Why an MDS Delete Demands Supporting Hardware

Removing the Multi-Displacement System (MDS) from a HEMI engine is a popular modification among enthusiasts who prioritize consistent, full-time V8 power over the fuel-saving cylinder deactivation that occurs under light loads. While the delete itself—typically accomplished by removing the MDS solenoids, installing a non-MDS camshaft, and updating the engine calibration—restores a traditional V8 firing order, it also exposes weaknesses in several valvetrain components that were designed for the lower RPM, variable-displacement demands of the stock configuration. Without supporting upgrades, the engine risks valve float, pushrod failure, and lost performance at high RPM. Three components stand out as essential complements to any MDS delete: upgraded valve springs, stronger pushrods, and high-ratio rocker arms.

Upgraded Valve Springs: Critical for High‑RPM Stability

Stock HEMI valve springs are engineered to meet a broad range of operating conditions, including the gentle cylinder deactivation cycles that reduce spring fatigue. After an MDS delete, the engine runs on all eight cylinders all the time, and many builders pair the delete with a hotter camshaft that increases lift and duration at higher RPM. The stock springs quickly become a limiting factor.

Preventing Valve Float and Ensuring Control

At elevated RPM—anything above the stock redline of roughly 6,200 RPM—the inertia of the valvetrain increases dramatically. Stock springs lack the necessary seat pressure and open pressure to keep the valves following the cam lobe profile. The result is valve float: the valve fails to close completely or bounces off its seat, causing a loss of cylinder pressure, potential piston-to-valve contact, and catastrophic engine damage. Upgraded valve springs from manufacturers such as Comp Cams, PAC Racing, or Manton provide significantly higher seat and open pressures, typically 30–50% greater than stock, ensuring the valve train remains stable well past 7,000 RPM.

Material and Design Considerations

Modern performance valve springs are typically made from chrome silicon or chrome vanadium alloys, which resist sagging under sustained high temperatures. Dual springs are common in HEMI builds, where an inner spring supports the outer spring, adding redundancy and reducing the risk of breakage. For extreme builds, conical springs offer a progressive rate that reduces resonance while maintaining high pressures. When selecting springs, pay close attention to the installed height and coil bind clearance—these must be matched to the camshaft lift and the retainer design.

Installation Tips and Valve Train Geometry

Installing upgraded springs requires a spring compressor tool and careful measurement of each spring's installed height. Inconsistent heights can lead to unequal valve control and cylinder-to-cylinder variation. It is also wise to replace the valve stem seals and inspect the valve guides for wear, as higher spring pressures can accelerate guide wear if clearances are excessive. Always verify that the spring retainer and locks are compatible—many aftermarket springs use 7° or 10° locks that differ from stock.

For further reading, consult the technical resources at Comp Cams for spring selection guidelines and PAC Racing for spring rate specifications.

Pushrods: The Unsung Heroes of the Valve Train

Pushrods transfer the motion from the lifter to the rocker arm. In a high-performance HEMI, they must handle extreme compressive loads and resist buckling under high spring pressures. Stock pushrods are thin-walled, often made from mild steel, and are designed for the reduced demands of a stock cam and moderate spring pressures. After an MDS delete with a performance cam, upgraded pushrods are a reliability necessity.

Material Strength and Wall Thickness

Aftermarket pushrods are typically made from chromoly steel (4130 or 4140 alloy) with wall thicknesses ranging from 0.080″ to 0.120″ or more. Thicker walls provide greater column strength and resist bending under heavy spring loads. Some high-end pushrods use heat-treated 3/8″ diameter tubes for maximum stiffness, though clearance with some cylinder heads can be an issue. Always choose pushrods that are rated for at least the peak spring pressure you are running—many builders aim for a safety margin of 20%.

Length: The Key to Proper Rocker Geometry

One of the most overlooked aspects of a HEMI build is pushrod length. If you are using aftermarket cylinder heads (especially those with milled decks or different valve heights), the required pushrod length can change dramatically. Using a pushrod length checker is essential. The ideal geometry places the rocker arm tip centered on the valve stem at mid-lift. Incorrect length leads to side loading, accelerated guide wear, and reduced power. Many HEMI performance suppliers—like Manton and Smith Bros.—offer custom-length pushrods to accommodate specific builds.

Heat and Fatigue Resistance

Performance driving generates intense heat in the valvetrain, especially near the exhaust ports. Chromoly pushrods handle thermal expansion better than mild steel and resist fatigue from high-RPM cycling. Some builders also opt for hardened pushrod tips to reduce wear in the rocker arm sockets and lifter cups.

For a deeper dive into pushrod selection, refer to the guide at Manton Pushrods.

