The Mercedes-AMG GT R is already a track-focused monster, packing a hand-built 4.0-liter twin-turbo V8 that produces 577 horsepower and 516 lb-ft of torque. But for enthusiasts who crave even more aggression, a Hennessey Performance upgrade transforms this beast into a hypercar challenger. Hennessey, a Texas-based tuner with decades of experience, offers a range of upgrade packages that unlock significant horsepower and torque gains while maintaining daily drivability. This guide provides a comprehensive, step-by-step installation process for a typical Hennessey HPE700 upgrade, along with a detailed cost breakdown and everything you need to know before you commit to the project.

Understanding the Hennessey Performance Upgrade

The Hennessey Performance upgrade for the AMG GT R is not a single part but a holistic package designed to increase airflow, reduce restrictions, and recalibrate the engine’s electronic brain. Unlike generic piggyback tunes, Hennessey’s engineering team develops specific software and hardware calibrations for each vehicle, often incorporating proprietary components. The most popular package for the GT R is the HPE700, which pushes output to an estimated 700 horsepower and 650 lb-ft of torque at the crank. Higher-tier packages like the HPE800 and HPE1000 exist for those who want to chase extreme numbers, but the HPE700 is widely considered the sweet spot for street and occasional track use.

The core components of a typical Hennessey HPE700 upgrade include:

  1. High-Flow Exhaust System: A cat-back or full exhaust system with larger diameter piping and low-restriction mufflers or resonators. Hennessey often uses 304 stainless steel with mandrel bends and optional carbon-fiber tips.
  2. Upgraded Intercoolers: Larger, more efficient air-to-water intercoolers that reduce intake air temperatures, which is critical for consistent performance when pushing higher boost levels.
  3. Engine Tuning (ECU Remap): A proprietary calibration that adjusts boost pressure, fuel mapping, ignition timing, and transmission shift points. Hennessey provides a handheld tuning device for reflashing the ECU.
  4. High-Flow Air Intakes: Cotton or synthetic filter elements with larger intake tubes that replace the restrictive stock airbox.
  5. Optional Upgrades: Many owners also opt for upgraded downpipes, a heat exchanger, and a blow-off valve kit, though these are often sold separately.

Hennessey backs their packages with a limited warranty on parts and labor, which is a significant reassurance considering the investment. Always visit the official Hennessey Performance product page for the most current package details and compatibility with your specific model year.

Tools and Materials Needed

Before you begin, amass the following tools and materials. Having everything within arm’s length prevents frustration and costly mistakes. The specific sizes given are common for the AMG GT R; confirm with your kit’s instructions.

Essential Hand Tools

  • Complete metric socket set (8mm – 22mm, including ½” and ⅜” drive ratchets)
  • Torque wrench (½” drive, range 20–150 ft-lb)
  • Combination wrenches (metric, especially 10mm, 13mm, 15mm, 17mm)
  • Flathead and Phillips screwdrivers (medium and long reach)
  • Trim removal tools (plastic pry bars for clips and panels)
  • Allen key set (hex bits for intercooler mounts)

Specialty Items

  • Low-profile floor jack (minimum 3-ton capacity)
  • Jack stands (pair, 6-ton capacity recommended)
  • Wheel chocks
  • Exhaust hanger removal pliers or pick set
  • Penetrating oil (e.g., WD-40 Specialist or PB Blaster)
  • Shop rags and gloves
  • Safety glasses and hearing protection

Supplies from the Hennessey Kit

  • Hennessey HPE700 upgrade kit (exhaust, intercoolers, intake, tuning device)
  • Installation manual (check that it matches your vehicle’s VIN)
  • Any specialty brackets or gaskets included

Optional Diagnostic Equipment

  • OBD-II scanner (to clear codes after installation)
  • Multimeter (for checking electrical connections on intercooler pump)

If you are not performing the installation yourself, a professional shop will have these tools. However, owning a torque wrench and socket set is valuable for future maintenance.

Step-by-Step Installation Process

This process assumes you have basic mechanical experience. If you are unfamiliar with removing exhaust systems or handling intercooler coolant, consider hiring a certified technician. Hennessey recommends professional installation for their packages, but many DIY owners complete the job in a long weekend.

Step 1: Prepare Your Workspace and Vehicle

Park the AMG GT R on a flat, level concrete surface. Allow the engine to cool completely – at least two hours – to avoid burns from hot exhaust components or coolant. Gather all tools and organize them on a clean workbench. Disconnect the battery negative terminal to prevent accidental electrical shorts or unintended activation of the cooling fans. Place wheel chocks behind the rear wheels. Lift the front of the vehicle using the front center jack point (consult your owner’s manual) and secure it on jack stands. For exhaust removal, you may need all four wheels off the ground; use a four-post lift if available. Otherwise, lift the rear as well and support with stands.

