Understanding the Mopar Hemi MDS System
Chrysler’s Multi-Displacement System (MDS) deactivates four of the Hemi V8’s eight cylinders during low-load, low-RPM cruising to improve fuel economy. Introduced in 2005, this feature is found on most Hemi-powered vehicles, excluding manual transmission Challengers, early truck variants, and high-performance models like the 6.2L Hellcat.
The transition from eight to four cylinders in the Hemi MDS system takes just 40 milliseconds, made possible by a combination of advanced electronic controls, Electronic Throttle Control, and sophisticated algorithms. MDS uses oil pressure to deactivate the lifters for cylinders 1, 4, 6, and 7, shutting their intake and exhaust valves and halting combustion. These cylinders become sealed air springs, minimizing pumping losses while maintaining smooth operation.
Fuel savings don’t come simply from fewer active cylinders, but from reduced mechanical resistance—specifically pumping losses caused by throttling. With fewer cylinders active, the throttle opens wider, improving efficiency. With the Hemi MDS system Chrysler estimates up to 20% better fuel economy under ideal conditions.
Technically, the Hemi is an OHV, pushrod 90-degree V8 with a cast iron block, aluminum heads, hemispherical chambers, dual spark plugs, and coil-over-plug ignition. The engine also uses an Integrated Air-Fuel Module (IAFM), reducing weight and complexity.
How to Identify MDS And How It Works
MDS-equipped engines can be identified by inspecting the valley beneath the intake manifold—MDS solenoids will be present unless removed and replaced with block-off plugs. The Hemi MDS lifters themselves have a distinctive oil port on the side.
The ECM shuts off fuel to selected cylinders and activates solenoids that send oil pressure to the lifters, decoupling them. These lifters no longer open the valves, but maintain enough tension to keep the valvetrain quiet. As the remaining cylinders take over, throttle and injector pulse are increased to maintain power. Deactivation occurs every 180° of crankshaft rotation, making the switch seamless to the driver.
Performance and Reliability
While MDS does not affect peak engine performance, it adds complexity. Lifters and camshafts in MDS engines are known wear points, especially in high-mileage or modified applications. Swapping a Gen III HEMI into a performance or off-road application often involves deleting the Hemi MDS system.
The real fuel economy gain isn’t just from disabling combustion, but from reducing the energy required to pump air through the engine under partial throttle. At cruise, an engine with fewer active cylinders operates with a wider throttle opening, reducing intake vacuum and pumping effort—making it more efficient.
Why MDS Fails
Precise timing and clean oil are critical to MDS function. Oil contaminants can clog the solenoids or lifter internals, causing incomplete or mistimed transitions between modes. A common failure mode involves the ECM commanding a lifter to engage, but it remains disengaged. The result? The roller slams against the cam lobe, leading to lifter, cam, and valvetrain failure.
Deleting MDS
Many HEMI owners opt to delete MDS, especially when modifying for performance or after experiencing failure. This involves replacing the MDS lifters with standard or high-performance versions, installing valley plugs (Mopar #53032221AA), and flashing the ECM to disable MDS control. Though you may lose a small amount of fuel economy, the trade-off is improved reliability and peace of mind.
