The first series (big turbo) of the Escort RS Cosworth was the "homologation" base used for the rally cars. The major competitors in WRC (those with the most sponsors and subsequently the most important budgets) had cars which remotely resembled the base (production) car but were light years away from it mechanically. Here are the major changes in the Group A car as compared to the production car:

Chassis are reinforced (roll cage), seam welded and many new welding points are applied.

The suspension housing tunnels are reinforced with welded upper aluminum/titanium plates to allow suspension angle adjustment

The brake system is completely free in GroupA. No ABS or servo assistance is used. There are two brake circuits and two master cylinders. The front brakes (depending on the type of rally) are usually Ø378mm with 8! pot calipers and 4 brake pads each. Rear brakes are Ø315 with 4 pot calipers. The front brake calipers are water cooled with separate water pumps, radiators and reservoirs. Braking force bias (front/rear) can be changed from the cabin

The gear box used is an MS95 7 speeder (non sequential) with the first ratio removed. Dual plate carbon dry clutches are used. The gear box uses an oil pump and an oil radiator to keep cool. (Note that this gearbox was replaced by an X-Trac sequential one since the New Zealand '97 rally)

The steering is a rack and pinion based high ratio (12:1) with 2.4 turns from lock to lock. Servo steering assistance is available. The steering oil uses a radiator for cooling

A bigger intercooler is fitted in a different location than in the production car. Additional cooling is provided by an external water spray system. Air intake temperature is always around 40°C ! Water injection is also fitted

The fuel tank is re-located inside the car's trunk and is compliant to FIA regulations (80 lt. capacity)

The engine is heavily modified. The crankshaft is made of a single piece of steel and is lightened and balanced. Titanium driveshafts are used as well as forged aluminum pistons. The outlet valves have lithium inlays. The engine head is polished and wider valves are used throughout. The camshafts are specific, the compression ratio must be around 8.5. An FIA 34 mm restrictor is used on the turbo. Two high pressure fuel pumps are used. Two supplementary liters of oil are used, compared to the production car (a total of 8lt). Magnesium oil sump. Turbo boost pressure is in the 2.5 bar+ region. Bang-bang switchable from the cabin

The transmission system is also completely different than the one fitted in the stock car. Three self locking differentials are used. The center and front ones are computer managed. Two hydraulic pumps feed them with pressure (0-50 bars). The computer management can be overridden by the driver who can adjust the torque distribution, between the front and rear axle using an onboard button. The center differential is automatically disconnected whenever the driver pulls the parking brake. All differentials are connected to oil coolers
 

Engine:
Four cylinder 16-valve two liter Cosworth, Garrett AiResearch T35 turbocharger with FIA 34mm air restrictor.

1997:Hybrid IHI turbocharger and 8 injector inlet manifold
Modified compression ratio, modified crankshaft, all internal cleaned and polished, altered flywheel to suit clutch. All engine internals prepared by Mountune. Cast magnesium sump with mounting for Motorsport 8.5 in front axle, Kevlar sump shield. Three stage intercooler with water injection. Intake temperature maintained at optimum, approx. 40 degrees. Anti turbo lag system, Weber Marelli engine management electronics.

1997: Ford/Pectel T6 engine management, Turbo boost set at approx. 3bar

Engines prepared by Mountune, by 1998 they were prepared by Tom Wilkinshaw Racing (TWR) who also prepared the Arrows F1 engines.

Max power: 312bhp @ 6250 rpm*. Max torque: 450 N/m @ 5000 rpm

Width: 1738mm, Height: 1360mm Wheelbase: 2552mm
Laden weight: 1230Kg

Varies depending on spec. Acceleration: 0-60 in approx. 3 seconds, no more than 3.5 sec. in any spec. Top speed: 160km/h to 220km/h.
 

Mid-97:

Six-speed sequential XTrac box. Electo-hydraulically controlled front and center differentials, Torsen rear differential for tarmac, ZF rear differential for gravel.

Racing seats fully tested for impact resistance, tailor-made to fit driver. Steering column adjustable for height, different length wheel bosses used to suit driver. Adjustable pedal box. Aluminum mounting bolts used wherever possible to trim weight. Aircraft-style self-sealing fuel cell with quick-release twin fuel pumps inside tank. All encased in aluminum shell. Fully fireproofed hosing runs throughout the interior, including fuel lines, water injection lines, electrical wiring. Aircraft-style wiring harnesses differ on each rally depending on conditions. Driver information limited to rev. counter and warning lights. Brake bias adjuster knob. System monitor top center dash used by navigator and downloads to service crews for computer diagnosis. Center dash switches for heated screen, fans, fuel pump, lights, starting, ignition. Coralba navigation equipment, radio and intercom set mounted on co-driver's door. Horn button in co-driver footwell useful for alerting spectators when out of control. Map pockets.