The 6.4L HEMI: The Modern Muscle Heart

Swapping a 6.4L HEMI into a classic B-body like a 1970 Plymouth GTX or 1969 Dodge Coronet R/T is one of the most rewarding upgrades a Mopar enthusiast can undertake. The 6.4L engine, factory rated at 485 horsepower and 475 lb-ft of torque, provides a massive jump over even the best factory big-blocks of the 1960s, while offering modern reliability, fuel injection, and overdrive transmissions. This guide covers the real costs, the mechanical hurdles, and the final payoff of making this swap your own.

Costs and Budget Planning

Engine and Transmission Options

The single largest cost is the engine itself. A new crate 6.4L HEMI from Mopar Performance directly retails for around $8,000. You can find used engines from wrecked 2011–2023 Dodge Challengers and Chargers for $4,000 to $7,000. When buying used, confirm that the engine is complete with intake, throttle body, injectors, and alternator. Missing accessories add hundreds to the final bill.

For the transmission, the eight-speed 8HP70 (Chrysler part number 68238652AA) is a natural match. It can handle up to 700 lb-ft of torque and comes with gearing that keeps the 6.4L in its sweet spot. A new 8HP70 with torque converter costs about $3,000. A stand-alone controller from Holley or US Shift adds another $500–$800. If you prefer a manual, the Tremec TR-6060 or T56 Magnum are common choices. A T56 Magnum kit from Rockland Standard Gear runs $3,500 including bellhousing and clutch.

Supporting Parts and Labor

Engine mounts designed specifically for B-bodies cost $200–$400 from sources like Schumacher Creative Services. Headers are a must because the factory manifolds from a Challenger will not fit in a B-body engine bay; custom headers from Bouchillon Performance or TTI Exhaust cost $800–$1,200. The cooling system demands a custom aluminum radiator with electric fans, adding $300–$600. The fuel system must support 58 psi constant pressure—a Walbro 255 lph in-tank pump and regulator kit runs $200–$400.

If you hire a professional shop to perform the entire swap, labor charges range from $2,500 to $5,000 depending on the shop's experience and your car's condition. DIY labor saves that cost but requires the right tools and a month of weekends.

Technical Challenges and Solutions

Engine Bay Fitment

B-body engine bays were designed for cast-iron wedge motors like the 383 and 440. The 6.4L HEMI is physically shorter but wider because of its deep-skirt block and variable valve timing actuators. To clear the steering linkage, you need specific motor mount plates that position the engine low and to the passenger side. Many builders use aftermarket oil pans, like those from Santos Industries or Mast Motorsports, to gain steering rack clearance. The passenger-side header frequently contacts the inner fender, which can be clearanced with a hammer or by denting the fender. Some builders install a mini-starter to free up exhaust space.

Electrical and ECU Integration

The original B-body cars used a simple 12-volt system with a ballast resistor and points or electronic ignition. The 6.4L HEMI requires a PCM that communicates with the transmission controller, throttle pedal, and anti-theft system. If you retain the factory Mopar PCM, you must send it to a tuner like HPTuners to delete the immobilizer and calibrate for your vehicle weight and tire size. A simpler path is to install an aftermarket stand-alone engine management system like the Holley Terminator X Max. This system manages both the engine and the 8HP70 transmission, supports self-learning fuel tuning, and works with common DBW pedals. Wiring the Terminator X into a classic Mopar chassis takes a solid day of work but avoids the black box of factory electronics.

Cooling, Fuel, and Exhaust

Modern HEMIs operate at 200°F and require a robust cooling system. The original radiator in a B-body is too small and often clogged after 50 years. Install a three-row aluminum core radiator made for B-bodies with dual 12-inch electric fans wired to a 180°F thermostatic relay. For fuel, the 6.4L uses a return-style system with a high-pressure pump. The simplest retrofit is an in-tank pump module from a 1990s Dodge truck that fits inside the B-body's fuel tank after minor modification. Add a filter and a fuel pressure regulator set to 58 psi. The exhaust system should be at least 2.5-inch pipe with an X-pipe and low-restriction mufflers. Stainless steel systems are worth the extra cost for longevity.

