On forced induction and on nitrous applications, which experience extreme shock loads, Kempf says they move the top ring down from the piston crown to around .300?. However, depending upon valve configuration and the positioning of the valve reliefs, the top ring can be moved down by as much as .450?. But on most small-block applications with standard in-line valves and a power adder, setting the top ring at around .300? protects the top land and the top ring from slight detonation and other conditions that may occur in forced-induction applications.
With the top ring positioned at a dimension of .260? down from the piston crown, the piston will accept 250 to 260hp shots of nitrous. The big consideration with nitrous, Kempf says, is air-fuel ratios. If they are inconsistent no amount of top-ring-down will save the piston.
On naturally aspirated engines a higher placement of the top ring is desirable. As a result the crevice volume between the piston and the cylinder is smaller. The higher placement of the top ring also induces the induction gases to enter the chamber faster, as smaller crevice volumes provide quicker reactions to the initial pull. Super Stock racers, whose engine changes are closely regulated, always place the top ring as high as possible, according to Kempf.
Courtesy of Diamond Pistons