Understanding the Mopar 340 Engine

The Mopar 340 cubic inch V8 debuted in 1968 as a high-performance small-block offering from Chrysler. It was designed to fill the gap between the 318 and the legendary 440 big-block. With a bore of 4.04 inches and a stroke of 3.31 inches, the 340 featured a forged steel crank, heavy-duty rods, and a high-lift camshaft in its earliest versions. Factory horsepower ratings ranged from 275 to 340, depending on compression and induction. The engine block’s thick cylinder walls, large journal design, and robust oiling system make it an excellent candidate for both street and strip upgrades.

Understanding the 340’s architecture is essential before selecting parts. The stock heads feature 2.02-inch intake and 1.60-inch exhaust valves, which flow well for their era but leave room for improvement. The factory intake manifold and carburetor were adequate for daily driving but choked the engine above 5,000 RPM. The exhaust manifolds are notoriously restrictive. These limitations are exactly why a targeted upgrade path can deliver a significant horsepower increase—from 275 to a reliable 340 horsepower—without requiring a full rebuild.

Key Upgrades for Increasing Horsepower

1. Upgrading the Intake Manifold

The intake manifold is the gateway for air entering the engine. The stock cast iron or early four-barrel aluminum intake on the 340 was designed for low-end torque and fuel economy. Swapping to a high-performance aluminum manifold can immediately improve airflow and power. Dual-plane manifolds like the Edelbrock Performer RPM or the Weiand Action+ provide excellent throttle response from idle to 6,500 RPM, making them ideal for a street-driven 340. Single-plane intakes such as the Edelbrock Victor series or Holley Strip Dominator are better suited for high-RPM racing applications but may sacrifice low-end drivability.

What to Look for in an Intake Manifold

  • Plenum design – Dual-plane for street, single-plane for strip.
  • Material – Aluminum for weight savings and heat dissipation.
  • Port matching – Ensure intake ports align with cylinder head ports to avoid turbulence.
  • Carburetor pad height – Taller manifolds require hood clearance but often produce more top-end power.

An aftermarket intake typically adds 10–20 horsepower on a mostly stock 340. When paired with a larger carburetor and headers, the gains multiply. A good reference for intake selection is the Edelbrock Performer RPM 7176, designed specifically for the 340/360 LA small-block.

2. Installing a Performance Carburetor

The factory Carter AFB or AVS carburetor on the 340 flowed around 600 cfm. While reliable, it restricts the engine’s potential at higher RPM. Upgrading to a 750 cfm Holley or a similar aftermarket carburetor improves fuel atomization and allows the engine to breathe deeper. A 750 cfm carburetor is a perfect match for a mild 340 with a cam upgrade and exhaust work. For engines with more aggressive cams and heads, a 780–850 cfm unit may be necessary.

Carburetor Considerations

  • Vacuum secondary vs. mechanical secondary – Vacuum secondaries (e.g., Holley 3310) are street-friendly; mechanical (e.g., Holley 4150) deliver instant fuel but can be tricky to tune.
  • Electric choke – Retain drivability in cold weather.
  • Float adjustment – Critical for consistent fuel delivery under acceleration.
  • Fuel pressure – Most performance carburetors need 5.5–7 psi; upgrade to a high-volume mechanical pump or an electric pump if necessary.

A properly tuned 750 cfm carburetor on a 340 with headers and a dual-plane intake can add 15–25 horsepower. Spend time tuning the idle mixture and accelerator pump circuits to dial in throttle response. For more information, consult the Holley 4776 750 cfm Street Avenger.

3. Upgrading the Exhaust System

Exhaust flow is the engine’s release valve. Restrictive factory exhaust manifolds and single-exit exhaust systems rob the 340 of significant power. Installing a set of long-tube headers designed for the A-body, B-body, or E-body platform (depending on your car) can reduce back pressure and improve scavenging. Combine headers with a free-flowing exhaust system—2.5-inch or 3-inch mandrel-bent pipes with an X-pipe or H-pipe crossover—to unlock the engine’s full potential.

Header Selection Tips

  • Tube diameter – 1 5/8″ primary tubes are ideal for 300–400 hp; 1 3/4″ for 400+ hp.
  • Collector size – 3″ collectors with 3″ to 3.5″ exhaust are standard.
  • Coating – Ceramic thermal coating reduces under-hood temperatures and extends header life.
  • Fitment – Check for steering shaft clearance on early A-bodies and power steering models.

Headers combined with a high-flow catalytic converter (if required) or a straight-through muffler like a Borla or MagnaFlow can yield up to 30 horsepower over stock manifolds. Don’t forget to recurve the distributor after exhaust changes, as reduced back pressure alters the air-fuel mixture and timing requirements.

4. Enhancing the Camshaft

The camshaft is the brain of the engine. The stock 340 cam was a mild grind with about 268–274 degrees of advertised duration and .430–.450 inches of lift. A performance camshaft with higher lift and longer duration can dramatically increase horsepower, particularly above 3,000 RPM.

Choosing the Right Cam

  • Duration at 0.050″ – A 224°–236° intake duration is ideal for a street performance 340 with stall converter and 3.55–3.91 gears.
  • Lift – .470″–.520″ lift works well with stock valve springs (upgrade springs recommended).
  • Lobe separation angle – 110°–112° provides a good balance of power and idle quality.
  • Hydraulic roller vs. flat tappet – Hydraulic roller cams reduce friction and allow more aggressive profiles; they require retrofitting on early 340 blocks.

