page_title: AMC V8 engine
section_title: Gen-3 AMC tall-deck (1970–1991)
hierarchical_section_title: AMC V8 engine / Gen-3 AMC tall-deck (1970–1991)
caption_attribution_description: 1970 Javelin SST 390 CID (6.4 L) 315 hp (235 kW) V8 engine bay. Automobile built by American Motors Corporation (AMC). A muscle "pony car". This shows the factory "Go Package" version that included the ram-air hood and other high-performance components.
context_page_description: American Motors Corporation designed and manufactured two distinct V8 engine designs. Their first V8 engine design was made from 1956 through 1967. The second design debuted in 1966 and was made through 1991. Chrysler kept the last design in production until 1991 for the Jeep Grand Wagoneer.
context_section_description: In 1970, all three blocks grew in deck height and gained a new head design. These changes made this the third generation of AMC V8, hence it is sometimes referred to as the "Gen-3" AMC V8. The stroke and deck height on the 290 and 343 was increased by 0.16 in (4.06 mm) on both engines, becoming the 304 cu in (5.0 L) and 360 cu in (5.9 L), respectively. The 1970 AMX 390 remained at the same displacement by using a special rod and piston for this year only. It is believed that AMC kept the 390 this last year due to the reputation it had garnered in the two seater AMX, which was discontinued after 1970. In 1971 the 390 was also stroked by 0.16 in (4.06 mm) to become the 401 engine.
  The other change in 1970 was the switch to the "dog-leg" heads. These heads flow 20% better on the exhaust side than the 1966-1969 rectangular port heads and are thus the best for performance. There are two reasons for the flow increase: (1) the area of the port is larger, due to the dog leg and (2) the shape of the port floor was changed from a concave to a convex curve. The concave floor tended to bend the exhaust flow upwards which caused turbulence when the flow was forced to go down into the exhaust manifolds. By switching to a convex floor the curvature of the flow starts in the head and proceeds much more smoothly into the exhaust manifold resulting in less turbulence and better flow.
  The center two intake bolts on each head were relocated to prevent accidental mix-ups of Gen-2 and Gen-3 intakes. The intakes can be interchanged by slotting the bolt holes, but the added deck height of the GEN-3 engine means that sealing and port match will be compromised. Gen-3 intakes can be machined to fit Gen-2 engines by surface grinding the intake flanges and slotting the center holes.
  There is a persistent myth about 1970-mid 1971 319 and 291 AMC heads. These heads have the dog-leg exhaust ports and 50-52 cc combustion chambers. They are commonly identified by the first three (319) or last three (291 for the 360-401 heads; 304 used a different casting) digits of the casting number. There was a U.S. auto industry-wide shift to lower compression ratios in mid 1971, so AMC increased combustion chamber size to 58-59 cc. The first three digits of the casting number on the large chamber heads are 321, 322, or 323 depending on year. The only difference between small and large chamber Gen-3 heads is the combustion chamber size.
  The advertised power drop from 1971 to 1972 ('71 304/210 hp and '72 304/150 hp, for example) is not due to any engine change, but to in the way the automobile industry adopted SAE power rating method. The 1971 and 1972 engines produce the same power. The 1971 and earlier versions were rated using "gross" horsepower, which is with the engines operating with no accessories or drive belts, air filters, nor any exhaust system restrictions. The change to the more realistic "net" horsepower ratings for 1972 and later required the engine to be driving all stock factory accessories and to use the factory air filter system and a simulated factory exhaust. Power is measured at the flywheel for both systems. For example, even with the modifications to more reduce emissions and increase efficiency, AMC's 401 cu in (6.6 L) engine was among the strongest factory-spec engines available in an American car in 1971, with more horsepower than Pontiac's standard 455 cu in (7.5 L) and Chevrolet's 454 cu in (7.4 L) that powered the Corvette.
caption: Engine bay of a 1970 Javelin with a Ram Air 390 V8
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