Technical evaluation - works Renault Maxi 5 Turbo.
Cars & Car Conversions. August 1985.

Tarmac Terror

Arguably, this year's Tour de Corse-winning Renault Maxi 5 Turbo is the fastest asphalt rally car in the world. Martin Sharp traces a remarkable technical and development story. Driving impressions by Chris Sclater .

May 1985-and the cream of European Group B rally car technology, together with its associated driving brilliance, left Verghia to do battle on the first special stage of the tortuous tarmac roads that form the Tour de Corse- Europe's most exacting road race. With the Lancias withdrawn after a tragic fourth test, and mechanical failure afflicting both Peugeot and Audi, cynics could easily dismiss Jean Ragnotti's victory in the Renault Maxi 5 Turbo as one of default.

But this is emphatically not so; the Maxi was fastest on three of the first four stages; when most of the serious opposition was still in the hunt, and after the cancelled fifth special stage, the Maxi was again fastest over 14 tests, second fastest over eight, and third over the remaining 25. Certainly Ragnotti is no stranger to this classic Corsican event, having won there in 1981, but it's worth remembering that this year (1981) he was driving a modified version of a car which contested its first rally on the 1979 Tour of Italy.

The measure of this year's remarkable achievement is that the Frenchman brought Renault's old underdog to Ajaccio a full 13 minutes and 12 seconds ahead of countryman Bruno Saby in a 430bhp 4WD purpose-built Peugeot supercar. The latter, with a massive 3621b.ft of torque, contested its first rally five years after the Renault. So not only is life left in the old dog; the Maxi 5 Turbo is arguably the fastest tarmac rally car in the world today.

After the still-born Ford GT70. and the final lancia Stratos, which left Chivasso in 1975, enthusiast appetites were whetted by an 'experimental' turbocharged mid-engined Renault 5 exercise which appeared at the 1978 Paris Motor Show. By September 1980 this appetiser was an entree. It was homologated into Group Four, and by January the following year Jean Ragnotti had taken it to win the 49th Monte Carlo rally. By now the scene was set and Renault knew that it had some appropriate equipment with which to contest World Championship rallies.

"The race isn't just on the road, it's also in the workshop. It was most important for us to change the brakes and the suspension , to cater properly for the increased power, but we just didn't have the time or facilities available, for instance, to engineer and homologate an alternative dashboard. It isn't the best thing in the car but at least it's got some of the right gauges"

Thus, after that Monte win, Renault began serious testing and development to hone its 2WD mid-engined turbocharged tarmac tearaway into a fully fledged World Championship winning machine. Renault determined to contest that year's RAC rally, and in preparation, took the car to Greece for its first serious gravel tests. Frantcois Bernard was with the project from the start and it was his responsibility to ensure that the resulting chassis specification would withstand the rigours of an RAC.

The team learned much in Greece, and it became plainly evident that the original French De Carbon gas-filled shock absorbers were insufficiently adaptable to their intended tasks. "The RAC was its first rally on Bilsteins, and the car wasn't so bad, although there was still a lot of suspension work to do". Even so, after a limited amount of gravel development for a car which had been originally conceived for asphalt, Ragnotti's fifth place overall confirmed its potential and pointed the way to more investment in the machine.

By 1982 the 5 Turbo had begun to make a name for itself, as customer versions started achieving notable results. Therier won the French Rally Championship, and Ragnotti took the honours in Corsica. But by this time the Renault team had developed its gravel abilities to the extent that they entered the machine in the tough Mille Pistes, and several other loose surface events. It even attempted the Bandama Rally in the Ivory Coast, and although Ragnotti's works car crashed out of the event, a 5 Turbo came home fourth, with Bruno Saby at the wheel. Bernard: "It could have been even better if Jeannot (Ragnotti) hadn't crashed, because his car was considerably better than Saby's".

At that time the Renault team had only one basic specification in Group Four - Ragnotti's Ivory Coast car was simply his Corsica winner with more underbody protection and slightly modified suspension. Yet after these gravel experiences, Bernard was somewhat surprised that he'd learnt a considerable amount about the tarmac handling of his charge because "the stress on the bodyshell during gravel events showed us some weak points in the shell which we had to reinforce for it to be satisfactory on gravel. This work increased the rigidity of the shell and improved its tarmac handling as well. We found that we needed to reinforce the suspension pick-up points and build some extra tubes into the front of the car. The rear suspension also needed reinforcement at its locations, and most of these modifications have been carried over on to the Maxi now".

