273 vs 318

The first LA was the 273, with a two barrel carb, producing 180 gross hp. Introduced to Americans in 1964, it had a thin-wall casting and simpler wedge-heads and valves, which saved around 50 pounds over the existing A-series V8. The 273 ended up being only fifty pounds heavier than the 225-cid slant six. In 1965, a four-barrel carb and high performance cam could push that up to 235 hp; and, in 1966, a limited edition 273 with a 700 cfm carb and .500" lift cam put out 275 hp.
The 318 (5.2 liters) was brought out in 1967; the main difference from the 273 was its larger 3.91” bore. The 318 was never used as a performance engine by Chrysler; the 273 four-barrel matched it in peak horsepower, and the more performance-oriented 340 was quickly released in 1968. It had hydraulic tappets from the start, but hardened valve seats were not used until at least 1973.
~~all above info. copied and pasted from Valiant.org ~~