Understanding the LA Engine Family: 318 vs. 340

The Chrysler LA (Lightweight A) engine family debuted in 1964 and quickly became the backbone of Mopar performance for decades. The 318 and 340 share the same basic architecture—thin-wall cast iron block, 90-degree V angle, and wedge-shaped combustion chambers—but they are far from identical under the hood. The 318 displaces 5.2 liters (318 cubic inches) and was engineered for durability and fuel economy, making it a staple in trucks, station wagons, and economy coupes. The 340, first released in 1968, displaces 5.6 liters (340 cubic inches) and was built specifically for high-performance applications, featuring a larger bore, forged internals, and upgraded cylinder heads.

Here are the most critical technical differences:

  • Bore and Stroke: The 318 has a 3.91-inch bore and a 3.31-inch stroke. The 340 uses a 4.04-inch bore and a 3.31-inch stroke—same crank, larger bore.
  • Main Journal Size: 318 blocks have 2.500-inch main journals; 340 blocks use 2.625-inch main journals (same as 360). This means the 340 crank is not a direct drop-in for a 318 block.
  • Rod Length: Both use 6.123-inch rods stock, but 340 rods are typically stronger forged pieces.
  • Cylinder Heads: Early 318s came with small-valve heads (1.78/1.50 inches). High-performance 340 heads (casting numbers 302, 308, 920, 596) feature 2.02/1.60 valves, hardened seats, and better flow characteristics.
  • Compression Ratio: Factory 318 compression ranged 8.5–8.9:1; the 340 ran 10.0–10.5:1 depending on year and carburetion.
  • Horsepower and Torque: A stock two-barrel 318 produces roughly 150–190 hp and 240–270 lb-ft. A factory four-barrel 340 delivers 240–300 hp and 305–345 lb-ft.

These fundamental differences make a straight 318-to-340 swap more than just an engine swap—it’s an upgrade to a completely different powerband. For many enthusiasts, the decision comes down to whether to find a genuine 340 block or build a 318 to 340 “spec” using aftermarket parts. This article covers both approaches, focusing on the most common Mopar Performance conversion path.

Why Upgrade from a 318 to 340?

The 318 is a bulletproof workhorse, but it was never designed to lead the pack at the drag strip. Switching to a 340—or building your 318 to 340 specs—yields tangible, seat-of-the-pants improvements:

  • Instant Power: Expect a 90–150 hp gain and 100–150 lb-ft torque increase over a stock 318.
  • Throttle Response: The 340’s larger valves and higher compression transform part-throttle response, especially with a properly tuned four-barrel carburetor.
  • Acceleration: Quarter-mile times improve by 1.0–1.5 seconds; 0–60 mph drops by a full second or more.
  • Sound & Feel: A 340 with a mild cam and true dual exhaust produces a distinctive, throaty idle that screams old-school Mopar muscle.
  • Resale Value: A correct 340-powered car (or a period-correct conversion) is worth significantly more than a 318 car in most markets.
  • Parts Availability: Because the 340 was a high-volume performance engine, aftermarket support is strong—pistons, cams, intakes, and heads are all readily available.

Of course, the upgrade is not without trade-offs. Fuel economy will drop, you may need to upgrade the radiator and cooling system, and emissions compliance can be tricky in strict regions. But for most builders, the driving experience is worth the expense.

Cost Breakdown: What to Expect from a Mopar 318 to 340 Conversion

Costs can swing wildly based on whether you source a used long-block, rebuild a core, or piece together a new assembly from aftermarket parts. Below is a realistic, itemized breakdown for a typical do-it-yourself conversion using a mix of factory and quality aftermarket components. Professional labor adds $1,000–$3,000 depending on shop rates and scope of work.

Core Components (Prices in USD, 2025 estimates)

  • Used 340 Block (Standard bore, no cracks): $1,500–$2,500. A good core is getting harder to find; expect to pay a premium for date-correct codes.
  • Cast Pistons (Hyperutectic, 340 spec): $250–$400. Forged pistons add $200–$400 more.
  • Rings, Bearings, Gasket Set, Oil Pump: $300–$500.
  • Machine Work (bore, hone, deck, line-hone, valve job): $800–$1,200.
  • Crankshaft (340 stock cast or forged): $200–$500 if you reuse the 340 crank; a new aftermarket forged crank runs $700–$1,000.
  • Connecting Rods (stock 340 or aftermarket I-beam): $150–$400.
  • Cylinder Heads (rebuilt 340 castings): $600–$1,000. Aftermarket aluminum heads (Edelbrock, Trick Flow) add $1,200–$1,600 but save weight and flow better.
  • Intake Manifold (Edelbrock Performer or Mopar M1): $200–$500.
  • Carburetor (Holley or Edelbrock 650–750 CFM): $350–$700.
  • Camshaft & Lifters (hydraulic flat-tappet or roller): $200–$500.
  • Exhaust Manifolds (stock 340 HP manifolds or shorty headers): $150–$600.
  • Timing Chain, Water Pump, Pulleys, Belts: $200–$350.

