Counter blowout or high-vacuum conditions.
Certain engines, such as small block and big block Chevys, can produce high-vacuum or blowout conditions that can be big problems for many valve cover gaskets. Fel-Pro Performance CorkLam gaskets are designed to deal with those challenges. They have a metal core for extra strength, with a special cork-and-rubber compound bonded to both sides. This produces tough gaskets that can deal with problem engines, while also providing a little extra clearance between the valve cover and the valvetrain.