De Tomaso made a decision to revamp the Pantera in 1989. It came on the back of a booming market that saw buyers clamouring to secure anything with the slightest hint of collectability. New supercars were seemingly being launched every other week, some coming from established players like Aston Martin and Ferrari whilst new firms like Cizeta sprang up practically overnight to cash in on the frenzied buying activity. By mid 1990 though, the speculator-driven boom had turned to bust and even the biggest firms had to tighten their belts.

Launched in 1990, De Tomao's heavily reworked Pantera SI arrived during the darkest days for supercar manufacturers since the 1970's energy crisis. A global recession massively reduced the number of customers looking to buy such an extravagant toy, however, the firm survived despite making a major financial investment in the new car and only a handful eventually rolling off the production line. The Pantera's traditional steel monocoque was fitted with new subframes, the double wishbone suspension being recalibrated with longer arms at the front (by 2.5-inches) while the rear lost around half-an-inch. There were also new Brembo brakes with drilled and ventilated discs and four-pot alloy calipers, no ABS or power-assisted steering highlighting the SI's old school roots. Gorgeous 17-inch Fondmetal five-bolt wheels were unique to this latest generation model and originally shod with Michelin MXX tyres. As normal, Ford supplied the engines. Fuel-injected for the first time, catalysed five-litre Mustang V8's arrived in Modena with around 225bhp, De Tomaso fitting new camshafts, cylinder heads, valves, intake manifolds and pistons. The net result was 305bhp at 5800rpm, enough to match Ferrari's V8 supercars, the Pantera SI excelling with its all-round flexibilty. Producing 333 lb ft of torque at just 3700rpm, it could pull smoothly in top from barely 20mph. Smaller and lighter than the outgoing Windsor V8's, the new 302 motor was coupled to a five-speed ZF gearbox and differential.

The cabin was subtly revised with new seats, switchgear and instruments, a new three-spoke leather-rimmed Momo steering wheel, electric mirrors and an air-conditioning system from the Maserati Biturbo. Otherwise, the familiar wood veneered dash remained practically unchanged. Leather was used to cover the seats, dash, door trim and transmission tunnel but there were no airbags. There were no official options either although customers could specify custom paint and leather to ensure they had a true one-off. Launched at the Turin Salon in April 1990, the SI remained in production until late 1993, by which time 41 had been completed. This was way short of the 75 per year originally anticipated and De Tomaso eventually decided to focus solely on the Guara. Chassis numbers ranged from 9601 to 9641, two cars being crash tested whilst four were official Targa conversions by Pavesi in Milan. One was completed in right-hand drive, this car being equipped with a twin turbocharged engine by the British De Tomaso concessionaires and dubbed the Pantera 200. Another SI was later converted to right-hand drive, chassis 9610, a 1991 car, being rebuilt by the factory between 2002 and '03 at a cost of €27,000. Finally, a mysterious French-registered SI was once kitted out in Police livery complete with lights and siren, however, little is currently known about this intriguing machine.