Mel Nichols with Hannu Mikkola in an Audi Sport quattro S1

Hannu Mikkola doesn’t smile. He just says “Ready?” then nods, whacks his foot onto the throttle, and pops the clutch. The Sport quattro S1 erupts like a bronc from a rodeo chute. Its wheels rip into the gravel. In seconds – about seven! – it’s topping 100mph. It bounds over a crest and, roaring like a hungry lion, charges down the track weaving into the forest.

The speed is unbelievable. The rate at which we're catapulting through the trees scrambles my senses. The thrust mashes me into my seat as if I'm trapped, helpless, in a runaway rocket blasting inches above the ground. Through the blur, all I can see are the pine trunks crowding the track and, where they thin, the steepness of the bank dropping away. Blind fear takes over. My panicked brain screams that it's impossible to drive so fast down a forest trail. Don't let this be the day the quattro breaks or Mikkola makes a mistake.

Him? He's just sitting there, tight in his harness, tweaking the wheel this way and that in precise little movements; snaking a hand out to snap the lever up or down the six-speed gearbox; dancing his left foot over the clutch and brake pedals; dabbing the throttle with his right. His face is impassive, a mask of concentration, his eyes reading every inch of the road.

Driving against the clock, he's looking always to unleash the quattro's potential. Every instant, every yard where there's a chance, he gives the 500 horsepower its head. The response is phenomenal: blasting from 4000 to the maximum 8500 revs in each gear takes barely a second. In moments, on the shortest straights, we're shooting up to the 120mph maximum that the Audi is set to run in these conditions.

For many of the corners, he slows earlier than you might expect – then tweaks the car sideways. He does it by upsetting the stability with a quick flick of the wheel while simultaneously braking with his left foot and keeping the throttle on with his right. With the car "cocked" for the bend, he balances the drift – often at well upwards of 100mph –with the throttle and steering, constantly adjusting both, within milliseconds, as he controls the slide. By mid-bend he's applying full power, exploiting the four-wheel-drive advantage. Lord, this thing is so fast through the bends; so bullet-like out of them!

For one tight blind bend, Mikkola treads hard on the brakes as he whips down through the gearbox. The deceleration is as ferocious as the acceleration, flinging me forward until the belts cut into my shoulders and my eyes feel as if they'll pop out of their sockets. The instant he's slow enough, Hannu is back on the power, the engine snarling in a new burst, and we're sliding again, the nose tucked in so tight it's almost scraping the bank on the inside, while the tail 's out under the branches and over the drop on the outside. A mistake with steering or power and we're dead.

My senses are still reeling as we charge full-tilt along a semi-straightaway cut across the mountain face. Just before a kink where Mikkola will get sideways again, the trunk of a fallen tree juts up over the track. Its tip is level with my head; it seems to hang in slow motion, waiting to smash through the windscreen. I feel the blood drain from my face. Mikkola doesn't let up. An instant before disaster, he tweaks the Audi into a 100mph slide and pokes its nose under the trunk. The tree passes six inches above the bonnet and clears the windscreen by three or four.

Ahead, the track is breaking up into a jagged mass of loose slate; it must rip the tires to shreds. We charge on. The car doesn't waver as it tears across the slate.  Here, as on some of the other rough sections we've blasted through, the Audi 's ride comfort is remarkable; there is none of the dreadful crashing you might expect; just a jiggling that conveys tautness. It feels like a car that will do precisely as its driver asks, within fractions of a second, without jarring.

But the noise! The engine's snarl, rising and falling as Hannu constantly goes on and off the power, up and down the gears, is wicked. Stones blast the underbody. Frequently, branches smash against the nose and flanks as he tucks the car beneath the pines.

The accuracy with which he places the car, time after time, makes you want to gasp. At the end of the longest straight, we're full chat, and he flicks the car sideways and takes it, still flat, across a kink where the track ducks into a pleat in the mountainside. The outer front wheel is over the precipice; it rejoins the track precisely where he wants it for the next maneuver, and all at 120mph. This is to witness, from inches away, a top sportsman walking the tightrope.

As suddenly as it does everything else, the track dives sharp right between stacks of freshly cut timber. The gap seems barely wide enough. Hannu brakes hard, weaving a little, puts the car sideways precisely the right amount, and sweeps through hard on the power. At 60mph there are inches to spare. Now it's full tilt through the gears again, in a charge up the short hill to the tape indicating the end of the section.

"It is over," he says matter-of-factly on the headset. I look at my stopwatch. Just under six minutes for just on seven miles of this slippery, treacherous forest track. We have averaged 70mph.

But back at the encampment, where the mechanics' van and a smattering of quattro sedans are arrayed around a clearing, Mikkola isn't entirely sagtisfied. At 500bhp, the engine response is too much for the loose surface. Even with four-wheel drive in its latest, leading-edge form, he's getting slippage and having to lift off, he tells development engineer Dieter Basche. I hadn't noticed. Willy, the chief mechanic, goes to the van to get a smaller turbocharger.

For a week, Hannu, Dieter, Willy, and three more mechanics have been in the forest developing the S1 quattro as part of the program for the '86 rally season. They've worked through a selection of suspension options, trying first one spring and damper combination, then another. Finally, Mikkola and Dieter, who rides with him on each test circuit of the course, are happy with the way the car is sitting on the road. They work as a team, Hannu relaying his views of the car's performance and Dieter coordinating it with his engineer’s knowledge, as well as with what his backside tells him.

