MG Metro 6R4
The Winged Wonder




W hatever else one may say about what is now the definitive car, there was never an MG like the 6R4, sorry, MG Metro 6R4. What, it is now obvious, was the firstthoughts four-wheeldrive rally prototype, powered by a mule V6 made by cutting and shutting a 3.5-litre Rover V8. has changed tremendously.
The biggest difference is the specially designed and built four-camshaft 24-valve normally aspirated 3-litre V6 which, as originally promised, is no relation of the GM-originating Rover or the coming Honda V6 of Project XX. That ex-Rover 2.5-litre V6 was said to turn out 320 bhp.
The new 3-litre is capable at the moment of up to 410 bhp at 9000 rpm. As is increasingly so with most of Austin Rover's future rivals, the pretences of what are really silhouette racers are thinner than ever, with even less of the first thin skim of Metro, and the differences are compounded by the most overt aerodynamic aids yet seen in a Group B rally car.

Austin Rover Motorsport (ARM) director John Davenport plans to have the 200 homologation examples built in time to enter the car for this year's RAC Rally in November.He hopes to offer what is officially called the Clubman 'production' 250 bhp version for around £35,000 although, like several rivals and unlike Ford's RS 200, ARM is not planning to get the car through Type Approval so, if you fancy buying a 6R4, you will have to register it somewhere not subject to Type Approval regulations.

How could it happen?

