The R31 Skyline introduced many new technologies and features. The HR31 was the first Skyline to be equipped with the new RB-series of engines. The HR31 RB engines are often referred to as "Red Top" engines because of the red cam covers. There were three variants. The earliest series of DOHC, 24-valve, RB engines used the NICS (Nissan Induction Control System) injection system with 12 very small intake runners, and a butterfly system to divide the intake ports in half for better low RPM performance. Later versions used ECCS (Electronically Concentrated Control System) engine management, discarded the twelve tiny runners for six much larger ones (though the cylinder heads still retained twelve individual intake ports separated by casting), and received a slightly larger turbocharger. Nissan's "RD28", a 2.8 straight-six engine, featured for a diesel option. Another technological first for the R31 was the introduction of Nissan's proprietary four-wheel steering system, dubbed HICAS (High Capacity Active Steering). The R31 series were also the only models in the Skyline family to feature a four-door hardtop variation. These models were generally badged as the "Passage GT".