Ferguson Formula Capri
Reviewed by Unique Cars and Parts
Our Rating: 5
Ferguson Formula All-Wheel-Control
One of the lesser known iterations of the iconic Capri was the Ferguson Formula. Only subtle clues marked this Capri as different from the thousands of other Cologne-built coupes. A skid plate neatly tucked under the front end and a small bulge to the right of the transmission hump hinted at the extensive modifications within.
On the rear deck was a discreet anodized plate with the script "Ferguson Formula All-Wheel-Control." Unique Cars and Parts
are not sure if any made it to Australia, and assume if any did, they were private imports. The story was different in the USA, where Simon Haberman of FF4 Automobiles Limited, New York imported legal conversions.
Major A.P.R. (Tony) Rolt
The FF Capri was shipped from Cologne, West Germany, to the shops of FF Developments Limited near Coventry in England. FF Developments was owned by former Jaguar racing driver Major A.P.R. (Tony) Rolt. who had been one of the principal developers of the Ferguson system. At Rolt's facility the front suspension was removed and the conversion to all-wheel drive began with the installation of a special sub-frame in place of the standard cross-member.
This performed the multiple function of providing attachment points for the steering and suspension control arms while allowing sufficient room for the aluminium-cased front differential and half-shafts. The custom front wheel hubs retained the stock disc brakes. The differential was hung from the front motor-mount attachment points and the engine oil pan was modified on the right to allow room for it. Moving rearward, the tailshaft of the Ford automatic transmission was removed and in its place a centre differential was bolted in.
The stock transmission mount was retained. The forward driveshaft snuggled along the right side of the transmission and a minor shortening of the rear driveshaft completed the basics of the conversion. The essential element in the Ferguson system was its automatically controlled centre differential, which divides driving torque between the wheels and permitted the variations in wheel speed needed for scrub-free cornering. While a conventional differential would have met these requirements, it would have suffered from a complete loss of traction if one wheel had insufficient grip. To surmount this problem, the Ferguson Formula employed a hydraulic slip-limiting unit which coupled the front and rear wheels together as a function of the proportion of speed difference between the unit's front and rear output shafts.
This limitation of differential action meant that in no circumstances could a single wheel - or the front or rear pair of wheels - spin under acceleration. Likewise, individual wheels or pairs of wheels could not lock under braking. This potentially useful behaviour was accomplished by a differential unit which had undergone significant development since its previous incarnation in the discontinued Jensen Interceptor FF
. By the time it was fitted to the Capri
, it was some 15 percent smaller, 30 percent lighter and 50 percent less expensive than the unit used in the Jensen. As before, the differential was of the epicyclical type which permitted an unequal division of torque front to rear. In the case of the Capri and most other conversions, torque was allocated 37 percent front and 63 percent rear.
This unequal division allowed the vehicle to handle conventionally and also permitted the use of a smaller front differential. The major change to the centre differential was in the means of obtaining limited-slip action. In the old Ferguson unit, slip-limiting was effected by a pair of ball-and-ramp clutches. One clutch locked if the rear wheels overran the fronts; the other if the front wheels overran the rears. No slip-limiting was available in reverse. Replacing the clutches in the Capri's differential was a viscous coupling. The torque resistance of this coupling went up as the velocity difference between its two halves increased. The effect was to lock the wheels together and to maintain a constant rotational rate for all four wheels. With the viscous coupling, slip-limiting was available in both forward and reverse and the unit also cushioned driveline shocks originating at the wheels or from the engine.
On the Road
Of course we have never had the opportunity to get behind the wheel of the Ferguson Formula Capri, but from reading reports from the handful that did, it was apparently a rather intriguing experience; unlike conventional 4-wheel drives with their brute force qualities, the Ferguson was apparently a study in agility and sophistication. At parking speed slight heaviness was felt in the steering but vanished above a brisk walking pace. There was no surge or tyre
scrubbing at either full-left or right lock but the car did have a turning circle greater than the stock Capri. On the highway few claimed that it felt much different to the standard Capri, which was a very good thing.
There was one small exception, some drivers claiming a slightly sharper response to tar strips and potholes. But at lower speeds understeer was no stronger than a standard Capri. Trailing throttle induced a pronounced but manageable tightening of the line. The FF Capri was fitted with stock Dunlop SP57 185-13 tyres
– although we would think plenty decided to improve on the rubber when they took possession.