Ancillary Modifications

  • Radiator upgrade (aluminum, 2- or 3-row): $200–$500.
  • Electric fan conversion: $100–$250.
  • Fuel system upgrades (mechanical pump, lines, regulator): $150–$400.
  • Ignition upgrade (HEI or Mopar Performance electronic): $100–$300.
  • Transmission stall converter (if automatic): $200–$500.
  • Exhaust system (dual 2.5-inch with performance mufflers): $400–$900.

Total Estimated Cost (DIY, no labor): $4,800–$9,500. With professional assembly and dyno tuning, add $1,500–$4,000.

For budget-minded builders, searching for a complete running 340 from a donor car (e.g., Dodge Dart, Plymouth Duster, Challenger) can lower overall cost, but you must verify that the block is a true 340 — not a rebadged 360 or 318. Check the casting number (2780929, 3418445, or 4006832 for 340) and the bore measurement.

Step-by-Step Conversion Process

Step 1: Preparation and Planning

Before touching a wrench, decide whether you will perform a direct swap (remove 318, install 340) or build your existing 318 block to 340-like specs. If swapping blocks, confirm that the engine bay can accept the larger motor mounts — they are the same for LA engines, but some 340 blocks use different mount bosses. Check transmission bellhousing bolt pattern (all LA small-blocks share the same pattern, but starter mounting may differ). Gather all gaskets, fasteners, and specialty tools (torque angle gauge, harmonic balancer puller, pilot bearing tool).

Step 2: Remove the 318 Engine

Disconnect the battery, drain coolant and oil, remove the radiator, fan shroud, and A/C compressor if equipped. Unbolt the transmission from the engine (leave the trans in the car if possible), remove the starter, exhaust manifolds, intake, carburetor, and all wiring harnesses. Label or photograph connections for reassembly. Lift the engine using a load-leveler to avoid damaging the firewall or front crossmember. Remove the engine mounts, then lift the 318 out carefully.

Step 3: Inspect and Rebuild the 340 Long-Block

If you sourced a used 340, tear it down completely. Have the block pressure-tested, magnafluxed for cracks, and measured for bore wear. Typical machine work includes boring to 0.030–0.060 over, align-honing the mains, surfacing the decks, and installing new cam bearings. For cylinder heads, perform a three-angle valve job and check spring pressure. Seat valve stem seals. If using aluminum aftermarket heads, note that they require shorter pushrods and different valve covers. Assemble the short block with new pistons, rings, bearings, and a quality oil pump. Clutch builders should select a cam matched to the rest of the combination — for street performance, a Comp Cams XE268H or Mopar Performance 280/280 works well. Degree the cam to ensure proper timing.

Step 4: Install the 340 into the Engine Bay

Clean and paint the engine bay while the engine is out. Install new motor mounts (use 340-specific mounts with the larger rubber isolators). Lower the 340 into the bay with a load-leveler, aligning the transmission input shaft splines. Bolt the engine mounts loosely first, then the transmission bellhousing bolts. Torque all fasteners to factory specs. Reconnect the starter, exhaust manifolds, alternator, power steering pump, and A/C compressor using correct brackets (some 340 brackets are unique).

Step 5: Fuel, Cooling, and Ignition Systems

Upgrade the fuel system if the 340 demands more volume. A stock 318 mechanical pump may suffice for mild builds, but a high-flow pump with a return-style regulator is recommended for engines over 350 hp. Replace the radiator with an aluminum unit that has at least two 1-inch rows. Use a thermal fan switch and electric fan for better cooling at idle. Re-route heater hoses if necessary. For ignition, the Mopar Electronic Control Unit (ECU) from a later 340 works well; aftermarket MSD or Pertronix systems simplify wiring.

Step 6: Intake, Carburetor, and Exhaust

Bolt on an intake manifold that matches the cylinder head port configuration. Edelbrock Performer RPM (dual-plane) is ideal for street use; the Mopar M1 single-plane suits higher-rpm applications. Set the idle mixture and curb idle after the engine first fires. For exhaust, factory 340 HP manifolds offer good flow and a classic look. For maximum performance, use 1⅝-inch primary tube headers (full-length or shorty). Connect to a 2.5-inch or 3-inch exhaust system with X-pipe and performance mufflers. Avoid excessive backpressure — the 340 breathes hard above 4,000 rpm.

