The Spec-R differed from previous Silvia models by featuring a 6-speed manual gearbox as well as a 4-speed automatic transmission. The Spec-R also included extensive chassis and suspension strengthening via the use of larger anti-roll bars and strut bracing. The S15 featured the same 4-piston front brake calipers that were found in the Z32 300ZX but included a larger brake booster.
One of the biggest changes to the S15 model of the Silvia fitted with the 6-speed manual transmission built by Aisin Seiki was the implementation of a helical limited-slip differential. The result was a safer, more track suited drive; in some contrast to its drifting heritage and subsequent media attention. All other versions of the Silvia (S14, S15 Spec S JDM) came with the viscous limited slip differential.
HICAS (High Capacity Active Steering) four-wheel steering was available as an option.
The Spec-S featured a 5-speed manual transmission (in addition to a 4-speed automatic available on both the Spec-S and the Spec-R). It lacked the additional chassis support of the Spec-R; featured 4-piston front brake caliper and a slightly smaller brake booster. The Spec-S came only with an open differential. Australian-delivered Spec-S models featured the same helical differential, chassis bracing and 6 speed manual transmission as the Australian market Spec-R models.
The S15 line was later expanded to include various luxury and upgrade option packages for both the Spec-S and Spec-R. Autech, a specialty car developer, also offered several tuned versions of the S15; one with body and interior trim modeled after the Ferrari 456, called the style-A, available in both Spec-S and Spec-R based trims; and a second tuned version was based on the Spec-S trim level with the engine output increased to 200 hp (150 kW) through the use of increased compression, more aggressive camshafts, and free-breathing intake and exhaust tracts, along with ECU tuning and upgrades to the chassis and suspension. This version also included the 6-speed transmission and other upgrades normally found only in the Spec-R.
Reference : Wikipedia