From Facebook: “Why was the 396 from the 1965 Vette rated at 425hp and all the later model 396's were lower even in the other Chevy cars? Just what was so special about that motor? And why didn’t they stick with it? I know the 427 had more potential but always wondered why that beast of a 396 went away, Thanks.”
It was the high “state-of-tune” that made the Corvette’s L78 engine produce 425hp with 396 cubic-inches. It was introduced mid-1965 and produced more horsepower than any of the other 396 engines manufactured from mid-1965 until 396 production ended. Moreover, exceptional measures were incorporated into it to help it survive the high amounts of horsepower that lower horse 396 engines didn’t get, and that’s what made it special.
To give you an example of how special, look at the accompanying photo of the block stripped clear to the bone and you’ll notice the L78 block reveals four-bolt main caps that came on the 425hp version. The structure was substantially beefed up by increasing the bulkhead above each main bearing. It was only the 425hp L78 that got four bolt mains in ‘65.
The cross-drilled crankshaft held firm from flexing within the four-bolt mains was a Tuff-Trided forged steel unit with large chamfered holes to ensure a healthy supply of oil. The rear main bearing was 10-percent larger than the 425hp 409 and the rod journals were 12-percent larger. A direct descendant of the much-fabled Mark II Mystery 427, the Mark IV L78 block resembled the Mystery motor block almost to a T.
From the crankshaft to the crank-piston assembly, weak links were eliminated starting with high tensile strength alloy connecting rods and reinforced high-stress areas with ribs between the flange and the special aircraft quality rod bolt heads.
As captivating as the specialness of the L78 short-block assembly was, it was the Mark II 427 Mystery motor inspired cylinder heads that really made the 425hp 396 engine intriguing. Appearing with an uncanny resemblance to the 427 Mystery motor’s heads, the intake and exhaust valves tilted away from each other to improve induction and exhausting, creating the expression “porcupine heads” and were responsible for extracting more than one horsepower per cubic-inch from the L78 396.
To answer why “that beast of a 396 went away,” had it not been for an across-the-board General Motors dictate that only full-size cars received engines over 400 cubic-inches, the first year big-block Corvette would have displaced 427 cubic-inches. Though typical for GM during those years, the repressive dictate expired for Corvette in 1966, so the lineup was able to include two big-block engine choices. The hydraulic lifter version was the L36 400hp/427 and the high-performance solid lifter version was the L72 450hp/427 that replaced the L78 396.
Interestingly, early into the 1966 production year the initial power ratings for the two big-block 427 Corvette engines dropped to 390hp for the L36 and 425hp for the L72. These were administrative decisions made by GM brass and didn’t reflect a change in specifications that would decrease horsepower output. At the end of the 1965 model year, 2,157 1965 L78 Corvettes counting convertibles and coupes were produced, and the L78 never surfaced again.