Thus the Maxi 5 Turbo is the final result of Renault's Antony-based rally team's original response to the introduction of Group B regulations. Always the bridesmaid, Renault's Departement Rallye consistently takes a budgetary back seat compared to the Regie's less than successful but hugely expensive Formula One programme. Although many projects and several preliminary tests with 4WD cars have convinced Renault's rally people that to be competitive in the World Championship, all-wheel-drive is a must, number crunching and corporate politics have precluded its chances of building a 4WD rally car to match obvious rivals like Peugeot.

So the rally team set itself a task of building the most competitive 2WD Group B car from what it already had available.The new car was to be ready for the 1983 season, and Renault planned it in the summer of 1982. A particularly poor aspect of the Group Four car was its comparatively narrow front track- a factor compounded by the confined dimensions of the basic R5 design. But after a lot of work the result was an extra 7cm of front track width. With even more body reinforcement, particularly at the rear end, and bigger front wheel arches to accommodate bigger front wheels for the gravel, the basis of the first Group B Renault 5 Turbo was established.

Access was now available to the latest tyre technology because wheels of the same size as those tyres which were doing the winning could be fitted. The old Group Four car simply couldn't use the taller wheels such as those used by Opel and Audi, with the result that the car had been at a constant disadvantage. "That has always been a problem with the development of the car".By making it a priority to go to larger diameter wheels, Renault was able to use the latest Michelin tyres. That first Group B R5 Turbo came to be known as the Tour de Corse version after its 265bhp 1397ccengine had been modified to give 300bhp. The latter unit was first used in Corsica in 1983, but without success because the team couldn't get its tyres to work properly. To begin with, the fronts were very tall and narrow, causing a multitude of problems until wheel diameter changes- to 15 inches- allowed them to become shorter and squatter.

Thus equipped, the car started to go well, and led the '83 Tour de France until a broken rocker put it out. It then led the Rallye d'Antibes until Ragnotti put it off the road in the wet.

In 1984, with exactly the same car, but with different tyres again, not only did Jean-Luc Therier win the French Championship, he was also fourth at Monte Carlo behind three Audis, which were at the time revelling in their heyday; and Ragnotti finished a remarkable fifth in Portugal.

But during 1983 it had become clear that the bridesmaid, although remarkably successful in view of its stilted conception, was a little long in the tooth; yet comparatively paltry budgets still precluded any major Group B developments for the Renault rally effort. By the autumn of that year the team had decided to cut its losses and develop the utmost available out of the 5 Turbo.

Work began at the beginning of 1984; first tests being undertaken in October. The original intention was for the new car to be homologated by February 1st 1985, but "we finished the 20 cars by the end of 1984 which was later than expected, with the result that they were only homologated on March 1st this year. "The two priorities we wanted from the Maxi were more power and better handling".

While Philippe Coble nee worked on the engine systems at his office and workshops in Viry Chatillon, where Renault Formula One engines are also developed and assembled, Francois Bernard and his team set themselves to establishing the ultimate out of the, by now, ageing Turbo chassis. "Throughout the development of this car we have been constantly increasing its wheel diameters, and because the earlier cars all had developments of the original Turbo 1 and Turbo 2 road car suspension - reinforced, but using the same geometry- by this time we were not OK with the geometry of the original suspension. The roll axis and so on were way out. So we had to completely redesign it.

"We've increased the rigidity of all the suspension parts and particularly the steering where we have gone for something much closer to a circuit car. We've increased the wheel travel in both tarmac and gravel trim, although for asphalt it's not so much increased in absolute value, just a better balance between rebound and bump travel. . . it's now in the middle. With the old car, as we started using bigger and bigger wheels so not to have the car too high on asphalt we were much too dose to the bump stops".

The result of this work is a wheel travel increase of between 30 and 35 percent, together with the relevant geometry. And a good indication of the brinkmanship employed in extending the comparatively restrictive R5 Turbo base is the result- an impressive 8cm extra rear travel; producing a great improvement on gravel. "It now seems that we've come to the logical end of our available tarmac development, but we're in the process of doing a lot of tests on gravel and want to enter the car on more gravel rallies. OK, we've only got 2WD but we want to show just how far our car has progressed, and while some Opel and Toyota results indicate what can be done with 2WD on gravel, I think our car is capable of good results too".

