Some modern performance engines – like the 5.7 and 6.4 HEMI – use cylinder deactivation technology. This helps increase fuel economy and lower emissions. It’s a part of what helps these large V8 engines meet stricter emissions standards of today. Shutting down cylinders sounds simple in theory, but there’s a lot that goes into cylinder deactivation. In this article, we explain 5.7 & 6.4 HEMI MDS, examine some pros/cons, and discuss MDS issues and delete kits.
Which Models Use MDS?
Most 5.7 HEMI and 6.4 HEMI engines use MDS technology, including the following:
- 2005+ Chrysler 300C
- 2007+ Chrysler Aspen
- 2005+ Jeep Grand Cherokee
- 2006+ Jeep Commander
- 2005+ Dodge Magnum
- 2006+ Dodge Durango
- 2006+ Dodge Charger
- 2009+ Dodge Challenger
- 2006+ Dodge / Ram 1500
One exception is 5.7 and 6.4 HEMI’s equipped with a manual transmission. MDS isn’t used with the manual. Additionally, Ram 2500 and 3500 trucks do not use MDS. The 6.1 HEMI in older SRT models along with the 6.2L Hellcat engine also don’t feature MDS. Otherwise, the technology is in any automatic transmission 5.7L or 6.4L HEMI listed above.
What Is HEMI MDS?
Chrysler HEMI MDS in its simplest form is cylinder deactivation technology. MDS stands for Multi-Displacement System. Some GM engines use a similar technology that they refer to as Active Fuel Management (AFM). Both systems use a very similar design and operation. However, in this article we’re simply focusing on the 5.7 and 6.4 HEMI MDS.
MDS shuts down four of the V8’s cylinders in certain conditions, such as driving at a steady speed. Essentially, the V8 HEMI becomes a 4-cylinder engine when MDS is activated. When you need power and dig into the throttle the MDS kicks the cylinders back to life so you have the full power from the large 5.7L and 6.4L engines.
Sounds simple, right? Well, the Multiple-Displacement System is more complex than simply shutting down the ignition. A number of different things must occur in rapid succession for MDS to operate seamlessly.
How Does MDS Work?
It seems the simplest approach to cylinder deactivation would be shutting down the ignition and fuel supply. That would be as easy as a quick tuning change, and wouldn’t require any special tech. However, that wouldn’t provide any real benefit to fuel economy or emissions.
When 4-cylinders are shut down the engine still needs to make the same amount of power to hold a steady speed. So, the remaining cylinders need more air and fuel once the others are shut down. The benefits of cylinder deactivation come mainly from a reduction in pumping losses.
This means you actually need to keep the intake and exhaust valves in the closed position. First, the ECM cuts fuel to the cylinders that are shutting down (MDS deactivates cylinders 1, 4, 6, & 7). The intake valve then opens for the last time to pull in air before it’s shut by the ECM. No ignition occurs on the power stroke, and the exhaust valve then opens for its last time before being shut down. This happens in a split second all while the ECM is commanding more throttle and fuel for the remaining 4 cylinders. Depending on RPM, the whole process takes roughly one to two tenths of a second.
How can you possibly shut the valves when the cams are still rotating as normal? The Chrysler 5.7 & 6.4 HEMI use MDS solenoids to send pressurized oil to a locking pin in the lifter. The locking pin allows the lifter roller to follow the cam profile. However, the motion isn’t transferred to the plunger so the pushrods don’t engage. It’s essentially special lifters and solenoids that allow MDS to work; the ECM ensures the process is smooth.
Chrysler HEMI MDS Pros
Some pros of the 5.7 HEMI Multi-Displacement System include:
- Better fuel economy
- Lower emissions
Pros of MDS are straight-forward. It allows for better fuel economy and lower emissions. As alluded to above, MDS doesn’t improve fuel economy due to 4 fewer cylinders receiving fuel. Instead, it’s due to less pumping losses. Engines are actually more efficient when the throttle is open further.
Shutting down 4 cylinders means the engine needs to open the throttle about twice as much to create the same power. That’s where the roughly 10-20% MPG improvement comes from.
HEMI Multi-Displacement System Cons
Unfortunately, MDS also comes with some potential downsides:
- More components to fail
- Colder cylinder temperatures
The HEMI uses a total of 4 MDS solenoids, which means 4 solenoids that can potentially fail. The locking pins in the lifter rollers are more parts that introduce room for issues. Point is – more moving parts means more things that can go wrong, and that’s something we’ll discuss in the next section. Before discussing 5.7 HEMI MDS problems it’s important to discuss another potential downside.
