Early in the '66 model year, the 427ci L72 engine was rated 450 hp at 6,400 rpm. Not long after production began, the rating was lowered to 425 hp at 5,600 rpm. There were no changes to the engine, but Chevrolet's central office apparently realized that 450 hp would attract the attention of safety minded politicians and insurance companies. Also, there seemed to be an unwritten agreement between Ford, Chrysler, and GM that no engine would be rated over 425 hp no matter how powerful it was. In 1967, Chevy broke that rule by rating the L71 (triple two-barrel carb, solid-lifter, 427ci) engine at 435 hp. The L71 was identical to the '66 L72 except for the intake system. The L72 had one four-barrel carburetor compared to the L71's Tri-power setup.
Over the years there has been much speculation about the actual horsepower output of these two engines. Some people believed the L71 made nearly 500 gross horsepower. However, performance and dynamometer numbers seem to tell a different story. All known magazine quarter-mile times for L71- and L72-equipped Corvettes averaged 13.38 seconds at 107.94 miles per hour. For cars of equal weight, quarter-mile trap speeds are an excellent indicator of horsepower. Magazine tests of LS1-equipped C5 Corvettes had the same trap speeds as L71 and L72 Corvettes (see sidebar). Because big-block C2 Corvettes are nearly the same weight as a C5, we can assume they equal the 345 to 350 SAE net horsepower that LS1 engines make.
In "Big-Block Power Tune" (Nov. '06), Corvette Fever chronicled a meticulous dyno tune on a correctly restored '66 L72 Corvette in order to determine horsepower gain from the tune. Before the dyno tune, the engine produced 289 rear-wheel horsepower (rwhp) at 5,000 rpm and 331 lb-ft of torque at 3,800 rpm. After the dyno tune the output went up to 301 rwhp at 5,000 rpm and 342 lb-ft of torque at 3,800 rpm. This car was restored to showroom original condition for acceptable street operation and was equipped with the (more restrictive) N14 side-mount exhaust system. A standard under-car exhaust would most likely have resulted in somewhat higher numbers, probably at a higher rpm. Maximum horsepower was reached at 5,000 rather than closer to 6,400 rpm, where an open exhaust would have produced peak power. On average, the 345 net hp LS1 engine also produces 300 rwhp. Equal rwhp for both the LS1 and L72 translates to 345 net horsepower for the L72.
Enter the 454
For the '70 model year, Chevrolet developed 454ci engines to replace the 427. (See the accompanying chart for the 427s, their 454 replacements, and power ratings.) With preparation for their introduction well underway, all 454 engines except the LS5 and LS6 were cancelled due to GM's de-proliferation program to eliminate costly options as well as high insurance rates, and top management's desire to tone down horsepower. Eliminating the Corvette's LJ1, LJ2, LS7, and LT2 engines from production was untimely, as it was too late to put the LS6 on the Corvette option list. These cancellations left Corvette in the unusual position of having less power than the '70 SS454 Chevelle, for which the LS6 remained an option. Chevrolet corrected this the following year by making a lower compression, 425hp version of the LS6 an option for the '71 Corvette while deleting it from the Chevelle's option list.
To prepare customers for the 1972 switchover from the gross to net method of rating horsepower, Chevrolet reported both gross and net ratings for '71 Corvette engines. The Corvette LS6 received a 425 gross and 325 net hp rating. The gross-to-net conversion factor for that engine was 0.7647 (325/425), which can be used to convert our previously determined 345 net hp for the L72 to a gross rating. Using that factor we come up with a gross rating of 451 hp (345/0.7647 = 451.16) for the L72, which for all practical purposes is exactly the same rating Chevy initially gave the engine.