Understanding the HEMI Engine and MDS Technology

The HEMI engine, trademarked by Chrysler, is defined by its hemispherical combustion chambers that permit larger valves and more efficient airflow than conventional flat-head or wedge designs. This architecture has powered everything from dragsters to daily-driven pickup trucks. In 2005, Chrysler introduced the Multi-Displacement System (MDS) on the 5.7L HEMI to improve fuel economy by 8–12% under light loads. MDS deactivates four cylinders by collapsing the lifters on cylinders 1, 4, 6, and 7, effectively running the engine as a 4-cylinder during cruising and deceleration.

While MDS delivers tangible fuel savings, it introduces complexity in the valvetrain and oiling system. The MDS lifter bodies contain internal solenoids that respond to ECM commands. Over time, these components can fail—especially in high-mileage engines—causing camshaft damage or lifter collapse. For performance builders targeting 600+ horsepower, the MDS system becomes a liability. The collapsing lifters cannot reliably control aggressive cam profiles used in high-rpm builds, and the reduced oil pressure to MDS lifters under sustained heavy load can lead to catastrophic failure. Removing MDS eliminates these failure points and simplifies the engine’s hydraulic lifter system.

Beyond reliability, MDS delete allows tuners to run a larger, non-MDS camshaft designed for peak power at higher RPMs. Factory MDS cams are limited by the need to safely deactivate cylinders; a delete opens the door to profiles with higher lift and longer duration. This is the foundational step for any HEMI build aiming to exceed 600 wheel horsepower.

Primary Benefits of a HEMI MDS Delete for High-Horsepower Builds

Unrestricted Power Delivery

The most immediate gain from deleting MDS is the ability to command all eight cylinders to fire at full capacity at all times. In an MDS-equipped engine, the transition between 8- and 4-cylinder mode introduces a slight delay and can cause momentary lean conditions in the deactivated cylinders when they re-engage. With MDS removed, the engine management sees a constant eight-cylinder firing order, simplifying fuel and ignition maps. This allows tuners to safely advance timing and lean out air/fuel ratios in the mid-range without worrying about cylinder deactivation artifacts.

Simplified Engine Management and Tuning

Modern HEMI ECUs are highly adaptive, but MDS adds layers of logic that can interfere with aftermarket tuning. Many standalone ECUs or flashed stock ECUs struggle to properly emulate MDS when running aggressive cams or high-flow fuel systems. A physical delete—removing the MDS solenoids, blocking oil passages, and installing non-MDS lifters—eliminates these software dependencies. Tuners can then focus on VE tables, spark advance, and knock control without MDS-related code patches. This reduces the risk of mis-tunes that cause misfires or detonation under power.

Enhanced Component Longevity

Stock MDS lifters use a two-piece design with a spring-loaded collar that disengages the pushrod. Under heavy boost or high-RPM operation, the collar can partially engage, causing the lifter to “pump up” and lose effective travel. This leads to loss of valve lift, reduced power, and potential piston-to-valve contact. Replacing MDS lifters with non-MDS solid or hydraulic roller lifters (often sourced from Hellcat or aftermarket manufacturers) eliminates this failure mode. Combined with forged pistons and rods, the bottom end becomes capable of handling 600+ horsepower comfortably while the valvetrain remains stable at 7,000+ RPM.

Essential Components for a 600+ HP HEMI Build

Forged Pistons and Rings

Stock HEMI pistons are hypereutectic castings that can withstand around 500–550 crank horsepower before cracking under detonation. For 600+ wheel horsepower (approximately 700+ crank), forged pistons are mandatory. Look for 2618 or 4032 alloy forgings from brands like Mahle, CP-Carillo, Diamond, or JE. For a boosted application (supercharger or turbo), choose a dish volume that yields 9.5:1 to 10.5:1 compression; for naturally aspirated builds, 11.0:1 to 11.5:1 is optimal. Piston ring gaps should be opened to accommodate thermal expansion: typically .0055” per inch of bore for top rings on forced induction setups.