Steering and Suspension Upgrades

With the extra power, the steering and suspension need attention. The original manual steering box will feel vague and heavy with sticky tires. Upgrade to a Firm Feel or Borgeson power steering box with a 12:1 ratio. If you have a manual steering gearbox, convert to power using a factory pump turned at the correct speed. The 6.4L can work with the stock torsion bars, but upgrading to 1.12-inch bars from Hotchkis or Firm Feel reduces body roll. For the rear, add a set of CalTracs traction bars or factory horsepower leaf spring packs to control wheel hop. Disc brakes on all four corners are strongly recommended; use a 1990s B-body spindle with modern calipers or complete aftermarket kits from Wilwood or Baer.

Benefits Beyond Horsepower

The 6.4L HEMI swap delivers more than just straight-line speed. The modern fuel injection means the car starts instantly hot or cold without pumping the gas. Driveability on the street improves dramatically: no cold stumbles, no flooding, and no vapor lock. The 8-speed automatic or the overdrive manual allows highway cruising at 1,800 rpm, reducing engine noise and improving comfort over long drives. Gas mileage can exceed 20 mpg on the highway, a massive improvement over the original big-block's 10–12 mpg.

The weight savings are also noticeable. The fully dressed 6.4L weighs around 580 pounds, compared to a 440's 640 pounds. That 60-pound reduction off the front end helps turn-in response and braking. The iron block 6.4L is still heavy, but the aluminum heads and intake offset the weight forward of the strut towers. Combined with modern brake upgrades like Wilwood discs, the car stops as well as it accelerates.

Resale value can also increase. A properly documented swap with a professionally tuned ECU and matching transmission can add $5,000–$10,000 to the car's value compared to a stock or carbureted version. Enthusiasts actively seek out HEMI-swapped B-bodies for their blend of classic looks and daily-driver reliability.

Project Preparation and Timeline

Parts Checklist

Before you start, verify you have all of the following: engine, transmission with bellhousing, flex plate or flywheel, torque converter or clutch kit, motor mount plates, headers, mid-pipes, oxygen sensors and bungs, wiring harness and ECU, coolant temp and oil pressure senders (if gauges are aftermarket), air intake system, PCM with tune or Holley Terminator X, electric fans and radiator, fuel pump, filter and lines, throttle pedal (DBW), cruise control cable (optional), and a complete list of hardware including bolts, nuts, and washers.

Workshop Requirements

You need a floor jack, four tall jack stands, standard and metric socket sets, a torque wrench, a transmission jack or strong assistant, a welder (for exhaust or motor mount modifications), and a scan tool or laptop to program the ECU. A lift is not required but saves your back. Budget for a full alignment after the swap, especially if you lowered the engine or changed ride height.

Aftermarket Support and Community Resources

One advantage of the Gen III HEMI platform is immense aftermarket support. Companies like Holley offer complete injection and timing control solutions. Bouchillon Performance produces B-body-specific headers and exhaust systems. For community advice, For B Bodies Only has a dedicated HEMI swap section with hundreds of build threads. Another resource is Moparts, where early adopters of the 6.4L swap share part numbers and troubleshooting tips. Mopar's official Performance Catalog lists every crate engine option and warranty information.

Final Thoughts

Installing a 6.4L HEMI in a B-body is a major undertaking, but the result is a car that drives like a modern sports sedan while looking like a 1960s muscle classic. The costs are comparable to a professional big-block rebuild but deliver more power, better gas mileage, and superior reliability. Careful planning, the right parts, and patience are essential. Whether you choose to do the work yourself or hire a shop, a 6.4L HEMI swap transforms the B-body experience. Start with a solid, rust-free car, buy your engine and transmission as a matched set, and lean on the forums for guidance. With the right approach, you will end up with a vehicle that out-performs any production car from the Mopar golden era.