As soon as I’d posed the question I felt a bit mean. After all we had just spent the best part of an entire Silverstone test day abusing the splendidly generous hospitality of the members of the Vintage Sports Car Club, Sub-division: Aero-engines. It’s just that in all that time, no-one had actually asked the obvious question: why?

Not “Why?” in a pejorative sense but “Why?” in the “What makes a man want to take the engine from a post-World War One ‘plane and bolt it to the chassis of a 1928 Riley?” sense. With the benefit of hindsight it’s clear that the answer was as obvious as the question. Like Mount Everest to a mountaineer, it was there and so it was done.

Not that engineering a 6-litre de Havilland Gipsy Moth engine into the narrow confines beneath the bonnet of a quintessentially English sportscar was as simple a feat as I make it sound. Far from it. To date its owner Dr Geraint Owen has spent a conservative 2000 man hours on his pride and joy and readily admits that there remains considerable work to be done. Bigger and better brakes are on the wanted list and the work of a good upholsterer is also required to replace the current bucket seat carved from what appear to be two pieces of wood.

If it all sounds a little crude well, that’s really because it is. Not perhaps as crude as the Monarch-Curtiss featured earlier on these pages but nevertheless there is no denying the Riley-de Havilland lacks certain luxuries found in modern motoring.

First indications that something was amiss came even before the early sighting laps on which Dr Owen was planning to explain to me the intricacies of his homebrew special… at 90mph from behind his full-face helmet. Passenger rides are part and parcel of this job and, although far from being the perfect riding companion, I am getting better at sitting in the wrong seat. But never have I sat waiting for the driver, only to watch with increasing anxiety as, underneath his race overalls, he straps on what can only be described as a bullet-proof jacket. “To stop me being impaled on that,” he grins, pointing at the large nut that holds the steering wheel in place. I tried, really very hard, to smile back at him. And I think I succeeded.

The easiest way to describe riding ‘two up’ in the Riley is to imagine sitting side by side in an iron maiden on wheels and being shot out of a cannon. Owen has accordingly attached two grab handles to augment further his passenger’s comfort. One rests on the dash to stop you being launched out the front or back under vicious acceleration or braking, the other rests behind the driver’s far shoulder to stop you being thrown out the side under cornering. Dr Owen is clearly an unusually considerate man. And they obviously work because here I sit telling you about them. They are also extremely effective at pulling your hands from your wrists which might explain the odd typographic error in this story. My agolopies.

After a brief introductory jaunt during which I nod inanely as Dr Owen shouts indecipherable but doubtless vital words of advice over the cacophony that is a Gipsy Moth engine, it is time to fly solo. Firstly let it be noted that from behind the wheel, the Riley is far more motor car than aeroplane, which I guess is something. The pedal layout is conventional clutch-brake-throttle, while the gearshift – although mounted outside the car’s body – is a straight H-pattern linked to a monstrous Rolls-Royce 20/25 four-speed gearbox.

Getting underway is not difficult. With in excess of 350lb ft of torque the Riley easily picks up momentum from standstill but what happens next is far from ordinary. Large unstressed ‘plane engines built to produce low-end power at constant revs take very little persuasion to turn beyond their means. The de Havilland engine has a cautionary limit of 2,200rpm and while it will go a few hundred past that in the heat of battle it manages to get there so alarmingly quickly that you have to change gear almost constantly and don’t dare touch the throttle during double-declutched downchanges. Move the pedal a thousandth of an inch with the clutch depressed and the needle will be heading off the scale.

While this is more than a little disconcerting to the driver, it’s pretty hard work for the Rolls transmission which sounds less like a crash gearbox and more like a full-scale motorway pile-up. The art to building an effective aero-engined special is not so much in ensuring the engine or chassis are suitable but coping with the uncommonly high torque outputs of the chosen motor. Owen has circumnavigated this problem by mounting a small step-up ‘box between the clutch and the main gearbox, which also conveniently allows him to run conventional back axle ratios. The system functions well enough because he has used the same gearbox since he first built the car, but it is not a transmission for the faint of heart. The trick, says a remarkably still-smiling Dr Owen after my first graunching sortie, is to yank the lever through to the next ratio without hesitation and, in so doing, keep the nasty noises as brief as possible.

Mechanical sympathy left firmly in the pits, the next stint brings some degree of rhythm to my driving and, the odd missed downshift apart, it is a chance to sit back and enjoy the effortless power of the Gipsy Moth engine and the amazing poise of the Riley chassis. Despite standard Riley axles and worm and screw steering the car copes well with the surfeit of torque, although its ready willingness to lift an inside front wheel even at low speeds as you cross some small undulations at the exit of the pitlane hints at the unforgiving nature of the suspension.

Feedback, however, is what makes this Riley driveable at racing speeds. Accept that the narrow, hard, cross-ply Dunlop racing tyres and unyielding leaf-springs are ill-equipped to cope with the forces put through them, resign yourself to the fact that the engine is more than capable of not just loosening the rear end but snapping it clean off and simply revel in the fact that all you need do is keep your right foot well in and progress is swifter than you might ever imagine.

In full race trim, with headlights and fittings removed, the Riley is is geared to pull comfortably in excess of 100mph. Even in ‘full drag’ mode three figures are easy to attain, and your first approach to Copse Corner at full chat is more than a little unnerving. Air rushing under the front mudguards creates rather undesirable lift and the nose displays a distinct unwillingness to follow exactly the direction in which the wheels are pointed. A sharp jab on the throttle, however, is enough to break rear wheel traction and bring the front into line. Wind on the opposite lock – there are just one and a half turns from one to the other – to ensure you don’t end up pointing in the wrong direction and while the car will not exactly circulate Silverstone on autopilot, neither do you feel an overwhelming urge to bail out.

Brakes, too, are more reassuring than you might imagine. The 13-inch drums are operated hydraulically within the 19-inch Rudge-Whitworth wheels and are efficient with some feel, much more so than you might imagine in this 70 year old veteran. Nevertheless, with the engine’s ‘on-off’ power band, braking points were religiously adhered to and even increased. The Riley-de Havilland is yet to win a race outright but its owner is quite sure that that day is not too far off. All it will take, he reckons, is a little fine tuning and a large slice of opposite lock. Until that day look for Dr Owen and his fellow aero-engine fanatics at VSCC meetings across the country. You can tell the type by the wide grins and the fact that they drive at the most extraordinary angles. Have a good flight…