Why the Hellcat Underdrive Pulley Kit Is a Smart Power Upgrade

Adding an underdrive pulley kit to a Hellcat engine is one of the most effective ways to free up parasitic drag on the crankshaft and redirect that energy to the wheels. Unlike aggressive cam swaps or forced-induction modifications, an underdrive pulley reduces the rotational mass and load from accessory drives — alternator, water pump, power steering — without compromising the core reliability of the engine. The Hellcat Underdrive Pulley Kit is engineered to maintain proper belt alignment and tension while delivering a measurable gain in horsepower, typically 10–15 hp at the wheels, with no tune required.

This upgrade is especially appealing for owners who want a straightforward, reversible modification that doesn't introduce the complexity or risk of internal engine work. When installed correctly, the kit supports daily drivability, retains factory idle quality, and keeps engine temperatures in check. The key lies in the installation process itself: precise torque specs, correct belt routing, and careful attention to pulley alignment. This guide walks through every step with the detail needed to complete the job confidently in a home garage or shop.

Before diving into the mechanical work, it's worth understanding that an underdrive pulley slows the rotational speed of accessories. This reduces the power required to spin them, but it also means alternator output and water pump flow are slightly decreased at low RPM. For a street-driven Hellcat, this is rarely a concern because the system is designed to handle the reduced accessory speed within normal operating ranges. However, if the vehicle is used for extreme idling or competition where electrical draw is high, it's smart to monitor voltage and coolant temperature after the install.

Tools, Parts, and Workspace Preparation

A clean, well-organized workspace saves time and prevents mistakes. Park the Hellcat on a level concrete floor, engage the parking brake, and chock the rear wheels. The following list covers everything needed for a full installation. Some items are optional but strongly recommended for ease of work.

Required Tools

  • Socket set — ⅜‑inch and ½‑inch drive, metric and SAE sizes (10 mm, 13 mm, 15 mm, 18 mm are common)
  • Torque wrench — capable of 20–150 ft‑lb, preferably a click‑type for accuracy
  • Pulley puller — a three‑jaw puller or dedicated harmonic damper puller that fits the Hemi crankshaft
  • Breaker bar — 24‑inch minimum for breaking stubborn crank bolts
  • Allen keys — some underdrive kits use Allen‑head fasteners
  • Trim removal tools — plastic pry tools to pop loose radiator fan shrouds or belt guards without scratching

Parts and Consumables

  • Hellcat Underdrive Pulley Kit — includes the underdrive crank pulley, new bolts, and often a belt alignment tool
  • Replacement serpentine belt — the kit usually specifies a shorter belt; confirm part number before purchasing
  • High‑temperature grease — a small amount for the crankshaft snout and pulley bore
  • Thread locker (medium strength) — blue Loctite 242 recommended for pulley bolts
  • Safety goggles and mechanic’s gloves
  • Shop rags and a catch pan — coolant may drip if the water pump housing is disturbed

Optional but Helpful

  • Impact gun — for removing the crank bolt (use with caution on the crank snout)
  • Flywheel/flexplate holding tool — prevents the engine from turning while loosening the crank bolt
  • Dial indicator or straightedge — to verify pulley runout after installation
  • Scanned copy of the factory service manual — belt routing diagram and torque tables

Once all tools are laid out, disconnect the negative battery terminal and secure the cable away from the terminal post. This eliminates the risk of accidental shorts and resets the ECU adaptation — which can help the engine relearn idle and fuel trims after the new pulley changes accessory load characteristics.

Step 1: Gain Access to the Crankshaft Pulley

In most Hellcat-equipped vehicles — Challenger, Charger, Durango SRT, or Ram TRX — the engine bay is packed tight. Getting to the crankshaft pulley requires removing several layers of shielding and the radiator fan shroud assembly. Work methodically and keep fasteners in labeled bags.

  • Remove the engine cover (pull upward on the oil fill cap area).
  • Unbolt the air intake tube and resonator, then set them aside.
  • Disconnect the electric fan connectors and unclip the wiring harness from the fan shroud.
  • Remove the radiator fan shroud by pulling it upward and out of the lower retention clips. This creates roughly 5 inches of additional clearance in front of the crank pulley.
  • If the vehicle has a belly pan or under‑engine splash shield, remove it to allow access from below.

With the fan shroud removed, the serpentine belt path and the factory crank pulley are visible. Use a strong light to inspect the belt for any cracking or glazing — if the belt is original, replacing it proactively is wise even if the kit includes a new belt.

Step 2: Remove the Serpentine Belt and Accessory Drive Components

Routing the belt off the stock pulley is straightforward, but tension is high. Use a breaker bar on the tensioner to release pressure. On the Hellcat 6.2‑liter Hemi, the belt tensioner is located near the top of the engine on the passenger side. Rotate it counter‑clockwise to relieve tension, then slip the belt off the crank pulley first. Let the tensioner return slowly to avoid snapping the tool or belt.

