The first 911 went on sale way back in 1963. In this improved rendition for the late '90s, a rear-mounted, 3.6-liter air-cooled flat 6-cylinder engine produced 270 horsepower. Both rear-wheel-drive and all-wheel-drive Porsche 911 Carreras were marketed, in hardtop coupe and Cabriolet (convertible) form. The engine switched from solid to hydraulic valve lifters and a new dual exhaust system reduced back pressure while softening noise. A new 6-speed manual transmission replaced the prior 5-speed. In addition to the manual, rear-drive Porsches could have a Tiptronic 4-speed automatic transmission, which permitted changing gears manually by flicking a console shift lever or pushing fingertip switches on the steering wheel. Antilock all-disc braking was standard. An optional limited-slip differential incorporated an automatic brake differential, which used the brakes to prevent wheel spin at low speeds.