Forged Connecting Rods

Factory HEMI powdered-metal rods have a well-known fatigue limit near 550–600 ft-lbs of torque. Upgrading to forged 4340 steel rods (e.g., K1 Technologies, Manley, Callies) provides a safety margin for sustained high power. For builds exceeding 600 wheel horsepower, choose rods with 7/16” ARP 2000 bolts and a cap-screw design for better clamping force. Rod length must match the factory 6.200” (6.1L HEMI) or 6.650” (6.4L or 6.2L stroker) depending on stroke.

High-Performance Camshaft

Cam selection is the single most influential component for power output. For 600+ hp naturally aspirated, use a cam with approximately 230–240 degrees intake duration at .050” and .600”–.650” lift with a 112–114 LSA. For supercharged or turbocharged builds, a wider LSA (116–118 degrees) and less overlap reduces reversion. Non-MDS lifters (typically from a Hellcat or aftermarket) allow the use of aggressive lobe profiles that MDS lifters cannot handle. Do not reuse factory MDS lifters on a performance cam—they will fail prematurely.

Upgraded Cylinder Heads

Factory HEMI heads flow well, but pocket porting and larger valves (2.20” intake/1.60” exhaust) can unlock another 30–50 horsepower. For builds exceeding 600 hp, consider CNC-ported heads from companies like Texas Speed, Thitek, or R/A Performance. Stiffer valve springs (dual or triple beehive) are essential to control the aggressive cam lobe ramp rates. Retainers should be titanium to reduce reciprocating weight at high RPM.

Performance Intake and Exhaust Systems

A 600+ hp HEMI requires free-flowing intake and exhaust paths. For the intake side, a ported stock intake manifold or an aftermarket sheet-metal manifold (e.g., Edelbrock Victor) paired with a 102mm or 105mm throttle body improves airflow. For forced induction, a water-to-air intercooler or an air-to-air unit with proper ducting is critical. On the exhaust side, long-tube headers (1-7/8” to 2” primary tubes) feeding a 3-inch or 3.5-inch full exhaust minimize backpressure. Catalytic converters should be high-flow or eliminated on dedicated track cars.

Fuel System Upgrades

Stock HEMI fuel injectors and pumps are insufficient above 550 crank horsepower. Use injectors sized to support at least 800 hp (e.g., 1000cc or 1300cc) and a fuel pump capable of delivering 340 LPH or more (e.g., Walbro 525 or dual in-tank pumps). For E85 or race gas, stainless steel fuel lines and brushed ethanol-compatible components are necessary.

Detailed Installation Process for MDS Delete and Forged Internals

Performing an MDS delete and installing forged pistons and rods requires complete engine disassembly. Work in a clean environment and document all clearances.

Step 1 – Engine Removal and Disassembly

Remove the engine from the vehicle using a leveler. Drain all fluids. Separate the intake manifold, valves, timing cover, water pump, and front dress. Remove the cylinder heads and place them on a clean surface. Remove the oil pan, oil pump, and windage tray. Unbolt the factory rods and caps, remove the pistons, and separate rod caps. Label each rod and piston according to the cylinder it came from.

Step 2 – Remove MDS Components

Locate the MDS solenoids on the sides of the engine block (two per side). Remove them with a hex wrench. The oil passages that feed the MDS lifters must be blocked. Commonly, builders install MDS delete plugs (available from BAP, Manton, or aftermarket) into the lifter oil galleries. Alternatively, the lifters themselves can be replaced with non-MDS units, and the solenoids can be removed and the ECM programmed to ignore the missing sensors. However, physically blocking the passages is more robust for a race engine.

Step 3 – Prepare Block for Forged Internals

Have the engine block cleaned and honed to the correct bore size for your forged pistons. Measure piston-to-wall clearance per manufacturer spec (typically .0035”–.0045” for 2618 alloy). Install the new main bearings and insert the crankshaft with proper thrust clearance. For rebuilds exceeding 600 hp, upgrade to ARP main studs for better clamping.

Step 4 – Assemble Forged Pistons and Rods

Balance the rods and pistons set as a pair. Install rings on pistons using a ring expander, staggering gaps 180 degrees. Lightly oil the ring compressor and insert piston/rod assemblies into the block. Torque rod bolts to manufacturer specification using ARP moly lube. Rotate the crankshaft to ensure no binding.

