What are the functional differences between vintage Mopar 318, 340 and 360 engines
Executive summary
The Mopar LA-family 318, 340 and 360 are closely related small‑block V8s that trade bore, stroke, head flow and balance decisions for different real‑world roles: the 318 as an economical, low‑output workhorse; the 340 as a short‑stroke, high‑revving performance engine with aggressive heads and compression; and the 360 as a later, larger‑displacement engine that used some 340/318 hardware but was externally balanced and tuned for broader torque and emissions compliance [1] [2] [3].
1. Origins and cubic‑inch math — how bore and stroke separate the trio
The 318, 340 and 360 share the LA small‑block architecture but arrive at different displacements by changing bore and stroke: the 340 increased the 318’s bore to 4.04 in while keeping the 3.31 in stroke to make 340 cu in, whereas the 360 used a slightly smaller 4.00 in bore with a longer 3.58 in stroke to reach 360 cu in [1] [2].
2. Heads, valves and breathing — where the 340 stands apart
The 340 is defined functionally by its higher‑flow heads: larger ports, larger valves and runner shapes aimed at high‑rpm power, which in original form came with higher compression and more aggressive cam choices; that head and valve package is the core reason the 340 historically out‑performed the 318 in horsepower for street and strip use [4] [5] [6].
3. Compression, cams and intended use — performance vs economy tradeoffs
Stock 340s were relatively high‑compression, high‑revving engines (examples include 10.3:1 figures in brochures) while many 318 applications were low‑output two‑barrel economy units, and the 360 was introduced as a more torquey, emissions‑conscious replacement of the 340 in the mid‑1970s — later 360s often ran lower compression and milder cams in production to meet regulations [5] [1] [2].
4. Bottom end, balancing and interchange limits — practical shop differences
Although the LA blocks are similar, important physical differences affect swaps and builds: the 360 is externally balanced, requiring a matched harmonic balancer and flexplate/torque converter setup, while 318/340 internal balance differences and main journal sizes (360 mains are larger) mean not all rotating assemblies swap freely without machining or rebalancing [7] [8] [3].
5. Parts interchangeability and upgrade paths — what swaps easily and what doesn’t
Many components (heads, cams, intakes, rods in some years) can be mixed across LA engines for performance gains, and a 318 can be made to approximate 340 performance by upgrading heads, cam and intake, though boring a 318 to 340 dimensions without sleeve work is not generally practical; conversely, the 360 used some 340 heads but with different valve sizing and required attention to balancing and main journal differences for full internal interchange [4] [9] [3].
6. Real‑world advice and competing viewpoints — pick by goal, not myth
Enthusiast consensus reflected in forums is consistent: for period‑correct high‑revving muscle the 340 is preferable, but for availability, low cost and broad street torque the 360 is often recommended; some argue the 318 is the best budget path when paired with better heads and cam, while others warn that rarity and collector value can make 340s expensive to source and rebuild [10] [7] [6]. Source motivations are visible: enthusiast forums favor practical, budget‑driven advice and occasional nostalgia for period correctness, while summary articles highlight design intent and spec differences [2] [10].
Exactly which engine is “better” is a function of objectives: build a lightweight, high‑rpm performer and prioritize 340 heads and internals; want broad torque, easier parts sourcing and daily‑drive manners and lean toward a 360 while watching balancing requirements; want thrift and retrofit potential and consider upgrading a 318 with 340/360 top‑end parts rather than sleeving or stroking [6] [8] [4].