Of course, the fundamental change for the team in making its decision to develop the ultimate out of the R5 Turbo, were the wheel size advantages in jumping the car to a higher engine capacity class in FISA's Appendix J rules which allowed wider driving wheels. Maximum legal widths are now established as the sum of the front and rear wheels on each side of the car.

Renault's resulting specification for tarmac is 15 inch diameter front and 16 inch diameter rear wheels, with 20 percent wider rubber at the rear. But tyre availability still remains something of a problem, and the team has not yet finalised the optimum available specification for its gravel surface rubber. At the moment, 390 section TRXs are being used on 15 inch wheels all round, but Renault has some 415 section TRXs, and if these come up to expectations, then there's a good chance that the Maxi will run the existing tarmac 15/16 inch wheel diameter set.up on gravel surface events as well.

To cope with these running gear changes, much strengthening has gone into the front axle, particularly around the ball joints, and while developing this new car, vehicle weight was kept under constant scrutiny, with the result that the Maxi weighs "slightly more than 900kg" - the 2.5 litre class minimum weight limit. .

With significantly stronger suspension, yet an ostensibly similar basic body construction to the old car, it's evident that this weight watching has been highly successful. The basic steel bodyshell remains in the metal construction of the Turbo 1 road versions, which also had an aluminium roof, doors and tailgate. The Maxi's tailgate, however, is plastic, as are its wings, bonnet, bumpers, side skirts and front wheel arches.

These plastic parts are either in Kevlar or carbon reinforced Kevlar depending on their functions, but the basic structures of the car- the load bearing parts - are still derived from previous Renault 5s. Even the Maxi's front frame bears some similarity to the first FWD R5. Its rear frame is still essentially similar to that of the roadgoing R5 Turbo, although extensively reinforced. And that reinforcement can be put into perspective when it's considered that the rally team's work in improving the torsional rigidity ofthe Maxi has resulted in a 35/40 percent improvement over its immediate predecessor, the 300bhp Tour de Corse version.

Some of this rigidity is imparted by the Maxi's aluminium alloy rollcage picking up at stress points, but the greatest influence has been the inordinate amount of effort expended by the development team in perfecting important build requirements under significant budget constraints. "The race isn't just on the road, it's also in the workshop. It was most important for us to change the suspension and the brakes to cater properly for the increased power, but we just didn't have the time or facilities available. for instance, to engineer and homologate an alternative dashboard to the standard Turbo 2 version which we use. It isn't the best thing in the car, but at least it's got some of the right gauges" .

Francois Bernard knew that corners would have to be cut if his latest 2WD all-synchromesh transmission and honed-to-the-limit chassis layout was to cope with the enormous BHP, gigantic torque, and remarkable throttle sensitivity of Philippe Coblence's 'new' engine. This is the sort of sensitivity which derives from many years of enormous investment in turbocharged Formula One engine technology, and Bernard and his team must be complimented for having produced a 2WD chassis which handles sophisticated engine characteristics in ways sufficient to generate envy even among 4WD motorsport chassis engineers.

Bernard Dudot is the man in charge of both Formula One and rally engine development for Renault, and both activities take place at the Amedee Gordini factory in Viry Chatillon, where ex-Matra supercharged diesel engine specialist Philippe Coblence choreographed development of the Maxi's engine.

Because the rally car needed bigger wheels to remain anywhere near competitive, it was Coblence's first task to take the 1397cc 300bhp Tour de Corse specification engine to a capacity which would slot the car into a higher class. The turbocharger equivalence factor put the Tour de Corse car at a nominal 1956cc, in the up-to-2 litre class. Thus, to slot in to the next class up - 2.0-to-2.5litres - would require the engine to have a minimum actual capacity of 1430cc. But because the regulations dictate that the original cylinder block must be used, Coble nee and his team decided to go for the absolute maximum capacity that they could get out of it - 1527cc. The latter figure equates to 2136cc with the turbo factor applied.