One asks that question not in the censorious tone of a scandalized shareholder but more in straight wonder. The actual cost to ARG of the 6R4 programme has, one may be sure, had to be kept down, even when it is perfectly right and proper to sell the idea of competing in the World Rally Championship from 1986 as a potentially powerful publicity builder.
Obviously there have heen some short cuts taken, both to save time, the aluminium castings for the cylinder block and heads are done by the immensely experienced Cosworth at Worcester, as will be the complete assembly of heads and to save money. by not seeking Type Approval.
Some beginnings of an idea of the investment is given by multiplying that £35,000 said to be near enough the actual cost price of each 6R4 Club - by 200 (£7 million), and remembering that doesn't include the 20'evolution' 6R4 International versions.
After the TR8's last event, the 1980 RAC Rally. Davenport started looking for the next rally car. There was nothing near right in the production range, and he decided there was no alternative but to build a special rally car.
"I managed" says Davenport today "to persuade our director Tony Ball to part with £50,000 for a special" (1982) "NEC Show project car. We nipped down the road to see Patrick" (Head, designer of the then partly BL-backed Williams Grand Prix team at nearby Didcot) "and said let's see if we can design a rally car."
It stood a better chance of board approval if it bore some relation to a production model hence the Metro hit. Metro size had sonic virtues. notably intrinsically less weight. and some restrictions, like its shortness and, from a rally-engineering point of view, lack of space.
The show car did not appear but, as is now well known, Patrick Head and Williams produced the car. At first, it was going to be front-engined, rear-drive, like the Escort RS 1700T, with, a rear transaxle, but "not long after, we decided to swap that round and go four-wheel-drive, which is very similar to the RS200 then we changed to the final layout," with the engine back to front driving a midgearbox behind the seats, and the rear axle line passing under the hack of the sump.
It is one thing to decide to build a special chassis like the largely spaceframe basis of the 6R4, but another to design and build a pure rally engine. ARM did consider the possibility of using any of the existing or future production engine range, and turbo or otherwise supercharging but, given what the opposition were doing, something special had to be provided.
In the end, the only Rover V8 parts used in any 6R4 engine were the connecting rods, and then only in the Club version. The rest is the design of David Wood, who earlier made a name for his work on various Cosworth racing engines.
The evolution of the final pair of engines is interesting. For a start, Wood and Davenport felt that the turbocharged route was wrong for the Metro. Its attractions as a power unit for what remains the smallest rally supercar to date included the thought of a relatively small primary engine which is not difficult to make very strong so that its output could be boosted easily to match the inevitably rising competition. But doubts arise (a) about how much of such spectacular power claims actually apply when the engine is moved from the controllable and unlimited cooling of the test house into the inevitably crammed engine compartment of a car. and (b) about how much of such power translates into improved power-to-weight ratio when it brings with it more and more cooling equipment weight. It is reckoned as a rough rule of thumb that, for every horsepower added in such cars, one third is lost in carrying the extra cooling.
The case against the high output turbo engine also includes evidence of elaborate controls; complication and care to guard against turbo failure; the driveability nuisance of turbo lag, the lack of instant and effective over-run braking for responsive car control, and the need for bigger brakes to cope with the weight of added engine ancilliaries and bigger tankage to keep up with worse fuel consumpsion and the extra left-foot braking required to keep this turbo spinning on the approach to corners.
In Austin Rover's case, it is also admitted that the smallness of the Metro basic restricts the amount of heat exchanger space available. The Austin Rover team is dominated by the idea of a simple car, simple to drive, simple to control.
An efficient unblown engine may not be capable of the same ultimate maximum power hut, if suitably designed, it should be instantly responsive both when opening or closing the throttles, uncomplicated and less heavy itself, therefore requiring a less heavy car to carry radiators, pipework, insulation and brakes.
Early experiments were with the cut-down Rover V8 with its 90 deg. block. Wood says these led to the replacement of the cut and welded Rover block with a specially cast block, and thence (after acknowledging that the 1.25 to 1 Rover bore to stroke ratio and 2.5-litre capacity were restrictive) to the final completely new design which, however, retained the 90 deg. V rather than the 60 deg. nearer ideal for a V6.
On the `International' or full rally version, the crankshaft is machined from solid En 40b where the same size `Club' one is also fully machined but from a part forging. One balances such a crankshaft as three separate 90 deg. V-twins as far as primary (reciprocating) forces, which leaves just the secondary imbalance to deal with.
The crank is thus kept short and stiff, with 2.23in diameter main bearings and 2.23ín dia by 1.7ín wide crank journals to minimise the effects of the secondaries with a three crankpin layout, and does not need any crank damper. Each crankpin bas two'5/8in dia bores to lower weight. To reduce the considerable power needed at top revs to drive the feed oil pump of the dry sump system, the crankshaft oilways are drilled eccentrically, close to the journal surface, so that the pump only has to counter roughly 1/4in of radical passage to force oil inwards against centrifugal force into each longways drilling.
The rally connecting rod is a thing of mechanical grace, with its deep, slender webs and shotpeened finish the 0.866ín dia gudgeon pin is fully floating, circlipheld in the three-ring Mahle slipper piston whose slightly domed crown is deeply recessed for valve clearance in a 12-to-I compression ratio space.
Deep clearances are necessary because of the comparatively high valve lift on the rally engine 0._512ín or 13mm (against 0.413m or 10.5mm for the Club).
Returning to the cylinder block, its skirts are continued well below the crank centreline to be further stiffened by the necessarily stronger than usual cast sump which has to sustain some transmission loads from the rear final drive bolted partly to its right hand rear (as mounted in the car) face, and partly to the corresponding face of the block, and the nearside drive shaft coupling on the end of the tubular 1-1/4in dia fixed shaft which runs across the sump in a cast tube under the rearmost main bearing cap.
Cylinders are dry cast iron liners.

Cast in the same heat-treated LM25 aluminium alloy as the block, each one-piece cylinder head carries its three bearing inlet and exhaust camshafts working conventional but largediameter inverted bucket tappets against double springs and 1.417in (36mm) dia inlet and 1.181ín (30mm) dia exhaust valves in aluminium bronze insert seats.
The valves are set at 38 deg. included angle. with straight inlet ports and exhausts curved down through 30deg. Each pair of ports merges into one just inside the head. In the rally engine, the ports are polished: in the Club. they are left as cast. Combustion chambers are typical of four-valve engines, mildly pent roof and fully machined, with a single central plug.
From on top, the stiff design of the upper side is seen, with its neatly cast-in open girder bracing round the plug tubes and across the space between the camshaft and tappet housings.
In its at-first-sight mass of belts at the (car-) rear of the engine, the 6R4 unit reminds one somewhat of the short-lived 400bhp Cosworth GA 24-valve 3.4-litre conversion of the old Ford Essex 3-litre for the racing Capris of 1974.
Starting at the innermost end, there are the two toothed timing belts, one per bank, then another toothed belt for the feed and scavenge lobed rotor oil pump mounted low in the car left and finally the ribbed belt running up to the alternator in mid-V.
The intention is for all 6R4s to have fully enclosed cam drive belts. The centrifugal water pump lives at the other end of the saddle between the banks, and has two outlets, one to each side of the block after which coolant runs along the blocks up into the (car-) back of the heads and forwards. It is belt driven off one end of the right hand inlet camshaft, the other end carries the breakerless distributor.