High‑Ratio Rockers: Maximizing Valve Lift Without a New Cam

High-ratio rocker arms increase the effective lift of a camshaft by increasing the rocker arm ratio. The stock HEMI rocker arms typically have a ratio of 1.55:1 or 1.6:1, while aftermarket high-ratio rockers are available in 1.7:1, 1.8:1, and even 1.9:1 configurations. This upgrade can unlock significant power gains without the expense and labor of a cam swap.

How Ratio Affects Lift and Duration

Valve lift is the product of cam lobe lift multiplied by the rocker arm ratio. A 1.7:1 rocker on a cam lobe with 0.350″ of lobe lift produces approximately 0.595″ of valve lift, compared to 0.560″ with a 1.6:1 rocker. That extra lift improves airflow through the cylinder head, particularly at high RPM, and can add 10–20 horsepower in a properly tuned HEMI. Duration is also effectively increased because the valve spends more time off its seat at higher lifts, though the actual cam lobe profile remains unchanged.

Compatibility and Piston‑to‑Valve Clearance

Before installing high-ratio rockers, you must verify piston-to-valve clearance. The added lift may cause the valves to contact the pistons, especially with a modified camshaft. Use a clay test or a dial indicator to measure clearance at the tightest point (typically at top dead center overlap). Minimum clearance for safety is 0.080″ on the intake and 0.100″ on the exhaust. Many high-ratio rocker sets—such as those from Harland Sharp, PRW, or Chrysler Performance—include instructions for checking clearance.

Throttle Response and Mid‑Range Torque

High-ratio rockers also improve throttle response because the valve opens faster relative to crank degrees. This can yield noticeable gains in mid-range torque, making the engine feel stronger during everyday driving. However, the effect is greatest when paired with a camshaft that has a moderate lift profile—if you already have a high-lift cam, the gains diminish and clearance issues become more acute.

Rocker Stability and Spring Pressure

As you increase rocker ratio, the effective spring pressure at the valve tip rises proportionally. A 1.7:1 rocker increases the load on the spring by roughly 6% compared to a 1.6:1 rocker, so the valve springs must be able to handle that additional force without binding or coil clash. Always match rockers to the spring package you have selected.

Additional Supporting Mods to Consider

While valve springs, pushrods, and rockers form the core of a post‑MDS delete valvetrain upgrade, other components merit attention for a fully reliable build.

Camshaft Selection

Most MDS deletes involve replacing the stock camshaft with one that lacks the MDS lobes. Many aftermarket camshafts are designed to work with the upgraded springs and rockers discussed here. Street-oriented cams (218–224 degrees duration at 0.050″ lift) pair well with 1.7:1 rockers for a broad power band.

Oil Pump Upgrade

Higher spring pressures and heavier valvetrain components increase the load on the oil pump. A high‑volume oil pump from Melling or Moroso ensures adequate lubrication and hydraulic lifter priming, especially if you rev the engine aggressively.

Lifters and Retainers

Stock HEMI lifters are durable, but if you are changing the cam, consider upgrading to a non‑MDS lifter set. The lifter bodies are the same size but lack the internal oil passages for the MDS solenoids, reducing the chance of oil pressure loss. Upgrade the retainer to a steel or titanium unit for weight savings and strength.

Head Studs and Gaskets

For boosted or high‑compression HEMI builds, head studs replace stock head bolts to provide more consistent clamping force. This is not strictly necessary for a naturally aspirated MDS delete, but it adds a safety margin for future power increases.

Putting It All Together: A Sample Build Scenario

Consider a 2015 Dodge Challenger 392 (6.4L HEMI) receiving an MDS delete. The builder selects a Comp Cams MDS delete camshaft with 224°/230° duration at 0.050″ and 0.575″/0.590″ lift. To support this cam, the builder installs PAC 1775 valve springs (seat pressure 145 lbs, open pressure 385 lbs at the cam lobe lift). Pushrods are custom-length Manton 3/8″ chromoly with 0.120″ wall thickness, set to 7.800″ after checking geometry. Finally, Harland Sharp 1.7:1 roller rockers are installed, increasing the effective lift to 0.660″ on the intake. The combination yields a strong, reliable street/strip engine that revs cleanly to 7,000 RPM.

For more detailed build specifications, the team at Modern Performance Nation offers extensive dyno results and part‑selection advice for HEMI platforms.

Conclusion: Reliability and Performance Through Thoughtful Upgrades

An MDS delete unlocks the full potential of a HEMI engine, but it also shifts the operating envelope beyond what production components were designed to handle. Upgraded valve springs prevent valve float and ensure consistent closure under high RPM. Stronger pushrods resist buckling and maintain correct valvetrain geometry. High-ratio rockers amplify the camshaft’s lift, improving airflow and throttle response without requiring a new cam profile. Together, these three upgrades form the foundation of a robust valvetrain that can withstand years of spirited driving. Invest in quality components, carefully measure clearances, and your MDS‑deleted HEMI will deliver the power and reliability you expect.