The Mercedes-AMG G63 is already a formidable machine, delivering a menacing presence and 577 horsepower from its handcrafted 4.0-liter V8 biturbo. For enthusiasts who demand more—more power, more acceleration, and a more visceral driving experience—the Hennessey HPE850 supercharger upgrade transforms the G-Wagon into a street-legal beast. Developed by the legendary Texas-based tuner, the HPE850 package replaces the factory twin-turbo system with a high-flow supercharger, raising output to 850 horsepower. This comprehensive guide walks through the step-by-step installation process, covering tools, preparation, mechanical procedures, tuning requirements, and what to expect after the upgrade.

Understanding the Hennessey HPE850 Supercharger System

Hennessey Performance Engineering has been at the forefront of high-performance vehicle upgrades for over three decades. The HPE850 package for the G63 AMG is a complete forced-induction system that employs a Roots-type supercharger to replace the stock twin-turbochargers. The result is immediate, linear boost delivery from idle to redline, with none of the lag typical of large turbos. The kit includes a custom intercooler, high-flow fuel injectors, upgraded intake, and a recalibrated ECU. Before diving into the installation, ensure you have received the full Hennessey HPE850 kit directly from Hennessey Performance or an authorized dealer. Verify all components are present: the supercharger unit itself, mounting bracket, belt kit, intercooler assembly, injectors, tuning module, gaskets, and hardware.

Tools, Materials, and Workspace Requirements

Installing a supercharger is a major mechanical undertaking. Prepare the following tools and supplies before beginning. A well-lit, clean workspace with adequate ventilation is essential. Lift the vehicle on a sturdy set of jack stands or a two-post lift to access the underside.

  • Complete Hennessey HPE850 supercharger kit (verify all parts)
  • Metric socket set (8mm to 22mm, with deep sockets)
  • Combination wrenches (8mm to 19mm)
  • Torque wrench (capable of 10-150 ft-lbs)
  • Ratchet extensions and universal joints
  • Screwdrivers (flathead and Phillips)
  • Pry bar set
  • Coolant catch pan (at least 5 gallons)
  • Engine oil catch pan
  • High-quality engine oil (0W-40 or as recommended by Hennessey)
  • Mercedes-approved coolant (G48 or equivalent)
  • Shop rags and brake cleaner
  • Safety goggles and mechanic gloves
  • Digital multimeter (for battery and electrical checks)
  • Torx bits (T25, T30, T45) for Mercedes fasteners
  • Pulley puller (if required for harmonic balancer)

Step 1: Pre-Installation Preparation and Safety

Disconnect the Battery

Begin by disconnecting the negative battery terminal. The G63 AMG uses a sophisticated electrical system; removing power prevents accidental airbag deployment, sensor damage, or short circuits during component removal. Use a 10mm wrench, insulate the terminal, and wait at least five minutes for any residual voltage to discharge.

Remove the Engine Cover and Air Intake

Remove the engine beauty cover by unscrewing the four Torx screws (T30). Set it aside. Next, unclip the factory air intake ducts and remove the air filter boxes on both sides. This exposes the throttle bodies and provides clearance for supercharger installation. Use a flathead screwdriver to release the hose clamps.

Drain Engine Coolant

Locate the radiator drain plug on the bottom passenger side. Place a large drain pan underneath. Open the plug and allow the coolant (around 12 quarts) to drain completely. For environmental safety, collect and dispose of coolant properly. Optionally, remove the lower radiator hose for faster draining. Also, open the coolant expansion tank cap to relieve pressure.

Drain Engine Oil

With the vehicle lifted, remove the oil drain plug on the oil pan. Drain the oil into a second catch pan. Replace the drain plug with a new crush washer and torque to 30 Nm (22 ft-lbs). Changing the oil before installation ensures fresh lubricant circulates through the new supercharger. Hennessey recommends Shell Rotella T6 5W-40 or Mobil 1 0W-40.

Step 2: Removing the Stock Twin-Turbo System

This is the most labor-intensive phase. The factory twin-turbochargers sit low in the V-bank, requiring removal of several components. Document the positions of all hoses and electrical connectors with photos.