MDS always shuts down the same cylinders – 1, 4, 6, and 7. Let’s say you’re cruising at a steady speed on a relatively flat road for an extended period. Those cylinders and nearby areas (like the head) are going to get much cooler. Suddenly, you need a lot of power and the cylinders spring back to life at high-rpm and high engine load. Drastic heat changes and loads aren’t a good thing.
MDS is designed to restart the cylinders every so often to prevent them from losing too much heat, so that does lessen potential concerns. Many HEMI’s live long lives without any real downsides. However, it still can lead to uneven cylinder wear. It might not be horrible for the engine, but it’s certainly not good either.
6.4L & 5.7L HEMI MDS Problems
Most of the issues with 5.7 and 6.4 HEMI MDS were briefly mentioned in the last section. In these next sections, we’ll expand on a few of the common issues with MDS. Then we move onto MDS delete options as this is becoming increasingly common as older HEMI’s continue to age and run into MDS problems. Anyway, some of the common HEMI MDS problems include:
- Solenoids
- Lifer rollers
- Valve seats
Before jumping in it’s important to note none of these issues are truly common. Instead, when 5.7 or 6.4 HEMI MDS related problems do arise these are a few common areas. MDS has been around for a long time and has proven to be pretty reliable. However, problems aren’t unheard of by any means and there’s a reason some HEMI owners delete the Multi-Displacement System.
1) HEMI MDS Solenoid Issues
Despite the fact the HEMI’s use 4 solenoids this is actually a fairly uncommon issue, so we’ll be quick here. MDS solenoid failures can and do happen. The sensors receive an electronic signal from the ECM, which means electronic failures sometimes occur. Dirty or contaminated oil can also clog sensors and cause issues.
Again, this isn’t a truly common MDS problem. However, the MDS solenoids sit underneath the intake manifold. Replacing the solenoids is fairly simple but it does require some labor. A single solenoid is only about $50. Labor can run in the $400-600+ ballpark if you go to a repair shop, though. It’s generally best to replace all the sensors (especially on older, high mileage HEMI’s) since they’re fairly inexpensive relative to the labor.
2) 5.7 HEMI MDS Lifter Roller Failures
Lifter and lifter roller issues are the most common MDS-related problem on this list. These problems are widely discussed on the 5.7L and 6.4L HEMI since they can be costly and potentially lead to complete engine failure. The main culprit is oil contamination.
If small pieces of dirt or debris block the circuit to the locking pin then the lifter may fail to change state, move slowly, or fail to fully change. The roller will then continue to follow the cam lobe until a speed where it cannot. The 5.7/6.4 HEMI roller then contacts the camshaft lobe. Metal on metal contact damages the lifter, lifter roller, and camshaft. In rare cases (if not caught soon enough), metal shavings can make it past the oil filter and cause further damage.
Best case you’ll be replacing the camshaft, lifters, and lifter rollers. This all lies under the cylinder head, so it’s a labor intensive job. Cost of parts also adds up quite a bit; it’s likely this job will to cost $1,500+ at a repair shop.
3) Multi-Displacement System & Dropped Valve Seats
Dropped valve seats are the least common problem on this list and primarily affected the original 5.7 HEMI (before it was updated to the 5.7 Eagle in 2009). However, this issue highlights the previous discussion about colder temperatures on the MDS cylinders.
After a long drive where MDS is active for extended periods those cylinders are quite a bit colder. You then stop at a gas station to fill up on gas and restart the engine shortly after, for example. The valve seats would shrink in diameter faster than the aluminum cylinder head. In turn, the valve seats could possibly drop.
This affected some 2005-2008 MDS HEMI engines, but problems were rare even back then. The newer 2009+ 5.7 Eagle uses a design to retain the valve seats, which eliminated the issues. Still, it’s a good reminder that MDS has some inherent flaws with varying cylinder and head temperatures.
5.7 & 6.4 HEMI MDS Delete
Deleting the 5.7 and 6.4 HEMI MDS is commonplace in the performance and aftermarket Mopar world. Even though MDS is generally reliable it doesn’t provide any meaningful benefit outside of better MPG. If you don’t care about fuel economy than it does nothing more than introduce the possibility of failures and other problems.
There are two primary ways to get rid of MDS – disable it or completely delete it. A simple tuner will allow you to deactivate the MDS system, and quickly turn it back on should you choose. Otherwise, there are MDS delete kits to get rid of the Multi-Displacement System altogether.
MDS deletes require a set of non-MDS lifters and retainers along with four plugs to install in place of the solenoids. This is standard practice for camshaft upgrades as most aftermarket cams are non-MDS. If you run into serious HEMI MDS problems (like the lifter rollers) then it might also be a good time to completely delete the system.
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