In an effort to appeal to smaller engine-building companies and independents, USAC had permitted "stock-block" pushrod engines (generally defined as single non-OHC units fitted with two valves per cylinder actuated by pushrod and rocker arm). The traditional "stock blocks" saw some limited use in the early 1980s, but became mainstream at Indy with the Buick V-6 by 1985. Initially, the stock blocks were required to have some production-based parts. However, in 1991, USAC quietly lifted the requirement, and purpose-built pushrod engines were permitted to be designed for racing from the ground up. Attempting to create an equivalency formula, both pushrod engine formats were allowed increased displacement of instead of , and increased turbocharger boost of 55 inHG instead of 45 (1860 hPa instead of 1520).