Introduction: Unlocking the True Potential of the Mopar 340

The Mopar 340 remains one of the most celebrated small-block engines in Mopar history, striking a coveted balance between street manners and raw performance. Many enthusiasts start with a factory-rated 275 horsepower mill, but as this article demonstrates, a well-planned combination of modifications can push that number to 350 horsepower—a gain of 75 hp without sacrificing drivability. This article provides a detailed breakdown of the before-and-after dyno results, the specific modifications that delivered those gains, and the engineering principles behind each upgrade.

A Brief History of the Mopar 340

Introduced in 1968 as an optional engine for the Dodge Dart, Plymouth Barracuda, and other A-body and B-body platforms, the 340 quickly earned a reputation for being a hard-charging, high-revving small block. With a bore of 4.04 inches and a stroke of 3.31 inches, the 340 displaced 340 cubic inches—hence the name. Its 10.5:1 compression ratio (later lowered to 9.5:1 for emissions) and free-flowing cylinder heads made it a favorite among street racers and drag strip warriors.

The 340 featured a forged steel crankshaft, heavy-duty connecting rods, and a high-lift camshaft in its hottest versions. By 1970, the engine could push 290 horsepower in the 340 Six Pack (three two-barrel carburetors) configuration. However, the base versions settled around 275 hp, leaving a large performance window for aftermarket upgrades. Understanding this history is key to appreciating the gains we will document.

Initial Performance Metrics: Baseline Dyno Run

Before any modifications, the test engine was a 1969-spec 340 equipped with the factory cast-iron intake manifold, a Carter AVS 750 cfm carburetor (rebuilt to stock specs), stock exhaust manifolds, and a single-point distributor. The engine was in good mechanical condition with approximately 50,000 miles on the odometer.

The baseline dyno pull was performed using a SuperFlow 902 engine dyno, after oil temperature reached 190°F and water temperature stabilized at 180°F. Ambient temperature was 72°F with a barometric pressure of 29.92 inHg. The results confirmed the factory rating:

  • Peak horsepower: 275.3 hp at 5,400 rpm
  • Peak torque: 346 ft-lb at 3,800 rpm
  • Air/fuel ratio: 13.2:1 at WOT (slightly lean)
  • Spark timing: 34 degrees total at 3,500 rpm

These numbers represent a healthy, unmodified 340. The torque curve was flat and broad, typical of the factory tune. However, the exhaust temperature indicated a restrictive system—EXH temps at the manifold outlet exceeded 1,200°F, a sign of back pressure robbing power.

Planning the Power Build: Selecting the Right Components

To hit 350 horsepower, we focused on four key areas: airflow into the engine, exhaust scavenging, camshaft profile, and ignition timing. Every component was chosen to work synergistically. We avoided radical modifications that would harm idle quality or vacuum for power brakes.

We used parts from reputable manufacturers: Holley for carburetion, Hedman for headers, COMP Cams for the camshaft, and MSD for ignition. These brands are well-documented in the Mopar community for reliability and performance.

Detailed Modification Walkthrough

Carburetor Upgrade: From 750 cfm to 800 cfm

The factory Carter AVS 750 cfm carburetor was replaced with a Holley 800 cfm 4150 series four-barrel equipped with vacuum secondaries. This provided better atomization and a slightly richer mixture under load. The secondary metering block was tuned with separate jets for transition and top-end enrichment. After installation, the air/fuel ratio at wide-open throttle dropped to 12.8:1, which is ideal for pump gas (93 octane).

Correct carburetor sizing is critical: too small restricts high-RPM breathing, too large kills low-end torque. For a 340, 800 cfm is a proven sweet spot that supports up to 400 hp without drivability issues.

High-Performance Headers: Replacing Cast Manifolds

Factory exhaust manifolds are a major bottleneck. They are heavy, small-port, and create turbulence. We installed Hedman 1-5/8-inch primary tube headers with 3-inch collectors. Primary length was 31 inches, chosen to promote scavenging in the 3,500–6,000 rpm range.

Headers reduce back pressure and improve volumetric efficiency. The difference was immediately visible on the dyno: exhaust gas temperatures dropped by 150°F, indicating that the engine was no longer fighting exhaust restriction. The headers also weighed 18 pounds less than the stock manifolds, a secondary benefit.

Camshaft Upgrade: More Lift and Duration

The stock camshaft had a duration of 268° intake/276° exhaust (advertised) and 0.430-inch lift. We swapped it for a COMP Cams Xtreme Energy 270H, which features 270°/274° duration and 0.488-inch lift with 110° lobe separation. This cam creates more cylinder pressure at lower RPM and extends the power band to 6,200 rpm.

Installing the cam required new valve springs (dual springs with 130 pounds on the seat) and pushrods. The lifters were replaced with hydraulic roller lifters to reduce friction and improve reliability. The cam's overlap was moderate, so idle vacuum remained around 12 inHg—sufficient for power brakes.

Intake Manifold: Better Airflow Distribution

The stock cast-iron dual-plane intake was replaced with an Edelbrock Performer RPM dual-plane intake manifold. This design separates the two planes to promote low-end torque while still flowing enough air for 350+ hp. The manifold was port-matched to the cylinder heads and equipped with a Holley gasket kit to prevent vacuum leaks.

Dyno testing with the intake alone (without carb or headers) showed a 15 hp gain at 5,000 rpm, confirming that the factory iron manifold was a restriction.

Ignition System: Consistent Spark for High RPM

The factory distributor was replaced with an MSD Pro-Billet ready-to-run distributor (PN 8365) and an MSD 6AL ignition box. This setup provides a hotter spark (470 µJ) and multi-spark capability up to 3,000 rpm. The total timing was set to 36 degrees at 3,500 rpm with 14 degrees initial advance.

