Jack K. Yamaguchi has this to say about the series 4 turbocharger: “The twin scroll turbocharger concept was developed jointly by Mazda’s Department Five and by Hitachi, and is manufactured by Hitachi having been designated HT18S-2S. It’s turbine-side bearing area is now water-cooled, and the main body is further protected by high temperatures by a dual-skinheat shield. The 1 blade turbine, of impact design, is 64 mm (2.52 inch) in diameter. It’s scroll area, the nry path for exhaust gas, is divided into two passages separated by an integrally cast wall; one, called the secondary scroll, has a trap-door-like gate operated by intake vacuum. The turbine thus has two cross sectional area (a) factors, A being the scroll’s smallest area as used in calculating turbocharger performance characteristics: 6.00 sq. cm (.93 sq. in) for the primary path and 9,48 sq. cm (1.47 sq. in) for the secondary. The other factor is R, the distance between the turbine-shaft center and the center of area A: 59.4 mm (2.34in) for the primary path, 60.4 mm (2.38in) for the secondary. Thus the twin-scroll turbocharger has two A/R ratios: 0.4 for the primary and 1.0 for the primary and secondary combined.”

As for the compressor side, “its 12-blade compressor is 63 mm (2.48 in) in diameter.