Buyer’s guide: Ford Sierra RS Cosworth

| 5 Apr 2019
Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

Why you’d want a Ford Sierra RS Cosworth

The RS Cosworth gave the Sierra range proper ‘street cred’. Ford Motorsport under Stuart Turner needed a Group A saloon to replace the Capri. Conveniently, Cosworth had developed a 16-valve twin-cam version of the Pinto engine that, once turbocharged, would create a stonking Touring Car in the light three-door fleet Sierra bodyshell.

Aerodynamic work yielded effective spoilers; a nose intake fed air to the intercooler. Four-wheel disc brakes with ABS and 7x15in alloys were specified, but the RS tag was used only in the UK.

Initial sales network reaction was poor, so Ford massaged the price and spec, restricting the model to one trim colour and three external: Diamond White, Black or Moonstone Blue.

Having built enough cars to homologate for Motorsport by the end of ’86, including 500 shells set aside to be the evolution RS500, Ford’s next Cosworth would be a more civilised road car, not a competition special. The new four-door Sapphire saloon shell proved no heavier than the old three-door, but more aerodynamic.

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth
Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

The RS500, modified from standard three-door Cosworths by Aston Martin Tickford, was the most successful on track and is now the most desirable, but was barely any quicker as a road car. All were right-hand drive, intended for the UK, and almost all were black. The engine featured a stiffer block, larger turbo and intercooler, twin-rail injection and modified inlets.

Road power only went up from 204 to 224bhp, but race trim soared from 350 to 550bhp. Spoilers increased downforce and the rear suspension allowed more adjustability.

Copies are common: Ford sold a vast range of development parts and it was possible to uprate to RS500 spec. Ford Motorsport part numbers begin with code 909. RS500 chassis numbers on the slam panel and beside the driver’s seat are WFOEXXGBBEGG 38600-39099; engine numbers (remove cambelt cover to see) are YBD 0015-0537.

Finally, in 1987, the new MT75 transmission was strong enough to mate the Cosworth engine to the 4x4 chassis. Ford wasted no time in developing the Cosworth 4x4, with a stronger block, revised head and manifolds, plus a slightly larger turbo than on the Sapphire, giving peak power of 220bhp to cope with the extra friction and 210lb penalty of the four-wheel drive.

Originality, meticulous servicing and lack of rot are the key areas when buying any Cosworth Sierra.

Ford Sierra RS Cosworth: what to look for

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth


See above for trouble spots

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth
Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth


Meticulous servicing is vital. Upgrades are common but cause problems: standard engines must stay below 330bhp. The cambelt should be changed every 30,000 miles or five years. Listen for piston slap when starting from cold, and have a compression test done. Misfires can be minor, but indicate serious issues. A rebuild costs £3-5000.

Careful maintenance, especially avoiding air leaks, prolongs turbo life. Remove the inlet hose and check fore-aft movement of the impeller (there shouldn’t be any).

Internal corrosion leading to head gasket failure is common: look for coolant leaks on exhaust side and signs of overheating. Confirm that fans work at full capacity.

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth
Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth


Ford’s strongest 71/2in diff was used, but isn’t indestructible; listen for noises. Discs warp with heavy use. Dampers don’t last that long; yellow Konis should be black.


Rubber suspension bushes need regular replacement (or polyurethane), especially in the steering track control arms and anti-roll bar mounts.

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth
Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth


Recaros are good but cabin is otherwise cheaply made. Look for cracked plastic dash top, worn seat bolsters and failed controls. Trim material is hard to find.


Borg Warner T5 transmission is strong, albeit notchy when cold. Check for worn synchro on second and third; baulking on all gears might be a failing clutch cable.

Ford Sierra RS Cosworth: on the road

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

Our feature car is one of the most original examples around, yet even it has had the Stage 1 tune – a simple remap that’s said to be better for the engine, which otherwise is a little strangled in standard form. A Cosworth should be reliable, dependable transport, although minor electrical issues can have a massive impact.

Poor supply to the fuel pump can make the engine run lean and melt pistons: a separate fused supply is vital. Check the fuel pressure regulator at the front of the fuel rail, especially its pipework and hoseclips: leaks, again, have disastrous results.

Overfuelling is equally dangerous, commonly caused by sub-standard ‘uprating’ chips: it dilutes the oil, causing rapid engine wear. It cannot be over-stressed how important is thorough maintenance by someone who really knows Cossies: be wary of buying a car that has no evidence of such care.

The cooling fans also need their own electrical supply. Engine bay wiring degrades with heat and age, sending intermittent signals to the ECU: again engine failure can result. Old and poorly installed alarms give many problems.

Misfires are common; the rotor arm, dizzy cap, leads and plugs must be 100% (or uprated to a wasted-spark system). Crank and cam sensor settings are also critical. Always compression-test a misfiring engine before buying, in case a holed piston or similar is to blame.

All Cosworths should feel taut and lively, with not too much lag and no wallowing or creaking – anything less needs attention. Check for blue exhaust smoke (valve seals) or white (turbo wear).

