Some 10 minutes after driving Chris Williams's Packard-engined Behemoth my hands were still shaking, my voice was croaking and the cool autumn wind was chilling my sweaty overalls. My face was cherry red from the infernal heat of the engine and my eyebrows singed from its 24 flaming exhaust stubs. In my entire career I have never driven anything as visceral, as physical or as sheer bloody terrifying as Mavis, the 42-litre Packard-engined Bentley.

This is third outing for Mavis, the tribute to the pre-Second World War aero-engined giants that lapped the Brooklands outer circuit with the bravest of the brave behind their huge, string-bound steering wheels. Williams built Mavis at weekends and evenings over the last seven years using a Bentley 8-litre chassis, with specially fabricated parts and a V12 Packard engine from an American Second World War motor torpedo boat.

Her debut was at this year's Cholmondely Pageant of Power, where she quickly gained a reputation as a stocking sizzler when flames from her exhaust stubs vapourised man-made under garments as well as welding the trainer laces of those unwise enough to stand too close.

By the time of her second run at Brooklands Museum Wings and Wheels event, she was clearly a lead contender for this year's Christmas road test. With a paddock full of mouth-watering machinery, the crowd voted with their feet and besieged Mavis, Williams and his engineering crew and friend Duncan Round cowering in a far corner. In fact Mavis was still misbehaving with her huge oil pump filling the cockpit and poor Duncan's trousers, but the sight and sound of this behemoth drew young and old by the score.

Let Williams take up the tale and explain how this extraordinary car came about. "I first saw the engine at Beaulieu," he says. "I think it was Duncan Pitaway who had it and he knew I had the Napier [the Napier Bentley, a fearsome device built in 1968 by David Llewellyn and owned and driven with gusto by Williams since 1999]. He said 'Hey Williams, don't be a tart, get yourself a proper engine like this'. At that time it had upswept pipes and when he fired it up, this great sheet of flame and snot all went into the air and landed on next-door's white linen table clothes. It made a great noise, got through two gallons in no time and when the hat went round, I thought, 'cooool'.