Here's a breakdown of factors affecting its strength:
* Factory Specifications (1967-1980): Factory 400s were known for their torque, but not necessarily their high RPM capability or overall strength compared to smaller displacement small blocks like the 350. Their larger bore and stroke compared to a 350 made them heavier and less rev-happy. They were generally considered durable for their intended use, but not as easily modified for extremely high horsepower applications as the 350. Stock, they are considered reasonably strong for their intended application (trucks and some passenger cars).
* Modifications: This is where the strength dramatically changes. With aftermarket components like forged internals (crankshaft, connecting rods, pistons), upgraded valvetrain (camshaft, lifters, springs, rocker arms), a better oiling system, and a stronger block (potentially aftermarket blocks designed for higher horsepower), a 400 can be incredibly strong and produce far more horsepower than a stock version. Many 400s have been built to handle well over 500, 600, and even 700+ horsepower reliably. It also significantly depends on the quality of the build and the tuning.
* Weaknesses of the stock 400: The factory 400's larger stroke can put more stress on the crankshaft and connecting rods at high RPM compared to a 350, making them potentially less durable at very high RPMs *without* upgrades. The thicker cylinder walls of the 400 block generally make it very strong, able to withstand more boost for example, than a 350 block.
In short: A stock 400 is reasonably strong for its era and intended applications. However, with modifications, its strength potential is very high, making it capable of significant horsepower and torque. A properly built 400 can be exceptionally powerful, but that requires careful planning and execution.
