20 When compression can’t be lowered via a larger combustion chamber, head gasket thickness, or changes to deck/compression height, a dished piston is the last option. The larger the dish, the lower your compression ratio. To raise compression, we sometimes have to dome the piston, the exact opposite of dishing. The more dome we have, the higher the compression ratio, because there is less volume above the piston at top dead center. You have to cram the same amout of air/fuel mixture into a smaller space.