The Plymouth Road Runner, introduced in 1968, remains one of the most beloved muscle cars ever built. Its no-frills approach—a big engine in a relatively light body—made it an instant hit on the street and strip. Decades later, owners still enjoy modding these cars to extract more power, improve handling, or simply modernize the driving experience. But performance modifications vary widely in cost, complexity, and actual horsepower gains. This guide breaks down realistic price ranges, installation challenges, and performance expectations for the most common Plymouth Road Runner mods, from a $200 cold air kit all the way up to a $3,500 supercharger system.

Understanding Your Performance Goals

Before spending money on any specific component, define your objectives. Are you building a weekend cruiser that turns heads, a daily driver with a little extra punch, or a quarter-mile monster that needs to hit low 12s? Your goals determine which modifications make sense and where your budget should go. For example, a cold air intake and a tuner might satisfy a casual enthusiast, while a camshaft or supercharger is necessary for serious power. Always match the mod to the intended use, and factor in supporting upgrades such as fuel system, cooling, and drivetrain reinforcement.

Cold Air Intake Kits: $200–$500

A cold air intake (CAI) is often the first modification owners try. It replaces the restrictive factory airbox with a free-flowing filter and smooth tubing, drawing cooler air from outside the engine bay. For the Plymouth Road Runner, this can add 10–15 horsepower on carbureted engines and slightly more on fuel-injected conversions. The installation is straightforward, requiring basic hand tools and about an hour.

Cost Factors

  • Brand and materials: Polished aluminum or carbon fiber tubes cost more than powder-coated steel.
  • Filter type: Dry synthetic filters are cheaper; oiled cotton gauze (like K&N) offers better airflow but requires periodic cleaning.
  • Complete kit vs. custom fabrication: Some Road Runners need a universal kit and custom mounts, which can increase cost.
  • K&N – industry standard, excellent filter media
  • Spectre – budget-friendly, good value
  • Airaid – includes a heat shield for true cold air

For best results, pair the CAI with a free-flowing air cleaner assembly that fits the Road Runner's original four-barrel carburetor. Expect a noticeable improvement in throttle response and a deeper induction sound. However, cold air intakes alone will not dramatically transform performance—they are a solid foundation for future upgrades. Installation is easy, and you can find guidance on popular Mopar forums.

Exhaust Systems: $500–$1,500

Exhaust upgrades are a classic way to unlock horsepower while giving the Road Runner that signature V8 rumble. A properly designed system reduces backpressure and lets the engine breathe out as freely as it breathes in. Gains typically range from 10–25 horsepower depending on the existing setup.

Types of Exhaust Systems

  • Cat-back systems: Replace muffler and tailpipes; easiest install, mild gains.
  • Axle-back systems: Only change mufflers and tips; cheapest but least effective.
  • Header-back systems: Full replacement from headers to tailpipe; best performance but requires headers.

Key Components and Costs

  • Headers: $300–$800 – Long-tube headers maximize top-end power; shorty headers easier to install.
  • Mufflers: $50–$200 each – Choices include chambered (Flowmaster), straight-through (Borla), or turbo-style.
  • Mandrel-bent tubing: $200–$500 – Smooth bends maintain exhaust velocity.

Many Road Runner owners opt for a 2.5-inch or 3-inch mandrel-bent system with high-flow mufflers. The result: an aggressive tone that announces the car's presence without being unbearable on long drives. Professional installation is recommended unless you have welding experience, as exhaust routing must clear the torsion bars and rear axle. Check out Mopar's official parts catalog for OEM-style upgrades.

Performance Tuners: $300–$800

For Road Runners that have swapped to electronic fuel injection (EFI), a handheld tuner is a powerful tool. It allows you to adjust air/fuel ratio, ignition timing, rev limiters, and transmission shift points. Gains can be 20–40 horsepower on naturally aspirated engines and more on forced induction setups.

Considerations for Tuners

  • Engine compatibility: Most tuners work with Holley EFI, FiTech, or factory ECU conversions.
  • Ease of use: DiabloSport and Superchips offer pre-loaded “canned” tunes you install in minutes.
  • Custom tuning: $150–$500 extra if you want a dyno-tuned file from a pro.

For carbureted Road Runners, a tuner is less relevant, but you can achieve similar results with a distributor recurve kit ($50) and adjustable vacuum advance. Performance tuning is about optimization, not just adding fuel – a skilled tuner can unlock hidden power while keeping the engine safe. Consider DiabloSport's official site for specific Mopar support.