Remove Exhaust Manifolds and Turbochargers

Unbolt the exhaust downpipes from the turbos. There are four M10 bolts per side, often rusted. Apply penetrating oil beforehand. Disconnect the oxygen sensor connectors. Remove the heat shields covering the turbos. Using a ratchet and 13mm socket, unbolt each turbo from the exhaust manifold. Carefully lift the turbos out—they are heavy (approximately 25 lbs each). Keep the coolant and oil lines capped to prevent dirt ingress. The stock intercooler and associated piping must also be removed; follow the BenzWorld G63 AMG forum for specific fastener locations.

Remove the Intake Manifold and Throttle Body

Disconnect the electrical connectors to the throttle body and remove the four bolts securing it. Remove the intake manifold by unscrewing the eight Torx bolts. The intake manifold gasket should be replaced. Clean the mating surface thoroughly with a plastic scraper and brake cleaner.

Remove the Harmonic Balancer (Crankshaft Pulley)

The Hennessey supercharger is belt-driven off the crankshaft. To install the new pulley (included in the kit), you must remove the stock harmonic balancer. Use a pulley puller to extract the balancer without damaging the crank snout. Apply a light coat of oil to the new balancer bore before pressing it back on. Torque the center bolt to 250 Nm (184 ft-lbs) plus an additional 90 degrees.

Step 3: Installing the Hennessey HPE850 Supercharger

Mount the Supercharger Bracket

Begin by attaching the custom mounting bracket to the engine block. Use the provided hardened bolts and torque them to 50 Nm (37 ft-lbs). The bracket positions the supercharger precisely in the V-bank. Apply thread locker (Loctite 243) to prevent loosening.

Install the Supercharger Unit

Lift the supercharger into place. It is lighter than the twin turbos it replaces. Use a new gasket between the supercharger and the intake manifold. Align the drive pulley with the belt path. Hand-tighten the mounting bolts, then torque to spec in a crisscross pattern: 25 Nm (18 ft-lbs) for the M8 bolts, 45 Nm (33 ft-lbs) for the M10 bolts. Verify clearance around all corners.

Connect the Intercooler System

The HPE850 kit includes an air-to-water intercooler that sits above the engine. Route the intercooler pipes from the supercharger outlet to the intercooler, and from the intercooler to the throttle body. Use the supplied silicone hoses and T-bolt clamps. Make sure all clamps are tight but not over-tightened (2 Nm for small clamps, 5 Nm for large). Fill the intercooler reservoir with distilled water mixed with a water-wetter additive (like Red Line), per Hennessey instructions.

Install Fuel Injectors

The kit includes high-flow injectors (typically 1,000 cc/min or larger). Remove the stock injectors by pulling the fuel rail. Install the new injectors with new o-rings lubricated with engine oil. Reinstall the fuel rail and torque the bolts to 12 Nm (9 ft-lbs).

Reconnect Hoses, Wires, and Belts

Reconnect all vacuum lines, coolant hoses (to the water pump and expansion tank), and electrical connectors. Install the new serpentine belt that drives the supercharger. Follow the routing diagram provided in the kit; it may include an idler pulley. Run the engine for a moment (dry, no start) to check belt alignment.

Step 4: Reassembly and Fluid Fill

Refill Engine Oil

Pour in 8.5 quarts of 0W-40 synthetic oil. Hennessey suggests using a high-zinc oil for break-in. Check the level with the dipstick after the first start.

Refill Coolant

Use Mercedes-approved coolant mixed 50/50 with distilled water. Fill the expansion tank to the “max” mark. Start the engine and let it idle; add coolant as the system burps air. Run the engine with the heater on high to circulate coolant to the heater core. Recheck level after driving.

Reattach the Engine Cover and Battery

Reinstall the engine cover and secure with Torx screws. Reconnect the negative battery terminal. Torque the terminal bolt to 10 Nm.