Thoughtful electronics

Metro 6R4 has an interesting degree of electronic engine management, designed to Wood's specification by Lucas Micos, a small research and development arm of Lucas. Because of the 90 deg engine disposition and the 1-6-3-5-2-4 firing order (cylinders are numbered 1,2,3 right and 4,5,6 left, with cylinders 1 and 4 nearest the engine's front), the ignition system has to deliver sparks at uneven intervals. at 90 and 150 deg. crank angle.
The system has a 'mapped' EPROM memory for optimum spark timing for all conditions. and takes its engine speed and crank position readings off a combination of a 24-lobe interruptor disc on the end of the flywheel and a 6-lobe interruptor ring on back of the (car-) left hand inlet camshaft.
Each generates signals via a Hall Effect transistor magnetic field sensor. The halfengine speed 6-lobe ring has one gap larger than the rest to indicate crank position, and since spark synchronisation is vitally important, it is used in conjuction with the crank sensor and the control unit to shut down the engine automatically if a timing problem is detected.
The same people are responsible for providing the comprehensive control system for the constant pressure variable injection time fuelling system. Using five sensors of ambient, coolant and fuel temperatures, atmospheric pressure and engine speed in addition to conforming to the usual load and engine speed signals, it is capable of modifying the amount of fuel injected correctly according to those other five variables.
It has other refinements. A rally car which is switched off when hot can be most reluctant to restart because of fuel vapour lock caused by engine heat soaking out into the injectors and fuel rail. The fuel temperature sensor, which is placed in the injector fuel rail, therefore bas another job besides its function when the engine is running of telling the control unit to inject more petrol to offset hot, less dense fuel. When the engine is stopped, and as soon as the fuel warms up with heat soak, it switches on the Bosch high pressure feed pump to circulate fuel round the system and prevent vapour lock.
The Micos system is selfdiagnostic in that a dashboard warning lamp will light if a fault occurs. The 6R4 fitter at the service point presses the appropriate button so that the warning light flashes a faultidentifying code of rip to 12 flashes.

Flexible output

The claimed maximum power output is 410bhp at 91O0 rpm in tarmac tune, with 270lb ft of torque at 6500rpm the alternative state in loose surface tune of 380 bhp at 8500 rpm is obtained with longer intake trumpets and altered camshaft timing. Beside the differences already mentioned.
The Club engine has different pistons, valve springs, camshafts and inlet manifolding the last is a conventional V-engine type one associates with a central carburettor, except that its single combined intake carries only a throttle butterfly to feed the multi-injector system.
This milder tuned engine still delivers a claimed 250 bhp at 7000 rpm and 225lb ft at 4500 rpm.
One does not associate engines delivering a claimed specific output of over 130 bhp per litre to be flexible. Yet that is exactly what this engine is designed to be. relatively speaking. Its makers claim that in the top performance form, it is already delivering 230 1b ft 85 per cent of the peak figure at as low as 3500 rpm. without dropping below that level until after 9000 rpm. 500rpm before the maximum safe engine speed (which is guarded by an electronic cut-out).
This is partly thanks to what sounds like good porting and combustion. but also to the comparatively generou, valve lift and the valve timing the inlet opening 52deg b.t.d.c.. shutting 72deg a.t.d.c., exhaust opening 74deg b.b.d.c. and shut-ting 50deg a.t.d.c., ;giving 304 deg of total inlet opening period. Corresponding figures for the Club camshafts are 20-52-52-20, riving a 252 deg period.

Thanx to Michael Scarlett from the carmagazine "Autocar".

 
















 

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july. 2004 © rwp