Camshaft Upgrades: $800–$2,000

A performance camshaft is a major step up in commitment and cost. It changes valve timing and lift to improve volumetric efficiency across the rpm range. A well-chosen cam can add 30–60 horsepower, but it also alters idle quality and low-rpm manners. This mod is best for owners who are building a dedicated performance engine, not a daily driver.

Camshaft Selection Criteria

  • Duration and lift: A mild street cam (210–220 duration) works with stock converter; more aggressive cams require a higher stall torque converter and possibly upgraded valve springs.
  • Engine displacement: Big-block 440 cams differ from small-block 383/400 cams.
  • Intended rpm range: 2,500–6,500 for street/strip; lower for towing, higher for all-out race.

Professional installation is highly recommended because you must degree the cam, replace lifters, and often upgrade springs, retainers, and timing chain. Many shops charge $600–$1,200 in labor on top of parts. Brands like Comp Cams, Lunati, and Crower offer extensive dyno data for big-block Mopars. Always check piston-to-valve clearance – a mistake here can destroy the engine. For technical details, explore Comp Cams' Mopar section.

Supercharger Systems: $2,500–$3,500 (and up)

For the ultimate power boost, a supercharger forces air into the engine, effectively increasing displacement. A centrifugal supercharger (like Procharger) can add 100–150 horsepower on a stock 440, while a roots-type blower (like Weiand) provides instant torque off idle. However, the base price often doesn't include necessary supporting upgrades.

Types of Superchargers

  • Roots-style: Sits on top of the intake, iconic look, great for low-end torque. Cost: $2,800–$3,500 for complete kit.
  • Centrifugal: Belt-driven compressor similar to a turbocharger; highest peak power. Cost: $2,500–$3,200 usually includes intercooler.
  • Twin-screw: Efficient, compact, but expensive ($3,500+).

Hidden Costs

  • Fuel system upgrade: $500–$1,000 – high-pressure pump, larger lines, bigger injectors or carburetor.
  • Ignition system: $200–$400 – MSD box, better coil, colder plugs.
  • Engine internals: Forged pistons, stronger rods – $1,500–$3,000 if the engine needs to handle 600+ hp reliably.

A supercharger is not a bolt-on and drive situation – tuning is critical, and the Road Runner's stock drivetrain (transmission, rear end) may need reinforcement. Many builders start with a built 440 or 400 block. Expect installation labor of $1,000–$2,000 at a competent shop. Despite the cost, the reward is a car that can dominate modern traffic and run low 11s at the strip. Procharger's site offers kit details for classic Mopars.

Other Essential Modifications to Consider

Upgraded Suspension and Brakes

More power demands better control. A basic suspension upgrade (sway bars, firm shocks, poly bushings) costs $400–$800. Disc brake conversions run $600–$1,200 but are essential for safe stopping. Never outrun your brakes – many Road Runners still have original drum brakes that are inadequate for modern highway speeds.

Transmission and Drivetrain

A high-stall torque converter ($200–$400) helps with cammed engines. For extreme horsepower, consider a manual transmission swap (e.g., Tremec TKO) or a built 727 automatic. Driveshaft loops and upgraded axles ($300–$600) improve safety.

Budgeting and Installation Tips

Start with a realistic total budget that includes labor and unforeseen costs. A $3,500 supercharger easily becomes an $8,000 project once you add needed upgrades. Many owners follow the “pay as you build” approach: buy one part, install it, drive and enjoy, then save for the next. Document everything – your Road Runner's value increases with a well-documented build.

Do not cut corners on installation – a poorly installed cam or supercharger can destroy an engine. Join local Mopar clubs or online communities like Moparts for advice and shop recommendations. Quality parts from reputable brands may cost more upfront but provide reliability and resale value.

Many modifications (especially headers, cat deletes, and superchargers) may be illegal on public roads in states that enforce emissions standards. Check local laws before ordering parts. For street-driven cars, retain catalytic converters and ensure the tune keeps emissions within legal limits. California and New York have strict smog rules – plan accordingly.

Conclusion

Performance modifications for the Plymouth Road Runner range from simple, affordable upgrades to complex, expensive systems. A cold air intake and exhaust system are ideal first steps for the budget-conscious owner, while a camshaft or supercharger delivers serious power for those committed to building a show-stopping machine. The key is to research thoroughly, set a budget, and choose parts that complement each other. With careful planning, your Road Runner can achieve the performance you've always dreamed of without breaking the bank – or the engine.