I'd appreciate some insight from anyone in the know about this engine. How do they "deactivate" the four non-contributing cylinders? Is it always the same four, or do they shift it around in order to spread the wear? When does the deactivation occur? Can the feature be manually locked out? Any known issues yet?
I remember the Cadillac 8-6-4 system from years ago, it was a POS throw-away engine. I'm looking at trading my '03 Ram Hemi before the $7K incentives run out next week, butI don't want to be a guinea pig for a technology experiment that is valueless or worse.
The system works with special lifters that can be disengaged, thereby isolating the intake and exhaust valves in four of the cylinders (the same four - #1, #4, #6, and #7, which are 1st, 3rd, 5th, and 7th in the firing order of 1-8-4-3-6-5-7-2). The engine controller also shuts off spark and fuel to the cylinders, making them completely dead. The engine switches from 4 to 8-cylinder mode based on engine load... it's something that's calculated automatically, and you won't notice when it's happening. Basically, during deceleration, idle, cruise, or light acceleration - you'll be in 4-cylinder mode. Mash the throttle, and you'll get all 8.
How long does it actually take for a lifter to collapse when the oil supply is interrupted? Also, is the oil turned off and on by one solenoid valve, or one for each cylinder (4), or one for each valve (8)? Are these interruptors easy to replace when they get gummed up or malfunction?
Saw your question in the other thread. The 03 just seems to have the rep of being faster and more responsive, like dodge has been taming down the hemi's over the years.
As to the mds. Don't care for the mds one little bit but dodge being dodge there is a wide range of quality to their engines. Some say it's a good thing. I guess I do know how it works, at least on mine. What are model are you interested in?
My '06 Durango Hemi mds works fine so far (7,000 mi.) Deactivation (switch to 8 cyls) occurs when there is increasing load (i.e. going uphill). I have read specs (somewhere) that is runs on 4 cyls about
56% of the time. You can notice the shift if you are watching for it; it's not really objectionable at all. Usually I have to be watching the tach to be sure it's switching. I live in a hilly area (Birmingham, AL) and have had not any problems or annoyance from it. It IS mandatory that 5W-20 oil be used, however - if not, shifting will supposedly take longer, and it will be more noticeable. (Neither the dealer nor owner's manual tell you that, but numerous online sources do - and you don't have the usual "use this oil weight for this temperature range" chart; it says use THIS. ) I won't swear that the cylinders don't rotate the "active" sequence, but would be quite surprised if they did - seems an incredible lot of trouble to go to, and it would be a nightmare to troubleshoot. Unknown how the mds feature could be locked out, or why anyone would want to - this is what keeps the fuel economy reasonable.
I will say that with my former vehicle, a Chevy Colorado, I would go nuts going up hills on cruise - very poorly set up (I have been a Chevy man most of my life, BTW). It would lag way back going uphill, then downshift two gears and give you whiplash, trying to catch up. I finally stopped even trying to use the cruise in hilly country. The
5.7 hemi, in contrast, has extremely smooth cruise control, and rarely drops off more than 1-2 mph, even when swapping from 4-8 cyls or vice versa. Very nice.
Thanks Larry, that's a good review. Regarding why you'd lock it out, it seems that it doesn't do much for the mileage anyway, so if it were noticeable switching I'd lock it out. My 2003 hemi averages 14.5/19, and once got 20.5 average both ways on a trip from Dallas to Shreveport and back. If I believe the claims, the new MDS hemi hasn't improved on that any.
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