Step 2: Remove the Stock Exhaust System

Begin from the rear. Remove the rear bumper cover or lower valance to access the exhaust tips and hangers. This step varies by year; many GT R models require removing the diffuser first. Use a trim tool to pop the plastic clips. With the valance out of the way, locate the exhaust hangers – usually three to four rubber isolators. Spray penetrating oil on the bolts holding the exhaust flanges (at the downpipes and mid-pipe). Unbolt the factory exhaust from the downpipes using a 13mm socket. Slide the exhaust off the hangers. The stock system is heavy (approximately 40–50 lbs), so have a helper or a transmission jack ready. Carefully lower the exhaust and remove it from under the car. Store it safely; some owners sell their stock exhaust to recoup costs.

Step 3: Install the Hennessey Exhaust System

Test-fit the new Hennessey exhaust before applying any sealant. The system typically comes in three sections: axle-back, mid-pipe, and connecting pipes. Match each section to the corresponding brackets. Apply a thin layer of anti-seize compound on the flange bolts to prevent galling. Lift the mid-pipe into position and loosely hand-tighten the bolts to the downpipes. Then attach the rear sections. Tighten all clamps and bolts evenly, working from front to back. Use a torque wrench to tighten the downpipe nuts to the factory specification (typically 30–35 ft-lb). Double-check that the exhaust tips are centered in the bumper openings. Reinstall the rear valance and bumper cover, ensuring all clips snap securely.

Step 4: Upgrade the Intercoolers

This step is more involved because the AMG GT R uses a charge air cooler (CAC) system integrated into the engine intake manifold, plus auxiliary low-temperature radiators in the front bumper. The Hennessey upgrade typically replaces the two side-mounted intercoolers with larger units. Start by removing the front bumper cover (refer to a factory service manual). Drain the coolant from the intercooler circuit – collect it in a clean pan for reuse if still fresh. Disconnect the electrical connectors for the intercooler pumps and fans. Unbolt the stock intercoolers from their mounts (usually 10mm bolts). Carefully lift them out. Install the new Hennessey intercoolers, which often have thicker cores and larger end tanks. Reverse the removal steps, ensuring all hose clamps are tight. Refill the coolant system with the correct Mercedes-specified G40 coolant mixed 50/50 with distilled water. Bleed the system by running the engine with the coolant cap off and monitoring the flow.

Step 5: Install the High-Flow Intake System

Remove the factory airbox and intake tubes. The stock airbox is restrictive; Hennessey’s intake replaces it with a larger filter housing and smoother intake tubes. Disconnect the mass airflow (MAF) sensor and set it aside. Unbolt the airbox from its mounts. Install the new intake by attaching the filter to the velocity stack and routing the intake tube to the turbo inlets. Secure all clamps. Reinstall the MAF sensor in the new housing. Ensure the intake is not rubbing against any hot engine components; use heat shielding if provided.

Step 6: Engine Tuning – The Critical Step

With all hardware installed, it’s time to upload the calibration. Hennessey provides a handheld programming device (usually an HP Tuners MPVI2 or similar). Connect the device to the OBD-II port under the dashboard. Turn the ignition to ON (engine off). Follow the device prompts to read the stock ECU file. This may take 10–20 minutes. Once the stock file is saved, the device will prompt you to upload the Hennessey calibration. Accept the terms and confirm. The flashing process takes about 15–30 minutes. Do not interrupt power during the flash. After completion, the device will confirm success. Disconnect it and start the engine. Allow it to idle and reach operating temperature. Check for any warning lights. Using an OBD-II scanner, clear any temporary codes that may have been stored. Take the car for a gentle drive, varying RPMs to let the ECU learn the new parameters. After about 50 miles, perform a few full-throttle pulls on a safe road to confirm performance.

Step 7: Final Checks and Documentation

Let the engine cool and recheck all fluid levels (coolant, oil, intercooler coolant). Inspect for exhaust leaks at every joint. Retorque the exhaust bolts after the first heat cycle. Check that all trim pieces are flush. Keep the handheld tuner stored in the vehicle in case future maintenance requires a reflash. Hennessey recommends logging the first three data logs and sending them to their support team for verification.