Ford Sierra RS Cosworth price guide

4dr RWD

  • Show/rebuilt: £14,000
  • Average: £9000
  • Restoration: £5000


  • Show/rebuilt: £19,000
  • Average: £12,000
  • Restoration: £8000


  • Show/rebuilt: £30,000
  • Average: £20,000
  • Restoration: £15,000


  • Show/rebuilt: £70,000
  • Average: £50,000
  • Restoration: £35,000

Ford Sierra RS Cosworth history

1983 Mar RS Cosworth specification set out

1985 Mar Launched at Geneva Motor Show

1986 Series production gets under way at Genk, Belgium; 5000 built by December

1987 Jan Homologation achieved; 5542 three-door Cosworths built, 2616 for UK

1987 Jun RS500 unveiled: 225bhp, 206lb ft, limited-slip differential; homologated 1 August; Cosworth wins World Touring Car Championship and RAC Open Rally Championship

1988 Jan Four-door added (Sapphire in UK), 13,140 built; RS500 wins European and British Touring Car Championships – and many more

1990 Jan 4x4 four-door announced, with louvred bonnet; deliveries from April, homologated 1 August. Engine YBG with cat, YBJ without

1992 Manufacture ends (with circa 12,250 Cosworth 4x4s produced)

The owner’s view

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

Owner Ben Corner in his Cossie

“I was 20 when it came out,” remembers owner Ben Corner. “I was walking through Hereford when a stunning white RS Cosworth drove past and I wanted one immediately. I love the massive wing on the back, the big gaping hole at the front and the whole attitude of the car.

“My friend Sean Puckle bought this one from its first owner, a Grimsby businessman who felt so strongly about it that he interviewed Sean before agreeing to sell it to him. Sean won lots of awards with it and said he’d never sell, but one day he weakened and I jumped in quick before he changed his mind.

“I’ve had the Cosworth four years now and love driving it. It was my first turbo car and I find the lag quite fun; you can feel it coming in and then it really takes off. And it’s better put together than an Integrale.”

Also consider

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth
Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

BMW M3 (left) and Mercedes-Benz 190E are alternatives


Extensive re-skinning for lightness and a purpose-built four-pot gave BMW a demon homologation special. Beware subframe cracks (and rust), especially on lowered/stiffened cars.

Sold 1986-’91 • No. built 17,970 • Mpg 20-28 • 0-60mph 6.7-6 secs • Top speed 143-154mph • Price new £22,750 (1987) • Price now £50-75,000+

MERCEDES-BENZ 190E 2.3/2.5-16

With a 16v Cosworth head, the 190 was only slightly slower than its rivals despite its heavy shell. A full history is vital and look out for rot, which can be extensive.

Sold 1983-’92 • No. built 26,128 • Mpg 22-33 • 0-60mph 7.4 secs • Top speed 138mph • Price new £24,670 (1987) • Price now £10-20,000+

Ford Sierra RS Cosworth: the Classic & Sports Car verdict

Classic & Sports Car – Buyer’s guide: Ford Sierra Cosworth

A good, unspoilt Cosworth is hugely rewarding to own; a botched, abused one is a nightmare. So buy carefully, especially if you’re determined to own a hatchback.

Prices are already sky-high for three-doors and well-preserved Sapphires are taking off, too. They are just as fast and arguably more usable, but will never be worth as much, so beware excessive prices.


  • Phenomenal performance
  • Iconic ’80s looks, particularly the hatchback
  • Practical and usable
  • Well supported by specialists



  • Fakes are common
  • Most original Cosworths have been modified
  • Many have been stolen and/or crashed, so a thorough inspection/history check is vital

Ford Sierra RS Cosworth specifications

Sold/number built 1986-’92/30,932

Construction steel monocoque

Engine iron-block, alloy-head dohc 16-valve 1993cc ‘four’, Garrett T03 turbo (T04 on RS500) and Weber/Marelli injection/engine management; 201bhp @ 6000rpm-225bhp @ 6000rpm; 204lb ft @ 4500rpm-214lb ft @ 3500rpm

Transmission Borg Warner T5 five-speed manual, driving rear/all wheels (via LSD on RS500)

Suspension: front MacPherson struts rear semi-trailing arms, coil springs, telescopic dampers; anti-roll bar f/r

Steering variable power assistance, rack and pinion, 2.4 turns lock-to-lock 

Brakes discs all round, front 11.1in vented, rear 10.7in (vented on 4x4), with servo and ABS

Length 14ft 5in-14ft 9in (4390-4495mm)

Width 5ft 7in-5ft 8in (1700-1720mm)

Height 4ft 51/2in-4ft 6in (1360-1375mm)

Wheelbase 8ft 7in (2608mm)

Weight 2629-2874lb (1195-1305kg)

0-60mph 6.6-5.9 secs

Top speed 142-148mph

Mpg 19-30

Price new £17,100 (1987)


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