MDS Delete: Why Manual and Cam-Swapped HEMIs Need This
If you're planning a cam swap on an automatic Challenger, the MDS delete isn't optional — it's a required part of the build. Here's what MDS is, why it has to go before a cam swap, and what the delete kit actually includes.
What Is MDS?
Multi-Displacement System (MDS) is Chrysler's cylinder deactivation technology fitted to automatic-transmission HEMIs (5.7L and 6.4L). At light throttle and highway cruise speeds, the PCM deactivates cylinders 2, 3, 5, and 8 by holding their intake and exhaust valves closed and cutting fuel delivery. The result is a 4-cylinder operating mode that improves fuel economy by 10–20%.
The MDS system uses special collapsible lifters in the deactivated cylinders — they have a mechanism to collapse inward, preventing the valves from opening when in cylinder-deactivation mode.
Why MDS Must Be Addressed Before a Cam Swap
Aftermarket performance camshafts are designed for standard, non-collapsible lifters. Specifically:
- Lift: Aftermarket cams have significantly more lift than stock. The MDS collapsible lifter mechanism cannot handle high lift — the increased load can cause the lifter to collapse unexpectedly, destroying the lobe and the lifter simultaneously.
- Lobe separation angle: Aggressive cam profiles have tighter lobe separation angles. The MDS system's timing assumptions don't account for aftermarket cam profiles.
- Valve timing interference: In deactivation mode, the MDS lifters hold valves closed. With a high-lift cam, this creates a situation where the cam lobe tries to push the valve open while the MDS mechanism tries to keep it closed — catastrophic for the valvetrain.
The MDS Delete Kit
An MDS delete replaces all 8 MDS lifters with standard, non-collapsible performance lifters and includes MDS port block-off plates — threaded plugs that fill the oil passages used to pressurize and activate the MDS mechanism.
A complete MDS delete kit includes:
- 8 non-MDS performance lifters (replaces the collapsible ones)
- 4 MDS oil port block-off plugs (threaded into the block)
- Typically: updated valley cover gasket
Cost of a standalone MDS delete kit: $150–$250
Cost in a full cam master kit: Usually included
Most cam manufacturers (Comp Cams, Melling, EPS) sell "master kits" that include the cam, lifters, springs, and pushrods together. Always buy a complete kit — mixing components from different manufacturers can cause incompatibilities.
Does the PCM Need to Know?
Yes — after an MDS delete, the PCM must be tuned to disable MDS operation. If left enabled in the PCM, it will still try to activate the MDS system during highway driving, which will result in misfires, rough idle when switching modes, and eventually a check engine light.
Your tuner disables MDS with a simple parameter change in the PCM calibration. This is a standard part of any HEMI cam/MDS delete tune.
Manual Transmission Cars
Manual transmission Challengers do not have MDS — it was never offered with the Tremec. If you own a manual, you don't need to worry about MDS delete. Your cam swap is simpler: new cam, new springs, new pushrods, tune.
Post-Delete Feel
After MDS delete, the car runs 8 cylinders 100% of the time. You'll notice:
- Smoother idle (no mode-switching vibration)
- Slightly higher fuel consumption (5–10% on highway)
- The mode-switching "shudder" at highway cruise is permanently gone
- No more sound symposer "V4 mode" sounds
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