And this is where the design headaches really started. The Tour de Corse engine was just about on its maximum safe bore diameter at 76mm, and the team could only extract a solitary extra millimetre diameter as a result of fitting on-the-limit wet liners in the iron block. And so with a bore of just 77mm, the stroke needed some significant lengthening to arrive at the required maximum capacity. The latter was limited by the proximity of the connecting rods to the block. But the team went as far as they reasonably could and added an extra 5mm throw to the new crankshaft. The resulting 77x82mm bore/stroke dimensions are not exactly ideal for a competition engine, although there is obvious torque-producing potential there.

Such regulation-induced compromise design might jar the sensitivity of a perfectionist engineer; that the design of the rest of the engine caters for this compromise is an object lesson in technological orchestration. The Maxi's 350bhp output is almost 17 percent more than its predecessor and its 311lb.ft of torque amounts to a remarkable 26 percent improvement. But the figures don't really tell the full story, because by adapting recent Formula One turbocharging technology to its new rally engine (for "new F1 Turbo technology read 'DPV - or Dispositif Prerotation Variable - an amazing form of Anti-Lag), Renault has achieved a competition power unit which is devoid of any perceptible turbocharger lag.

Much of the Maxi engine's induction equipment is exactly the same as that fitted to last year's Renault V6 Formula One engine albeit with different settings, and indeed the same technology can be found in its cooled piston design. Here Coble nee's experience with supercharged diesel engines was of benefit, and although these types of pistons have been used in diesels for some time, it's only comparatively recently that they have found their way into most turbocharged Formula One engines.

The Maxi's pistons have galleries just under their crowns through which oil flows via inlets and exits beside the oil ring. The motion of the pistons creates the oil flow, which, as it passes below the super-hot piston crown, dissipate much of the heat generated by the very high combustion pressures. Further combustion cooling comes from the Maxi engine's water injection system. This is virtually identical to that which Renault uses in its Formula One V6, and also serves to reduce the chances of detonation on boost.

 It's an automatic system which injects pre-set water quantities- different to that of the V6- into the charge air stream at the water/air intercooler's outlet. Injection begins at 2.4 bar boost pressure,and although there's no fail safe reserve system, a green light on the dashboard adjacent to the boost gauge and in front of the' co-driver illuminates when the system is in operation. So if that light doesn't come on and' the gauge shows more than 2.4 bar there's a problem.

But there is little need for such fail safe insurance.  During circuit work, such as at our test session, the maximum water consumption is in the region of 4 lires / 100 km. Under actual rally conditions the engine would be doing far less full-boost work, and normally consumes between two and three litres per 100km, so with a seven litre water tank capacity, regular service attention will ensure that sufficient liquid is available.

While Renault's Formula One engine is equipped with all-electronic fuel injection and electro-magnetic injectors this season, the Maxi rally engine uses the electronically regulated Bosch mechanical system which was on the F1 V6 from 1982 to the end of last year. The system has a two in-tank lifter pumps supplying a swirl pot, and then a two stage fuel pump to provide pressure and distribution, with flow regulated by a camshaft and electric motor, governed by an electronic black box which reacts to information provided by five pressure/temperature/air-flow sensors. The only basic difference between this unit and the Formula One set-up is that the pump, distributes to four instead of six cylinders.

The Maxi's aluminium two-valves-per-cylinder head has revised combustion chambers and its single chain-driven camshaft has slightly more lift but a similar duration to its predecessor. And the torque output of this long stroke slogger is nothing short of amazing - 311 Ib.ft at 5000rpm is important enough, but this 1527cc four cylinder engine never pumps out less than 282 Ib.ft between 4000 and 6000rpm.

This sort of output places acute emphasis is­ on engine temperatures, and apart from the water/air intercooler and water injection, the' Renault, team have made great efforts to perfect the Maxi's other cooling media. Compared with cooling systems on the Tour de Corse version, the Maxi's brakes required Extra ducting, as well as its transmission and engine. While more cooling air is now forced through larger diameter ducts to the brakes, there are two large water/air radiators in the airstream at the front of the car. One serves' the engine, while the other is an extra radiator for the engine-compartment-mounted water/ air intercooler.

Also in the engine compartment are two water/oil heat exchangers which transfer transmission-generated heat into the coolant for the engine and intercooler. So, each drive train area is cooled by a single main water­-based cooling system with water/air radiators mounted in the front air stream not conceptually complicated and very effective.

