Complex sentence: Turbochargers were not permitted until the mid- 1977 season, and became permitted after protests by Porsche motorsport department after inspecting Al Holbert's AAGT winning Chevrolet Monza, which had won two titles.
Split into simpler sentences: Turbochargers were not permitted until the mid- 1977 season. They became permitted after protests by Porsche's motorsport department after inspecting Al Holbert's AAGT winning Chevrolet Monza, which had won two titles.