Step 5: ECU Recalibration and Tuning

This is the most critical non-mechanical step. The HPE850 kit includes a handheld tuner (or a mail-in ECU flash). Do not attempt to start the engine without uploading the new calibration. The stock ECU will not provide proper fuel maps or ignition timing for the supercharger, risking engine damage. Follow Hennessey’s instructions to connect the tuner to the OBD-II port. Upload the supplied calibration file. This process takes approximately 20 minutes. Verify that no error codes appear. If using a custom dyno tune, consult a reputable Mercedes tuner such as Eurocharged Performance for fine-tuning.

Step 6: Post-Installation Testing and Break-In Procedure

After installation, perform a thorough inspection before the first drive. Check all fluid levels (oil, coolant, intercooler water). Look for leaks around the supercharger, coolant lines, and fuel injectors. With the engine idling, listen for abnormal belt noise or supercharger whine—some whine is normal, but screeching indicates belt tension issues. Check that the intercooler pump is running (listen near the reservoir).

Take the G63 on a gentle 20-minute drive, varying RPM between 1,500 and 3,500. Do not floor the throttle. This allows the supercharger bearings and piston rings to seat. Return home and re-torque the supercharger mounting bolts and intercooler clamps. Recheck coolant level after the engine cools. For the first 500 miles, avoid prolonged full-throttle runs. After break-in, consider a professional dyno session to verify power output and air/fuel ratios. Hennessey claims the HPE850 delivers 850 horsepower at the crank (approximately 700-720 wheel horsepower on a dyno).

Performance Expectations and Driving Character

The transformation is dramatic. The G63 AMG accelerates from 0-60 mph in approximately 3.5 seconds (down from 4.5 seconds stock). The quarter-mile time drops to under 12 seconds at 120 mph. But peak numbers don’t tell the whole story. The supercharger provides instant thrust from idle—there is no turbo lag. The engine sound changes from a distant turbo whistle to a deep, aggressive supercharger whine that rises with RPM. Throttle response becomes razor-sharp. However, the additional weight of the supercharger (about 30 lbs) slightly affects front-end weight balance, though the G63’s heavy frame masks it. Fuel economy will drop to around 10-12 mpg combined; that’s the price of 850 horsepower.

Be aware that the increased torque (over 700 lb-ft) can stress the Mercedes nine-speed automatic transmission. Hennessey includes a transmission tune in some kits; if not, consider an aftermarket transmission control unit (TCU) tune. Also, the stock brakes may feel inadequate; upgrading to a big brake kit is recommended for track use or aggressive driving.

Common Challenges and Troubleshooting

  • Belt slip: If you hear chirping from the supercharger under load, tighten the belt tensioner or replace with a wider belt if the kit allows.
  • Overheating: Ensure the intercooler system is completely bled of air. Missing coolant flow will cause intake air temps to skyrocket.
  • Check engine light: Often due to a small vacuum leak. Smoke test the intake system.
  • Hard starting: The ECU may need a full flash if the battery was disconnected during tuning. Re-upload the calibration.
  • Boost leaks: Loose intercooler clamps are common. Retighten after heat cycling.

Maintenance Considerations

The HPE850 supercharger requires slightly more attention than the stock setup. Change engine oil every 5,000 miles with a high-quality synthetic. Inspect the supercharger drive belt at every oil change—replace every 30,000 miles. The supercharger itself is sealed and lubricated for life, but Hennessey recommends an inspection at 60,000 miles. Keep the intercooler reservoir filled with distilled water and water-wetter; flush annually to prevent corrosion. For complete peace of mind, register your kit with Hennessey to validate warranty and get technical support.

Conclusion

Installing the Hennessey HPE850 supercharger on the G63 AMG is a challenging but rewarding project. It transforms an already iconic SUV into a genuine supercar contender. By following this step-by-step guide—from draining fluids and removing the twin turbos to mounting the supercharger, refilling fluids, and performing the necessary ECU recalibration—you can achieve a professional-grade installation. The end result is explosive power, instant torque, and a driving experience that few vehicles can match. Whether you undertake the install yourself or work with a certified shop, the HPE850 package delivers on Hennessey’s reputation for pushing boundaries. Prepare for a new level of adrenaline behind the wheel of your G63.