These different types of Probe pistons allow for a range of engine outputs. The dished-top or "reverse dome" (left) has a compression ratio (CR) of 9.5:1 or higher, depending on the cylinder heads used. It's ideal for street use on pump gas. The flat top (center) typically has 12.5:1 CR for bracket racing application. The domed type (right) is solely for high-horsepower race engines with a 14:1 or higher CR.