Once the belt is free, inspect the idler pulleys for roughness or play. While the belt is off, spin each idler by hand — any grinding or wobble should be addressed before reassembly. Worn idlers can cause belt chirps after the underdrive install, leading to a false diagnosis of pulley alignment issues.

If the vehicle is equipped with a supercharger, the accessory drive layout is similar, but the crank pulley is larger. The underdrive replacement will be noticeably smaller in diameter — this is normal. Do not mistake the reduced diameter for a manufacturing defect; the difference in size is what reduces accessory speed.

Step 3: Safely Remove the Factory Crank Pulley

With the belt off, the factory crank pulley is exposed. The single hex bolt holding it to the crankshaft is torqued to a high value from the factory — typically 130–140 ft‑lb. Attempting to break this bolt loose with the engine’s starter or by putting the car in gear can damage the crank snout or transmission. Use one of these methods:

  • Flywheel holding tool — insert through the starter opening to lock the flexplate.
  • Impact gun — a ½‑inch impact rated for at least 600 ft‑lb of breakaway torque can usually rattle the bolt loose without needing to hold the crank.
  • Breaker bar and socket with a helper holding the crank bolt socket while a second person bumps the starter (use extreme caution and only if experienced).

Once the bolt is out, thread it back in two or three turns by hand. This protects the crankshaft threads when the puller applies force. Place the three‑jaw puller behind the factory pulley hub and center the forcing screw against the crank bolt head. Turn the forcing screw evenly until the pulley pops loose. If the pulley resists, apply penetrating oil around the crank snout and let it soak for ten minutes. Never hammer on the puller or use a torch on the crank snout — this can anneal the forged steel and cause catastrophic failure.

After the pulley is free, remove the crank bolt completely and slide the factory pulley off the snout. Clean the crankshaft snout with a lint‑free rag and inspect it for scoring, rust, or burrs. Even small imperfections can cause the new pulley to seat incorrectly, leading to wobble or premature belt wear.

Step 4: Prep and Install the Hellcat Underdrive Pulley

Take the new underdrive pulley out of the packaging and inspect the bore for a snug fit on the crank snout. It should slide on easily with hand pressure after a light coating of high‑temperature grease. If it feels tight, use fine emery cloth to polish the snout — do not force the pulley or drive it on with a hammer. The hub face must sit flush against the crank shoulder with zero gap.

Apply a thin layer of grease to the crank snout and the inside of the pulley bore. This prevents galling and makes future removal easier. Slide the pulley onto the snout by hand until it stops. Thread the new crank bolt (provided in the kit) into the crankshaft by hand to ensure the threads start cleanly. Use a torque wrench to tighten the bolt to the manufacturer’s specification — usually 130 ft‑lb for the Hellcat platform. Some kits specify a two‑step torque sequence: first 80 ft‑lb, then an additional 60‑degree turn. Always follow the instructions included with the specific kit.

Critical check after tightening: place a straightedge across the face of the new pulley and measure the gap to the block or timing cover at four points around the circumference. The gap should be uniform within 0.005 inch. Excessive runout indicates the pulley is not seated squarely and must be removed, inspected, and reinstalled.

Step 5: Install the New Serpentine Belt with Correct Routing

The Hellcat Underdrive Pulley Kit requires a shorter belt because the reduced crank pulley diameter changes the effective belt length needed to maintain tension. Using the old belt will result in a loose fit, and the tensioner may bottom out or fail to maintain proper tension. The kit typically includes the correct belt, but if sourced separately, confirm the part number against the manufacturer’s recommendation.

Route the new belt around the crank pulley first, then work outward following the factory belt diagram. A common mistake is routing the belt wrong on the water pump or power steering pulley, causing the belt to ride on its edge. Use strong lighting and mirror to verify the belt sits centered on every pulley groove. On the tensioner, rotate it counter‑clockwise again and slip the belt over the final pulley — usually the alternator or supercharger snout — then release the tensioner slowly. The tensioner arm should rest within the indicated travel marks (if present) or sit at roughly mid‑travel.

After routing, spin the engine over by hand using a socket on the crank bolt for two full rotations. This confirms there is no binding, the belt tracks true, and the tensioner does not bottom out or contact the belt guard. If any rubbing or chirping is heard during this manual rotation, stop immediately and inspect the belt path.

Step 6: Reassemble the Engine Bay and Perform Final Torque Checks

With the belt installed and verified, reinstall all components that were removed. This is the time to be methodical — rushing reassembly can lead to loose fasteners or pinched wiring.

  • Reattach the radiator fan shroud, ensuring the lower clips snap home and the wiring connectors click securely.
  • Bolt the air intake tube and resonator back into place, tightening clamps evenly.
  • If the belly pan was removed, reinstall it with all fasteners.
  • Reconnect the negative battery terminal and torque it to 8–10 ft‑lb.

Before starting the engine, double‑check the belt tensioner position, the crank bolt torque, and that no tools or rags are left in the engine bay. It is easy to leave a socket or rag under the fan shroud, which can cause immediate damage on startup.