And, if it is considered as a series of linked supplementary systems which go together to form a major system, then the Maxi's turbocharger layout can be accurately described by exactly the same phrase. As with the Formula One V6, the turbocharger is a Garrett unit, although of a different size. The rally engine uses a T31 turbine and a T04 Compressor which produces a normal maximum boost value of 2.85 bar, with a dashboard-mounted rotary control to allow adjustment between 2.6 and about 2.85 bar at present.

There are no electronics involved in the regulation of the Garrett's wastegate, which is controlled only by pneumatics. Over the whole engine range the electronic black box provides optimum fuel flow to the injectors based on the engine operating conditions it receives from its sensors, and Magnetti Marelli's ubiquitous Microplex systemcontrols advance and retard. Therefore, whenever any different boost pressures are dialled in, there is no need to make any ignition or injection adjustments manually - the black box does it all. Fuel is supplied to the injectors at a comparatively low 2.0 bar pressure by two low pressure pumps in the tank and two variable pressure pumps outside, with valve regulation. The dashboard fuel pressure gauge shows between 3.5 and 4.0 bar when the engine is on full boost.

And it's when the Maxi's engine is boosting that the major benefits of race and rally engineering collaboration can be seen. As reported in CCC (June 1984), on January 1st 1984 Renault's Formula One engine wizard, Jean Pierre Boudy, was 'poached' by PTS for its works rally team. This was the reason why the 205T16's architect, Bernard Perron, was confident in stating that Peugeot "don't know what turbo lag is". Neither does Renault with its Formula One V6 engines, because before he left Boudy had developed some aviation engine turbocharging technology for the V6.

This was first used at the Monaco GP in 1982, and developments of the same system are still used in Formula One. But because Boudy defected to Peugeot with his ideas, the 205T16 was the first turbocharged rally car to use his system (The T16 being out before the final evolution R5 Turbo – The Maxi 5 Turbo). Which Renault calls DPV, or Dispositif Pre­rotation Variable (Variable Pre-rotation Device).

"At the very beginning we had made the decision to use it on the Group B R5 Turbo. It's not really necessary for road cars and a little expensive for them, but it's not so complicated". It may not be "so complicated" but the DPV system is an 'extremely effective way of ensuring near-instantaneous throttle response from a competition turbocharged engine’. The system provides for variable air areas and direction angles at the intake to the compressor wheel in advance of the rotation period during which the compressor turbine will require those conditions.

It's not a by-pass system, it's essentially a multi-variable nozzle arrangement which is mechanically linked to the throttle and which changes the direction and area of air which hits the compressor wheel according to throttle position. The rate of change of air flow area and direction is not exactly linear, but it's progressive and always increasing toward full flow at maximum compressor wheel speed.

This irregular mid-compressor-speed flow, which constantly changes the way the air hits the compressor vanes in the mid-range, improves the compressor's efficiency under part-load, because it is arranged so that the turbine will idle at a higher speed than it would given a fixed intake air direction and area. And of course, the higher the compressor idle speed, the shorter the time it will take to reach its important operating RPM. Result: less lag.

The DPV is inoperative at each end of the turbine's speed range. Under pedal-to-the metal full boost conditions it has no effect because there's regular full flow to the compressor, and with the throttle closed the DPV is shut down as well. But it is not just retained inertia and completely eradicated turbocharger lag which form the real benefit of the DPV system, because there's another major advantage in that as a result of there being no by-pass or blow-off to atmosphere, and because it's an internal function to the turbocharging system, there is no pressure wastage in the all­important part-load condition (ie. The blades don't stall during gear changes, or on/off throttle movements).

Renault's ground-braking, turbo-lag-removing and
beautifully crafted DPV seen from the inside.

High technology and innovative solutions may have sophisticated the operational gaps' in the Renault 5 Turbo's specification, but without those six years of on-event' experience, and progressive development of its comparatively under-financed but totally dedicated team of professionals, there would have been no chance of achieving such astounding tarmac ability from this a car.

We've already said that the Maxi 5 Turbo is arguably the quickest tarmac rally car around today. Just to add weight to that argument, consider the fact that over this year's 15 special stages on the Tour de Corse after Vatanen had retired his Peugeot, Ragnotti's Maxi was giving away two driving wheels, 80bhp and 51 lb.ft of torque to Saby's second evolution 205 T16 and yet he was still an average of 18.73 seconds quicker than his Contryman per stage !