The 1971 Chevelles got fresh front-end styling that included large Power-Beam single-unit headlights, a reworked grille and bumper, and integral park/signal/marker lights. New dual round taillights were integral with the back bumper. Because SS models suffered heavy insurance surcharges, Chevrolet introduced the "Heavy Chevy" at midyear, which was based on the base Chevelle, and was available with any V8 engine except the 454, which was exclusive to SS models. The Heavy Chevy (RPO YF3) was ONLY available with the base Chevelle sport coupe (13437) and was primarily a dress up option and even it was limited to options available on the standard Chevelle sport coupe; no carpeting, no bucket seats, etc. For 1971, the SS option was reduced to one RPO code, RPO Z15, and was only available for the Chevelle Malibu. This RPO code required any optional engine and transmission available in the Chevelle lineup. Since the 307 V8 was the standard base V8 in 1971, it could not be ordered with the SS option; one had to order one of the two 350 V8 engines (L65 or L48), the LS3 402 or the LS5 454.
The minimum Chevelle SS engine was a two-barrel 350-cubic-inch V8 rated at 245 gross (165 net) horsepower. Optionally available was a four-barrel carbureted version of the 350 V8 rated at 275 gross (200 net with dual exhaust and 175 net with single exhaust) horsepower. The 402 cid big-block engine continued to be optional as the SS 396 but was only available in one horsepower rating, 300 gross (260 net) horsepower, and was not available with cowl induction. The base LS5 454 V8 produced 365 gross and 285 net horsepower, but cowl induction was available that produced more power because of the air induction and louder exhaust system. The LS6 had been planned for 1971, but was dropped before production. As a result the LS6 was only available in the 1971 Corvette. Similarly, the high performance 455 cid engine that was in the 1970 Buick Skylark GS/GSX Stage I had been planned for 1971, but was dropped before production. The parts that would have been used for the 1971 Buick engine were sold to buyers as spare parts delivered in the trunk for the buyer to use and was called the Stage II option.
Chevrolet specifications for 1971 included both "gross" and "net" horsepower figures for all engines. The SS option could be ordered with any optional V8 and became more of a dress-up option than a performance option. GM mandated all divisions design their engines to run on lower-octane regular, low-lead or unleaded gasoline. To permit usage of the lower-octane fuels, all engines featured low compression ratios (9:1 and lower; well below the 10.25-11.25:1 range on high-performance engines of 1970 and earlier). This move reduced horsepower ratings on the big-block engines to 300 for the 402 cubic-inch V8 but surprisingly, the LS5 454 option got an "advertised" five-horsepower increase to 365. The LS6 454 option, which was originally announced as a regular production option on the Chevelle SS for 1971, was dropped early in the model year and no official records indicate that any 1971 Chevelles were assembled with the LS6 engine.
It's interesting to note that both 350 V8 engines, as well as the dual exhaust 402 cid V8 engine, were available without the SS option; only the LS5 454 V8 required the SS option. A single exhaust version of the 402 cid engine existed in 1970 with 330 gross hp and in 1972 with 210 net hp. In 1971 the single exhaust version of the 402 cid engine produced 206 net hp, but only appeared in the full size Chevrolet brochure. Although not appearing in the 1971 Chevelle and Monte Carlo brochures, no doubt it could have been available as a special order.