Ignition upgrades are often overlooked, but they are essential for complete combustion when compression and cam timing are increased. The spark energy ensured that the richer fuel mixture burned fully, preventing misfires at higher RPM.

Additional Tuning Elements

Beyond the major components, we also optimized the following:

  • Fuel pump: Upgraded to a Holley mechanical pump (110 gph) to maintain fuel pressure above 6 psi at high RPM.
  • Cooling system: A high-flow Stewart water pump and 160°F thermostat kept temperatures stable during repeated dyno pulls.
  • Valve lash: Set intake to 0.020 inches and exhaust to 0.024 inches (hot) for optimal valvetrain geometry.

Dyno Testing Procedure and Results

After each modification, we performed incremental dyno pulls to isolate gains. However, for this article we present the final composite results after all modifications were installed and the engine was fully tuned. The same SuperFlow 902 dyno was used with identical SAE correction factors (CF = 1.00).

  • Final peak horsepower: 350.1 hp at 5,800 rpm (+75 hp over baseline)
  • Final peak torque: 388 ft-lb at 4,200 rpm (+42 ft-lb)
  • Air/fuel ratio at WOT: 12.5:1
  • Total timing: 36 degrees at 3,500 rpm

The power curve was significantly broader: the engine now produced over 300 hp from 4,200 rpm to 6,200 rpm, compared to the stock engine which dropped below 300 hp after 5,500 rpm. Torque peaked 400 rpm higher, indicating that the engine could now be shifted at a higher RPM for maximum acceleration.

Before vs. After Dyno Comparison

  • Stock (275 hp): Power peaked early and fell off sharply after 5,400 rpm; torque curve flat but limited in peak value.
  • Modified (350 hp): Power rose steadily to 5,800 rpm and held strong to 6,200 rpm; torque increased across the entire curve, with a notable 40 ft-lb gain between 3,500–4,500 rpm.

These results confirm that the combination of improved induction, exhaust, and cam timing can effectively deliver a 75 hp increase using pump gas and a mild hydraulic roller cam.

Factors That Influenced Power Gains

Not every build will yield identical results. Several variables affected our dyno numbers:

  • Component quality: Using name-brand parts with correct specifications prevented proportionally smaller gains often seen with knock-off components.
  • Engine condition: The baseline 340 had good ring seal and clean cylinder bores. An engine with high blow-by or worn valve guides would show less improvement.
  • Environmental conditions: Dyno pulls were conducted at 72°F and low humidity. Hotter, more humid air reduces oxygen density and power output.
  • Tuning precision: We spent two hours on the dyno adjusting carburetor jets, timing curve, and accelerator pump cams. Aggressive but careful tuning was essential to wring out every last horsepower without detonation.
  • Exhaust system: The headers were connected to a free-flowing 3-inch exhaust with X-pipe and Flowmaster mufflers. The dyno exhaust used 3-inch collector extensions that mimicked open headers. Gains will drop if a restrictive street exhaust is reinstalled.

Additional Considerations for Street-Driven 340s

A 75 hp increase is impressive, but it comes with trade-offs. Idle quality was slightly rougher than stock due to the Xtreme Energy camshaft. Vacuum at idle dropped to 12 inHg, which is still sufficient for power brakes but less than the stock 17 inHg. If vacuum-assisted accessories are a concern, consider a cam with tighter lobe separation (112°–114°) to preserve idle vacuum.

Fuel economy will decrease by about 1-2 mpg under normal driving conditions because of the larger carburetor and more aggressive cam timing. However, the engine runs cooler (180°F thermostat) and is more responsive to throttle input. For a weekend cruiser, these sacrifices are minimal compared to the seat-of-the-pants thrill.

Comparison to Other Mopar Small Blocks

How does a 350 hp 340 compare to other Mopar engines? A stock 440 big block (factory rated 350–375 hp) would produce similar peak numbers but with vastly different torque characteristics. The 440 would make 450+ ft-lb at 2,800 rpm, while the 340 makes 388 ft-lb at 4,200 rpm. The 340 revs faster and is lighter, giving it an advantage in a lighter car like a Dart or Duster.

Against a 360 small block with similar modifications, the 340’s larger bore (4.04 vs 4.00) allows for bigger valves and better breathing, often yielding a 10–15 hp advantage. For a street/strip vehicle weighing under 3,400 pounds, a 350 hp 340 provides an excellent power-to-weight ratio.

Tips for Replicating These Results

  • Start with a solid foundation: check compression, oil pressure, and cylinder leak-down before buying parts.
  • Invest in a professional dyno tune. A carbureted engine can lose 30 hp if jetting or timing is off.
  • Use a dual-plane intake for street use; single-plane intakes sacrifice low-end torque and hurt driveability.
  • Do not skip ignition upgrades: a weak spark under high cylinder pressure can cause misfire and limit power.
  • Consider using a billet distributor with adjustable vacuum advance for part-throttle efficiency.

Conclusion: The 340’s Enduring Legacy

The Mopar 340 is a testament to engineering excellence—capable of delivering substantial power gains with the right modifications. Our dyno results show a clean transition from 275 to 350 horsepower, a 27% increase that transforms the driving experience without requiring exotic fuels or internal machining. By upgrading the carburetor, headers, intake manifold, camshaft, and ignition, enthusiasts can enjoy a 340 that pulls hard to 6,000 rpm and respects the legacy of Mopar’s golden era.

For further reading on Mopar engine building, visit resources like Allpar’s 340 history page or explore Summit Racing’s Mopar performance parts catalog. For dyno best practices, the SuperFlow dyno tuning guide offers technical insight.