Cost Breakdown

The investment for a Hennessey upgrade on an AMG GT R goes beyond the kit itself. Use this realistic table to budget:

ItemEstimated Cost (USD)
Hennessey HPE700 Upgrade Kit$10,000 – $12,500
Optional: Upgraded Downpipes$1,200 – $1,800
Optional: Heat Exchanger Kit$800 – $1,200
Professional Installation Labor (12–16 hours @ $125–$175/hr)$1,500 – $2,800
Coolant and Other Consumables$100 – $250
Tools (if not owned)$200 – $500
Dyno Tuning (if not included)$500 – $800
Total Estimated Range$14,300 – $19,850

Note that prices fluctuate based on current Hennessey pricing, dealer discounts, and regional labor rates. The kit itself includes the tuning device, so no additional dyno time is required for the base calibration, but post-install verification is recommended. The Car and Driver review of the AMG GT R provides a baseline performance context – a stock car runs 0–60 mph in about 3.5 seconds. With the Hennessey HPE700, owners report 0–60 in the low 3.0‑second range and ¼‑mile times under 11 seconds at over 130 mph.

Performance Gains and Dyno Results

Based on independent dyno tests and owner reports, the HPE700 upgrade delivers the following average increases at the wheels (compared to stock):

  • Peak Horsepower: +120–140 whp (from ~510 whp stock to ~640–660 whp)
  • Peak Torque: +150–170 lb-ft (from ~500 lb-ft wheel torque to ~670–700 lb-ft)
  • Boost Pressure: Increased from about 15 psi to 19–20 psi (with upgraded intercoolers)
  • Intake Air Temperatures: Reduced by 20–30°F under sustained load

It’s crucial to use 93 octane premium fuel (or equivalent) at all times. Hennessey also offers an E85 conversion for additional power, but that requires fuel system upgrades and is not covered under the standard warranty.

Warranty and Reliability Considerations

Hennessey provides a limited warranty on their parts and labor – typically 12 months/12,000 miles for the HPE700. However, the factory Mercedes-Benz warranty on the powertrain will be voided for any components directly affected by the modification (engine, transmission, cooling, and exhaust). If your GT R is still under factory warranty, you may want to wait until it expires or purchase a used vehicle. Hennessey’s warranty is transferable to a second owner for a fee, which adds resale value.

Reliability is generally excellent if the installation is performed correctly and maintenance is elevated. Oil changes should be done every 5,000 miles with full synthetic (0W-40 or 5W-50 as specified by Hennessey). Spark plugs should be inspected at 30,000 miles and replaced if necessary. The upgraded intercoolers require periodic cleaning of the radiator fins. Avoid prolonged idling or track sessions without proper cool-down laps to protect the turbos.

Alternatives to the DIY Installation

If you lack the time, tools, or confidence, several shops specialize in Hennessey installations. Hennessey’s own facility in Sealy, Texas performs turnkey installations, including shipping and logistics. Many high-end European performance shops (such as RENNtech, Weistec, or VF Engineering) also offer similar tuning packages. The advantage of professional installation is the included warranty on labor and the ability to handle unforeseen complications like seized bolts or wiring issues. However, expect to pay a premium.

Maintenance After the Upgrade

Post-installation, commit to a stricter maintenance schedule. Monitor oil levels more frequently, as increased boost can push oil past the turbo seals if the crankcase ventilation isn’t optimal. Check intercooler coolant level weekly. Clean or replace the high-flow air filter every 12,000 miles or annually. Use a quality fuel system cleaner every 5,000 miles to keep injectors clean. If you track the car, consider upgrading the transmission cooler and adding a differential cooler – Hennessey offers a track package for serious owners.

Also, invest in a quality OBD-II data logging tool to monitor air/fuel ratios, boost pressure, and knock counts. Hennessey provides baseline logs, but proactive monitoring can catch issues before they become expensive.

Conclusion

Installing a Hennessey Performance upgrade on your Mercedes-AMG GT R is one of the most effective ways to unlock its true potential. The HPE700 kit transforms an already brilliant supercar into a fire-breathing street monster, capable of embarrassing exotics costing five times as much. The installation, while involved, is achievable for a competent DIY enthusiast with the right tools and patience. For those who prefer a turnkey solution, professional installation is readily available. With a total investment ranging from roughly $14,000 to $20,000, the cost is substantial but justified by the engineering quality and the resulting driving experience. If you respect the car’s limits and adhere to the recommended maintenance, your upgraded GT R will deliver years of adrenaline-filled motoring. Check the Mercedes-Benz official AMG GT R page to understand the baseline before you commit – and then go unleash the beast.