Popular camshaft choices for the Mopar 340 include the Comp Cams Xtreme Energy 268H for a noticeable lope and strong mid-range. Expect a 20–30 horsepower gain from a cam swap alone, especially when paired with the other upgrades listed. Always degree the cam during installation to confirm valve timing.

5. Improving Ignition Timing

An optimized ignition system burns the air-fuel mixture more completely, extracting every ounce of power from your upgrades. Start by checking your distributor’s advance curve. The stock Mopar distributor often has too much initial timing and a slow mechanical advance, leading to sluggish performance.

Ignition Upgrades

  • Performance distributor – MSD Pro-Billet or FBO Ignitions drop-in systems feature adjustable mechanical and vacuum advance.
  • Ignition coil – A high-output coil like an MSD Blaster 2 or 3 increases spark energy.
  • Spark plug wires – Low-resistance spiral core wires (e.g., Taylor 8mm) reduce radio interference and deliver consistent spark.
  • Timing adjustment – Set initial timing to 12°–16° BTDC, with total mechanical advance at 32°–36° by 3,000 RPM. Dial back vacuum advance for highway cruising.

Upgrading the ignition system can be worth 5–10 horsepower, but its real benefit is improved throttle response and reliability. A properly timed 340 will be less prone to pinging and will achieve a cleaner idle. Use a dial-back timing light and a distributor machine to dial in the curve accurately.

Supporting Mods for Optimal Performance

To fully realize the potential of your Mopar 340 upgrades, consider additional supporting modifications. These can enhance overall performance and reliability, ensuring your engine runs strong for years.

Fuel System

Stock fuel pumps deliver around 4–5 psi, which is marginal for a performance carburetor. Install a high-volume mechanical fuel pump (e.g., Carter M6903) or an electric pump with a regulator to maintain consistent pressure. Use a sump or a pickup extension in the fuel tank to prevent starvation during hard corners.

Cooling System

Increased horsepower generates more heat. A high-performance radiator (aluminum cross-flow), a 160–180° thermostat, and an electric fan upgrade will keep your 340 cool even in stop-and-go traffic. Consider adding an oil cooler if your engine sees prolonged high-RPM use.

Valve Springs and Pushrods

When upgrading the camshaft, you must upgrade the valve springs to prevent valve float. High-load springs with a seat pressure of 110–130 lbs. are typical for street-strip cams. Replace the factory pressed-in rocker studs with screw-in studs for added strength. Hardened pushrods may also be required to avoid flexing under high valve lift.

Cylinder Heads

For a real leap in power, consider upgrading to aftermarket aluminum cylinder heads such as the Edelbrock Performer RPM or a set of ProMaxx heads. Aluminum heads flow better, weigh less, and allow higher compression ratios without detonation. A set of aluminum heads can add 40–50 horsepower over stock cast-iron heads on a 340.

Engine Tuning and Dynamometer Testing

After completing the upgrades, tuning your engine is essential. A proper tune will ensure that all components work together harmoniously, maximizing your horsepower gains. Consider using a dynamometer to measure performance and make adjustments as needed.

Dyno Tuning Steps

  1. Set ignition timing and curve before touching the fuel mixture.
  2. Adjust idle mixture screws for peak vacuum (typically 15–18 inHg).
  3. Dial in the carburetor’s secondary opening rate—too early causes bogging, too late kills top-end.
  4. Read spark plugs after full throttle pulls—tan color indicates near-ideal AFR; white or blistering indicates lean (dangerous), while black soot indicates rich (loss of power).
  5. Check wideband oxygen sensor readings: target 12.5–13.0 AFR at wide-open throttle, 14.0–14.7 at cruise.

A dyno session can reveal hidden gains. Many build combinations that look good on paper need jet changes or timing adjustments to reach their true potential. Expect your 340 to produce 330–360 horsepower after a comprehensive upgrade—well within the 275-to-340 target range, with careful tuning pushing you toward the higher end.

“The Mopar 340 is one of the most responsive small-blocks to bolt-on modifications. With a good intake, cam, and headers, you can reliably hit 1 horsepower per cubic inch on pump gas.” — Mopar engine builder, Jim K.

Budgets and Priorities

Not all upgrades need to happen at once. The most cost-effective path to 340 horsepower starts with headers and a performance exhaust, followed by an intake manifold and carburetor. The cam and ignition should come next. If your budget allows, aluminum heads will push the power over the top. A typical budget breakdown:

  • Headers + exhaust system: $400–$800
  • Intake manifold: $200–$400
  • Carburetor: $300–$600
  • Camshaft + lifters + springs: $250–$500
  • Distributor + coil + wires: $200–$400
  • Aluminum heads: $800–$1,500

Total: $2,000–$4,000 for a strong pump-gas street build.

Final Thoughts

Boosting your Mopar 340’s power from 275 to 340 horsepower is an achievable goal with the right upgrades. By focusing on key components such as the intake, exhaust, and ignition systems, you can unlock significant performance improvements. Remember to tune your engine after modifications to ensure optimal performance and reliability. Whether you’re building a weekend cruiser or a bracket car, the 340 rewards smart part selection and careful assembly. With patience and the right parts, you’ll feel the difference in the seat of your pants.