Welcome to winter, down the road in Calgary.... 89 octane is the way to go, my hemi dose not have mds. keeping the Od locked out will disable mds, 4 cy is not enough to move a big heavy truck, once your on the highway, your momentum dosent need all 8 cylinders to keep going.

Once you hit a hill, or towing, Mds is useless, by locking out the OD, your telling your truck its going to needing all 8 cy, and your going to get better city milage with the od locked out.

I don't know the time frame when mds kicks in at idel.

 

Mds does not kick in at idol, however it does kick in just befor 2nd gear under verry easy acceleration around 10 m.p.h. good luck.

 

Right after I bought my 08 hemi i went straight to the muffler shop and made it louder. As I was driving home it hit the mds and i thought I had broke the damn thing, it definitely makes a difference in sound with a loud muffler on it. So i drove around in tow/haul for a week before I figured out what it was. Now I dont mind

 

so if i put some magna flows on it ...is it gonna sound bad? or should i go ahead and upgrade the exhaust?

 

I have a magnaflow on mine. The sound was irritating at first but i turn the radio up and im overall used to it now. Its a matter of what you can tolerate when it comes to that.
Alot of guys on here have a modded exhaust and MDS. Some disabled it with tuners and others like me have not.