In 1970, GM lifted a restriction on engines over 400 ci, but in 1971, it required all cars to run on unleaded gasoline and to meet strict emission standards. So what's a muscle car supposed to do? Chevy responded by shrinking the base engine in the 1971 Chevelle, taking it down to a 350-ci V8 rated at 245 horsepower. The top-of-the-line was the LS5 454-ci V8 engine rated at 365 horsepower, down almost 100 horses from the previous year. Luckily, buyers could special order LS6 engines which generated 325 net horsepower, and 425 gross horsepower. Phew! Just short of a quarter of the 80,000 Chevelles sold that year were bought with the 454 engines (19,292 were sold).