They’ve sorted out a new ultra-high-performance braking system made by the English company Alcon. The temperature tags on the brakes have been reading 650 degrees Centigrade: red-hot. In days, the brake maker's engineers have come up with pads that give Mikkola the feel and braking power he wants. “Getting pads that can withstand constant left-foot braking while you've got 500 horsepower charging hard is a hell of a challenge," says one of the young engineers. "But we've done it now.” Hannu nods. “Yes, the brakes are terrific,” he says.

They've spent days on one of the most difficult tasks of all – evaluating new centre and rear differentials, which split the torque to the front and rear wheels in various ways and lock up with different degrees of progression.

At the level of sophistication that four-wheel-drive rally cars have reached, this split and this progression are crucial to handling balance, not just grip. Working painstakingly, logging and comparing the results through Dieter's little computer, they decide first on the centre diff and then, after another two days' work, the rear diff. The mechanics are so adept that they get the diffs in and out and send Mikkola off on another blistering circuit within 29 minutes. At last, Hannu is happy with the handling. As we finish our circuit and trundle back to base he says: “The balance is very good now. I can start to go very fast.”

Now there's just the engine to finalise, to bring into line with the drivetrain and the suspension settings for these sorts of loose surfaces. To cut the power and temper the five-cylinder enqine's response, so Dieter tells his mechanics to mount a smaller turbocharger and a different Motronic engine management system. Mikkola wants power – now it will be around 460bhp instead of 500 – but doesn't want it developing so fast that it sets the wheels spinning mid-bend, forcing him to lift off, wasting time.

While Willy and the boys work, Mikkola takes a 90 quattro and drives the circuit, slowly, checking the surface. His lines in the S1 are etched into the soil as if a huge vacuum cleaner has sucked off the top few inches along two perfectly flowing tracks, clean and neat as a railway line. Before the bends the tracks swerve out and then in, where he's tweaked the car sideways.

"Rally driving is a bit like downhill ski racing," he says. "We have to take similar lines through the bends, turning before the corner so we can accelerate into the next bend earlier. Skiers use their bodies; we use the wheel and the throttle.

"To win in rallying, you have to be very single-minded. Most of all you need the will to win, often against great pressures when things aren't going right. You need the gift, obviously, but you need very fast reactions, and you must have balance; to be able to feel what the car is doing.

"When I'm driving fast, and it all goes the way I like it, that's a wonderful feeling.”

But spending weeks testing in some remote forest, trying something and failing, then waiting while the mechanics pull the car apart – the drudgery behind the scenes – seems almost excruciatingly tedious. In 1985, Mikkola spent almost 40 days out testing; his colleague Walter Rohrl stayed similarly long in the forests too.

"Sure, I'd rather be with my family in Finland or Florida. But if you want to win, you have to do it; you have to get the edge on the other cars and the other teams. It becomes enjoyable when you see that you can make the car better and faster, so that it's not just the driver who makes the difference.

"Because the layout of the S1 is so similar to that of the road Audis, I also like to know that the work we're doing is valuable to customers. This is a very quick way to develop new systems and components. If we can use them in the production cars, it justifies the money we're spending. One of the new differentials we’ve been developing is likely to go into other quattros.

"And although this new S1 is a long way on from the original quattros we rallied – the big wings give terrific stability, the response from the new turbo engines is now incredibly quick, and the handling is in a different league – there's a long way we can still go.

"While the 500bhp we've achieved now can be too much for really slippery surfaces, I think we'll get to 600bhp before we reach four-wheel drive's limits.
"That's going to be very interesting. With our current performance, the cars are very challenging to drive. They're so fast, you have to concentrate really hard every inch of the way.”

Dieter Basche, who has to ride with Hannu as he pushes the quattro day in, day out, during testing, shows visible signs of stress near the end of the session.

“Sometimes, " Dieter says, "when I reflect on how fast we're going and what could happen if something broke or Hannu made a mistake, I have to force myself into the car. The speeds are so high now that even some of the seasoned navigators are admitting to being scared.

"But we're determined to keep on winning rallies, and I have a job to do. So I get in and, like Hannu, just concentrate on how the particular component we're evaluating is performing. In the end, it's worth it."

Willy signals that the car is ready again. Hannu and Dieter don their headsets, Dieter sets his computer, and they strap themselves into the flimsy-bodied beast. The engine snaps into life and idles with a sound as guttural as a World War I fighter plane. Mikkola runs an eye round the cabin while the engine warms. He has gauges that cover everything from fuel, oil and turbo boost pressure to engine, gearbox, and differential temperature. There's a speedometer, of course, so that the drivers can keep an eye on speeds on public roads between stages.

But the main instrument, easiest to see, is the tachometer, reading all the way to 9000rpm. Across the centre of the dash are a mass of fuses, and ahead of the passenger – in rallies, the navigator, of course – the radio equipment and the computer for plotting times, speeds, and distances.

Hannu decides that all is well. In a few moments, the S1 is belting around the mountain in the evening light, its roar rolling through the valleys like thunder. You can hear the eagerness of the engine. This time, though, those curious fluttery whistles of the wastegate dumping the excess boost are less frequent; Mikkola isn't backing off as much.

Six minutes later, Hannu and Dieter climb out grinning. 'It's perfect now,' says Hannu. 'I can drive it now. Let's go home.”