Step 7: Startup, Bleed Procedures, and Performance Validation

Start the engine and let it idle for 15–20 seconds, then shut it down. This first start allows the belt to seat and the tensioner to settle. Listen for belt chirps, squeaks, or any rhythmic ticking from the pulley area. If chirps are present, a slight misalignment is usually the cause — but often the belt just needs a few thermal cycles to relax. If the noise is severe, shut down and check that all pulleys are coplanar.

After the initial check, restart the engine and let it reach normal operating temperature. Monitor the voltage gauge on the dash — if it drops below 13.2 volts at idle with the headlights and HVAC on, the alternator may not be spinning fast enough. In most Hellcat applications, voltage remains stable because the underdrive pulley still spins the alternator at an adequate speed for street driving. If voltage consistently reads low, consider an overdrive alternator pulley to compensate.

Take the vehicle on a road test that includes stop‑and‑go traffic, steady cruising, and a few full‑throttle pulls. Pay attention to:

  • Belt slip at high RPM — a squealing sound under hard acceleration indicates insufficient tension or a worn tensioner.
  • Coolant temperature — if the water pump is underdriven too far, coolant temps may rise in hot weather or during extended idling. Hellcat engines have robust cooling systems, and this is rare unless a thermostat or fan controller fault exists.
  • Engine idle quality — the ECU will adapt to the reduced accessory load within a few drive cycles. Initially, the idle may hunt slightly; this is normal and usually resolves after 50 miles.

After the road test, let the vehicle cool and recheck the crank bolt torque. Some builders recommend retorquing after the first heat‑cool cycle because the pulley can settle slightly. It is a good habit to recheck at 100 miles and again at 500 miles.

Common Mistakes and How to Avoid Them

Even experienced DIY enthusiasts can run into trouble with underdrive pulley installations. Here are the most frequent issues and ways to sidestep them:

  • Using the wrong puller — a three‑jaw puller that slips off the pulley edge can gouge the hub and ruin the new pulley. Invest in a puller that grabs the inner ring or use a damper‑specific tool.
  • Over‑greasing the crank snout — too much grease can hydrolock the pulley, preventing it from seating fully. A thin film is enough.
  • Neglecting to clean the crank snout — old corrosion or debris causes the pulley to sit crooked, which leads to belt vibration and premature bearing wear in accessories.
  • Skipping the belt alignment check — a belt that runs at an angle will squeak, shed rubber, and eventually fail. Use a straightedge or laser alignment tool for confidence.
  • Reusing old belt tensioner — a tensioner with 80,000 miles may not have enough damping capacity to keep the new belt tight. If there is any doubt, replace it.

Expected Performance Gains and Reliability Considerations

On a bone‑stock Hellcat, an underdrive pulley typically adds 10–12 horsepower at the wheels, with a modest torque improvement in the mid‑range. In vehicles with a supercharger pulley upgrade, the underdrive crank pulley can complement the setup by reducing parasitic loss without forcing the blower to spin faster. The gain is most noticeable during part‑throttle acceleration and in‑gear roll‑ons because the engine is not working as hard to spin accessories.

Reliability is maintained because the internal engine components — rods, pistons, bearing clearances — are unaffected. The only long‑term consideration is alternator output at idle. If the vehicle is used for heavy electrical draw applications (audio systems, auxiliary lights, winches), a high‑output alternator or an overdrive alternator pulley may be necessary to keep the battery charged. For standard street driving, the factory alternator handles the reduced RPM without issue.

Water pump flow is reduced proportionally, but the Hellcat’s electric cooling fans and high‑capacity radiator compensate effectively. Owners who track their cars in high‑ambient‑temperature regions should monitor coolant temps during extended sessions. If the temp rises above 230°F, consider a lower‑temperature thermostat or an auxiliary electric water pump as a complementary upgrade.

Final Thoughts and Further Reading

Installing a Hellcat Underdrive Pulley Kit is one of the few modifications that delivers a tangible power increase while preserving the engine’s original reliability profile. The installation is well within the reach of an experienced home mechanic, provided the right tools are used and torque specs are respected. The key is to move deliberately through each step, verifying alignment and belt tension before calling the job complete.

For additional technical data and community experience, consult the Hellcat Forum for real‑world install reports and troubleshooting threads. If you’re sourcing the kit, American Muscle and Modern Performance carry validated kits with clear instructions. Always cross‑reference the kit part number with your vehicle’s VIN to ensure compatibility.

For those planning a full suite of bolt‑on upgrades, pairing the underdrive pulley with a cold‑air intake and a cat‑back exhaust creates a well‑rounded power package that responds well to a custom tune. And if the goal is maximum reliability for track use, consider adding a catch can and upgrading the oil pump to the Hellcat Redeye version — but that is a topic for a separate build guide.

Take your time, keep the work area clean, and enjoy the sharper throttle response and the knowledge that every rotation of the crankshaft is now delivering more usable power to the pavement.