Bentley Mulsanne Turbo

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SINCE W.B.’s words in April’s “Matters of Moment”, we have had the chance to sample the turbocharged version of the Bentley Mulsanne, albeit briefly. Unashamedly aimed at restoring some of the individuality to the marque, lost when the last R-Type rolled off the production line more than a quarter of a century ago, the Mulsanne Turbo will widen the appeal of Rolls-Royce Motors’ products as well as providing the first Bentley to be more than a radiator shell different from an equivalent Rolls-Royce since 1955.

Externally, the new car is distinguishable from the normally aspirated Mulsanne (which continues in production) only by the painted radiator shell, twin exhaust pipes and the very discreet “Turbo” badges low on the front wings and on the boot lid. With the exception of a leather rimmed steering wheel and another small “Turbo” badge, the interior is indistinguishable from a normal Mulsanne.

The turbocharged version of the 6,750 c.c. V8 Rolls-Royce engine is, like all RR products, the result of a long gestation period. RR engineers first looked at the possibilities of turbocharging back in 1974, but the project aimed specifically at providing the then unnamed successor to the T-Type with considerably enhanced performance did not get underway until 1977. The result is a vehicle fully in keeping with RR practice, yet with that sparkle of extra performance so much in the Bentley tradition.

A Garrett AiResearch turbocharger is situated at the front offside of the already crammed engine bay and is fed directly from both exhaust manifolds. It blows through a Solex four choke carburetter which is situated in the V in a sealed air chest which is finished in the traditional black stove enamel with “Turbo” cast on the cover. A waste-gate limits the maximum boost (a figure not quoted by the manufacturers) but in addition there are sophisticated control systems to ensure that the set up is running at the optimum efficiency at all times. An air dump valve is fitted to the compressor side of the turbo unit so that when there is a depression in the inlet manifold (such as when cruising on part throttle), the output from the compressor is returned to the inlet, thus allowing the engine to operate as a normally aspirated unit but at the same time keeping the turbine running so that when further demand is made on the engine by opening the throttle further, there is no delay in providing boost for the carburetter.

Within the engine itself, special pistons are used which have been designed to withstand the higher mechanical and thermal stresses associated with a turbocharger installation — steel struts are inserted into the piston casting to control the differential expansion of the cast iron cylinder liner and the aluminium piston to ensure that optimum running clearances are maintained throughout the operating range. The compression ratio, static, is 8:01, but this rises effectively to some 11:1 at full boost. The fuel system and exhaust systems have been re-designed to cope with the additional demands placed on them, while the distributor is new and includes a knock sensor as well as having advance and retard characteristics designed to suit the turbocharged unit.

The transmission is mainly unaltered, although an uprated torque converter is fitted to the automatic transmission to cope with the 50% increase in power claimed for the engine and the rear axle ratio has been raised to 2.7:1 to improve high speed cruising and give better fuel economy. The half shafts have also been uprated and specially designed Avon VR tyres are used exclusively.

The experienced driver can tell straightaway that the cache is driving is turbocharged, even if there are no badges on the vehicle to tell him so before he sets off — or rather that used to be the case, for there is absolutely no hint of a turbocharger at work in the Bentley Mulsanne. No whine, no lag, no surge, no strange exhaust note, just smooth continuous power whenever it is required. Those familiar with the Spirit or Mulsanne in normally aspirated form would notice a slight increase in exhaust noise within the car, nothing unpleasant, just a slight but noticeable burble, and they would certainly remark on the increase in performance, but nothing more. When one considers that the all up weight of the vehicle approaches 2 1/2 tons, an acceleration capability of to 60 m.p.h. in 7 1/2 sec. with an automatic gearbox is very impressive. Top speed is well in excess of 130 m.p.h. — we were able to push the needle right off the 140 m.p.h. speedometer without any trouble, at which speed a colleague, travelling in the back, was able to write notes in a perfectly legible hand. Perhaps most impressive of all is the pick up in the middle of the range — just where it is needed for safe overtaking. Power is on tap instantly and 50 to 70 m.p.h. is achieved in a mere five seconds without using the kickdown.

The standards of comfort, of course, are all that one expects from a Rolls-Royce, and the new Avon tyres have not affected the ride despite their much stronger construction. It takes quite a while to get used to the Mulsanne Turbo, for it is so effortless that one is invariably travelling at about 20 m.p.h. faster than one thinks. The brakes are well up to this, but the steering is not as convincing as it might be, lacking feel and making it difficult to place the car absolutely accurately. To get the best out of the power available firmer suspension would be an advantage, for the amount of body roll when cornering fast is uncomfortable. Fuel consumption is not going to worry the owner of a car costing £58,613, but RR claim that the turbocharged Mulsanne shows an overall improvement on its normally aspirated brother of some 5%. A completely different package to the last “Blower” Bentley, which was a speed machine pure and simple, the new car from Crewe provides the rushed hush of the eighties just as the 8-litre Bentley gave discerning motorists of the thirties the chance to travel very quickly, in comfort and quiet. — P.H.J.W