before the downshift is 11.5
MPG in the 1.00 gear on the 2% hill just after the downshift is 11.3

As the engine is more loaded climbing the hill
the higher rpm does not hurt MPG as much because
MAP, intake manifold absolute pressure is high, pumping losses are low,
and increased cylinder pressures and faster burn allow ignition timing to be retarded, which decreases negative work. "Negative work" is when at high degrees of ignition advance, the piston pressure pushes downward before the piston reaches top dead center. When this happens, the other cylinders have to apply work to push this piston 'over the top' where its pressure begins to push in the right direction to make power.
Attach a scanner to your vehicle and you will see ignition timing advance as high a 40 degrees at low throttle openings or when EGR is allowed to flow.

This is "common sense"
as auto engineers would try to make a downshift occur
about at the right load and MAP
that MPG would stay about the same in either gear.

With 3.92 diff on level highway
Reserve Torque goes up to 44%
and MPG on level grade will go down to 16.2
(due to pumping loss from intake manifold vacuum & greater internal engine friction at higher rpm)
Reserve Torque declines to 15% when the hill is 3.1%

With 4.10 diff, 4.7V8 Reserve Torque goes up to 47%
and MPG on level grade goes down to 15.8
Reserve Torque reaches 15% on a 3.7% grade hill

With 4.56 diff Reserve Torque goes up to 53%
and MPG on level grade goes down to 14.8
Reserve Torque reaches 15% when the hill grade is 4.9%

These are using new stock engine specifications.
Since the 4.7 engine is well broken in,
the piston ring tension is worn down,
and the tire tread may also be partially worn,
which reduces rolling resistance
so it may be a bit better than this.

 

I've got 3.92s and averaged 17.1 mpg on the trip to NC (~725 miles driven). At the time my truck had 102k miles on it and ~80% tread life remaining.

 

I've not noticed any gear searching on my '04 pre-MDS Hemi. But I've had 4.56 gearing, which tends to get the engine rpm's into the powerband faster and keep it there.
I can't, however recall it being an issue before the gears, but that was almost three years ago...

 

Some people like the mds some people don't, i am one who really has zero desire to own an mds system, I seen what a mess they ended up being in the cadillacs. I just have to keep my liitle 4.7.

 

MDS is nothing like the 8-6-4 caddy put out 20 years ago...you would know that if you knew anything about either.