Step 7: Final Assembly and Tuning

Fill the engine with break-in oil (non-synthetic) and coolant. Prime the oil system with a drill and a priming tool. Start the engine and break-in the cam at 2,000–2,500 rpm for 20 minutes. Do not let it idle during break-in. After the cam is set, retorque the head bolts, adjust valves (if solid lifters), and check the timing. Set initial timing to 12–16 degrees BTDC, total vacuum/mechanical advance to 34–38 degrees all in by 3,000 rpm. Road-test the car, then add a 50-mile and 500-mile oil change.

Step 8: Drive and Enjoy

After proper break-in, retune the carburetor and check for leaks. You can expect a dramatic increase in throttle response and overall zip. Consider chassis upgrades to match the new power — tighter steering box, sway bars, and rear suspension links (pinion snubber) will help put the power down.

Expected Power Improvements and Performance Gains

The following table summarizes typical output from a well-executed 318-to-340 conversion using a stock 340 block with mild bolt-ons versus a fully built 340 with performance heads, cam, and intake.

Configuration Horsepower (crank) Torque (crank) 0–60 mph ¼-mile ET
Stock 318 (2-bbl) 150 hp 265 lb-ft ~10.2 sec ~17.5 sec @ 78 mph
Stock 340 (4-bbl, stock cam) 275 hp 340 lb-ft ~7.5 sec ~15.0 sec @ 92 mph
340 with Edelbrock heads, mild cam (280°) 350–380 hp 380–400 lb-ft ~6.2 sec ~13.5 sec @ 102 mph
340 stroker (416 cu in) with roller cam 450 hp+ 450 lb-ft+ ~5.5 sec ~12.0 sec @ 112 mph

Even a mild, correctly assembled 340 will completely transform a passenger car or light truck. The torque curve comes on strong from 2,000 rpm, making daily driving effortless. Expect top speed to increase by 15–25 mph, depending on gearing. Many builders report that their converted cars “feel 500 pounds lighter.”

Common Pitfalls and How to Avoid Them

  • Mixing 318 and 340 Parts: The 340 block has a different deck height (9.60 inches vs. 318’s 9.600? Actually same deck height, but piston pin height differs). Never assume 318 pistons will work in a 340 bore — you need 340-specific pistons.
  • Wrong Oil Pan: A 318 pan has a different sump shape and rear seal architecture. Use a 340 pan with the correct dipstick location. Early 340 pans are rare; aftermarket solves this.
  • Ignition Timing Confusion: The 340 distributor shaft length is the same as 318, but the advance curve should be tailored. A stock 340 distributor is fine; adjust the mechanical advance to 12–16° initial.
  • Carburetor Selection: A 600 CFM carb is adequate for a mild 340; a 750 CFM works best with high-flow heads and a large cam. Over-carbureting causes bogging — tune carefully.
  • Transmission Matching: The 340’s extra torque can destroy a slushbox 904. Either rebuild the 904 with a high-stall converter and shift kit, or swap to a 727 TorqueFlite. For manuals, the O-345 heavy-duty output shaft is recommended.

Is a 340 Conversion Right for You?

If you own a vehicle that originally came with a 318—especially a Mopar from the 1960s or 70s—the 340 swap is one of the most satisfying upgrades you can make. It preserves the Mopar lineage while giving you a power level that rivals many big-blocks. The initial investment may feel steep, but the return in driving excitement and collectibility is substantial. For those who prefer building on a shoestring, consider the “318 to 340” approach using a 340 crank and 4.04-inch pistons in a 318 block — but remember the main journal size mismatch. You can grind a 360 crank to fit? Not easily. A simpler path is to buy a ready-to-run 340 short block from reputable suppliers like Mancini Racing or Summit Racing.

For detailed rebuild and tuning guides, the Allpar website has extensive factory specs, and For A Bodies Only and Moparts forums are goldmines of real-world experience. Always cross-reference your engine casting numbers before purchasing parts — a mistake here can cost hundreds of dollars.

Ultimately, whether you are building a street beast, a weekend warrior, or a meticulous restoration, the Mopar 318 to 340 conversion remains a time-honored path to genuine horsepower. With careful planning, realistic budgeting, and attention to detail, you can transform your Mopar into what it was always meant to be — a muscle car that demands respect.