Step 5 – Install New Camshaft and Lifters

Coat the camshaft lobes with assembly lube. Install non-MDS lifters (with proper offset orientation at #1 and #4 cylinders). Reinstall the timing chain and cam retainer. Verify camshaft end play (.005”–.015”). Install a new timing chain tensioner and guides.

Step 6 – Reassemble Cylinder Heads and Valvetrain

Install new valve springs, retainers, and locks. Check installed spring height and coil bind clearance—especially important with aggressive cams. Use a spring compressor to install valve seals. Torque cylinder head bolts in three stages (20 ft-lb, 50 ft-lb, then final angle per service manual). Reinstall pushrods and rocker arms. Adjust valve lash if using solid lifters; with hydraulic rollers, tighten until zero lash, then add 1/2 to 3/4 turn.

Step 7 – Final Assembly and Tuning

Install new oil pump (Melling high-volume recommended), oil pan, front cover, and water pump. Reinstall intake, injectors, and throttle body. Prime the oil system by spinning the oil pump with a drill until oil reaches the rockers. Install the exhaust headers. Reinstall the engine and connect all sensors. Do not start the engine without a proper base tune—the aggressive cam and increased airflow demand revised injector pulse width and timing. Have the tune performed on a dyno by a certified HEMI tuner.

Performance Expectations and Real-World Results

A properly built 6.4L HEMI with MDS delete, forged internals, stage 2 cam, ported heads, and long-tube headers can reliably produce 580–640 wheel horsepower on 93 octane. With a supercharger (e.g., Whipple 2.9L or Vortech) on a built bottom end, numbers rise to 700–900 wheel horsepower. The MDS delete itself does not add power directly; it allows the use of components that do add power. On a 6.1L HEMI, simply deleting MDS and installing a high-lift cam can increase power by 80–100 horsepower compared to the stock MDS-restricted cam.

For example, a 2013 Chrysler 300 SRT-8 with a 6.4L HEMI running a MDS delete kit, forged pistons, Manley rods, BAP oiling system, and a 236/242 .635” cam dyno-tested at 624 whp and 577 wtq through a 5-speed automatic. The same build with MDS left intact would have been limited to a milder cam and would likely peak around 530 whp.

Durability is also improved. Owners report that MDS-deleted engines with forged internals can sustain multiple drag strip passes in 90+°F weather without oil temperature exceeding 280°F, whereas stock MDS cars often experience lifter tick or misfires after hard sessions.

Common Pitfalls to Avoid

  • Incorrect lifter selection: Using Hellcat non-MDS lifters (part number 53022207AF) is a direct swap, but they have a different offset at cylinder 1 and 4 positions. Confirm alignment before final rocker installation.
  • Oil galley blockage: If you remove solenoids but leave the vertical oil passages open, the engine will lose oil pressure. Use MDS delete plugs or block the passages with threaded plugs.
  • Tuning without delete: Some tuners try to disable MDS in software only while leaving physical components in place. This is risky because the MDS lifter sensors may still cause false knock readings. Always perform a physical delete on high-power builds.
  • Skipping piston-to-valve clearance check: Aggressive cams require fly-cut valve reliefs in the pistons. Failure to check clearance at overlap can cause catastrophic valve–piston interference at high RPM.

Conclusion

Performing a HEMI MDS delete is not merely about turning off a fuel-saving feature—it is the gateway to unlocking the true capability of the Gen III HEMI. Pairing that delete with forged pistons, forged rods, a performance cam, and supporting components allows builders to confidently hit 600+ horsepower while maintaining streetability. The result is a reliable, high-output engine that retains the legendary HEMI roar and responds harder than any factory MDS-equipped powertrain. Whether you are building a project car or a serious race piece, the combination of MDS delete and forged internals is the foundation for triple-digit power gains.

For more information, refer to Mopar performance guides from Mopar Performance, technical articles from CP-Carrillo, and tuning resources from HP Tuners.