Fitting an engine many times the size of the original is never a sure fire recipe for racing success. Mark Walker and Duncan Pittaway have done just that with their GN-based, Vintage Sports Car Club-eligible specials, using components originally intended for the aviation industry. Joseph Mark Bates evaluates the results…
The noble art of building a special raises a smile in most enthusiasts of motor sport. There is something in us all that admires and appreciates the skill, imagination and sheer boldness that goes hand in hand with building a one-off car and then racing it. To step outside of the safe confines of what has been done before and to enter an arena where it’s no one else’s fault is the unique realm of the special builder.
Mark Walker and Duncan Pittaway are both, if you’ll pardon the pun, driven men. Both are hardened VSCC members and competitors, and when they decided to build their own individual interpretation of an ideal road and racecar, both chose the extremely adaptable GN platform as a base. Both cars use the same basic GN chassis and original chain drive transmissions, but there the similarities end…
The brainchild of Archie Frazer Nash and Ron Godfrey, the original GN was one of more sophisticated cycle cars, and, like the three wheeler Morgans of the day, the benefits of a powerful engine coupled to a lightweight chassis gave the GN considerable sporting potential and a great deal of racing success. Various GN-based specials were created with larger and larger engines in the pursuit of more performance, however Mark and Duncan’s cars have taken the principle of “big engine in a small car” to new extremes: Mark’s yellow and black “Thunderbug” features a unique 4.2 liter V-twin up front, while Duncan’s orange and purple GN boasts an 8.2 liter Curtiss X5 V8 aeroplane engine squeezed between the chassis rails.
Mark Walker’s GN Thunderbug
In building his special, Mark Walker drew his inspiration from two quite different sources; firstly, Mark was inspired by a cartoon of a V-twin cyclecar featuring two absurdly large cylinders, and secondly, a well known GN-based special called BHD. Right from the start, Mark knew that he wanted a large capacity V-twin up front, and a 90-degree V-twin at that, simply because all the larger V-twin engines seemed to be built with the cylinders opposed at that angle. He started with a 1908 Riley crankcase, which gave him the correct angle for the cylinders, and started a search for two suitable pots to attach to it, at first imagining two cylinders from a Tiger Moth engine would do the job. Instead Mark sourced two suitable cylinders that were originally part of an early 1920s 9-cylinder Samson ariel aeroplane engine, which, with a bore of 125-mm and a stroke of 170-mm, gave the new engine a monstrous 4.2 liters of cubic capacity.
Mark coupled the Samson pistons to a pair of con rods taken from a Liberty V12, which is a 27-liter American aeroplane engine from the correct period, the rods being each over a foot in length. Mark fabricated a crank himself and used needle roller bearings throughout. He also made up some adaptor blocks to house the cylinders on top of the crankcase, which were then recast at a local foundry.
Mark mounted the engine in the GN chassis, which he shortened by one foot to give him the same dimensions as the aforementioned BHD special, and then mated the motor to the standard four-speed GN transmission. The GN transmission transmits the power to the rear axle using a separate drive chain and sprocket for each gear. It looks deceptively complicated, but is actually a very simple system, with the lack of differential giving the cars an extremely narrow rear track.
The Thunderbug has never had its brake horsepower accurately measured, but Mark did tell me some fantastic statistics about the huge V-twin, which should give you some idea of how the engine behaves; maximum revs for the engine is approximately 1,500 rpm, which isn’t at all high by any standards, but by the time the revs have got up to just 1,000 rpm in top gear, the Thunderbug is already doing 66 mph. Top speed for the little car is about 100 mph, at which speed the engine is firing once every five feet!
From a standstill, you can literally feel each cylinder pushing the car forward in turn, the massive amount of torque the engine delivers arriving with a deliberate shove on each firing stroke. The revs rise steadily and quickly to their maximum and it’s time to change gear. In hardly any time the Thunderbug is in top and really flying, the amount of torque meaning there’s very little reason to change gear. The car is extremely light, and this, coupled with the narrow tires, the lack of differential and the amount of power delivered low down in the rev range, all mean that it’s very easy to break traction. The Thunderbug power slides at will. Fortunately, the handling of the GN is very forgiving and any wayward break of traction is easily countered by the GN’s very light and neutral steering.
