Wide variations in exhaust hydrocarbon concentration have been traced to the angular position of the top compression ring gap. Furthermore, with a fixed gap position, equally large changes in exhaust hydrocarbon concentration have been correlated with blowby flow rate, which was chiefly determined by the smallest of the two compression ring gap areas. Enlarging gap area increases blowby volume, which lowers exhaust hydrocarbon output. It is believed that the air-fuel mixture in the piston-bore-ring crevices escapes combustion and that both gap location and blowby flow influence the amount of this hydrocarbon-laden gas which makes its way into the exhaust gas.
A modification of the piston and top ring, called the sealed ring-orifice design, has demonstrated the technical feasibility of reducing exhaust hydrocarbon concentrations by minimizing the crevice effect, while cutting blowby flow below that possible with production rings.