Mark never intended to win races with his GN special, instead he built his car first and foremost so he could drive to the racetrack, race all weekend, and then drive home. He has certainly achieved his aims, but the end result is much more than that. The Thunderbug excels as a road car and is an incredibly fun car to race, the steady thump of the motor and the amazing power slides adding another dimension to VSCC events. For many, the Thunderbug is the ultimate GN special, but others may prefer a slightly different style and even more cubic capacity.
Duncan Pittaway’s GN Curtiss
The engine in Duncan’s GN is a Curtiss X5 that originally found home in the fuselage of a First World War fighter plane called the Curtiss Jenny. Duncan bought the engine at the time that Mark was finishing his Thunderbug project, and it was Mark who first suggested putting the huge motor in a GN chassis. Without being too concerned about how well the end result would actually work, and rather taken with the idea of an 8.2-liter cycle car, Duncan set to with earnest.
Duncan wanted a low slung car, and so bent the standard GN front axle as per racing GNs. The Curtiss engine is much longer than the Thunderbug’s, and so Duncan left the chassis the standard length. Like Mark’s car, Duncan’s GN uses the standard transmission, but he has fitted stronger drive chains to cope with the extra power. Duncan has also fitted some front brakes, which, in order to keep within the Vintage Sports Car Club regulations, came from a pre-1930 Bullnose Morris.
The inspiration for the paint scheme came from an old black and white photograph of a racing GN, sporting a striking chequered design. Duncan had fabricated similar looking bodywork for his car, and so translated the pattern on to his sheet metal. Easier said than done as it turned out, because not only do the “squares” have to cope with the contours of the body and still look square, but they have to be the right size to start and finish at the front and back of the car, and they have to meet opposite color squares from the other side. The end result, I have to say, is nothing short of stunning, and creates quite a dash as the car belts around the circuit.
Given the engine’s origins, it may come as no surprise that Duncan’s GN sounds exactly like a World War One fighter plane. The Curtiss engine is double the size of the Thunderbug and is therefore the quicker car, and, being a V8, is therefore more balanced and much smoother. At 90 mph, the car is revving at just 1,300 rpm, which, contrary to appearances, makes Duncan’s GN a superb motorway car. Producing lots of power at low revs is precisely the engine’s design brief, and because the car weighs a little over half a ton, there is an abundance of power from a mere whiff of throttle. Duncan has plans to fit a different cam, which in theory should increase the engine’s power output by about another third and should see the GN Curtiss really fly.
To get Duncan’s car to corner quickly is a case of promoting understeer, and then using the massive amount of horsepower to get the back end to drift and correct the steering. The Curtiss V8 spins the rear wheels effortlessly at almost any speed, which makes for a lot of fun. It’s not just the paintwork that makes the Curtiss GN stand out at VSCC events; the Curtiss will be the car going through the tight bends with the front wheels hard over and, at times, nearly on opposite lock, rear tires fighting for grip and emitting blue smoke, Duncan wrestling with the huge steering wheel as the car drifts sideways.
Road or track
While both cars are quite obviously racecars and can be seen competing at Vintage Sports Car Club races and sprints either in the UK or in Europe, what makes Mark and Duncan’s cars different from most cars competing at these events is that they will have been driven to the racetrack. Both cars are registered, tested and road legal, which is staggering in itself, but what is even more incredible is just how well these cars perform on the open road. There aren’t many road cars, of any vintage, that can keep up with either GN special. The impression of speed is entirely relative to your surroundings, and hurtling along in either of these mammoth-motored GNs feels indecently quick. You sit down low in either car, squeezed into a seat made from nothing more than a piece of sheet metal, feet stretched out before you down the narrow cockpit. You look out over the tiny screen and down the bonnet with the only things interrupting your view of the open road being the engine sticking out the side of the car and the narrow, exposed wire wheels. The bellow from the huge engines coupled with wind in the face and the intoxicating aroma of Castrol R makes for extremely exhilarating motoring.
Mark and Duncan’s GNs have both been built with ingenuity and not a little daring. Both machines capture a spirit, an exuberance and a sense of fun that the pioneering motor racing gentlemen of yore would have recognised and applauded. They raise a smile indeed.
