{"id":497839,"date":"2018-10-02T15:31:57","date_gmt":"2018-10-02T15:31:57","guid":{"rendered":"https:\/\/sportscardigest.com\/\/?p=65892"},"modified":"2024-01-14T16:04:29","modified_gmt":"2024-01-14T16:04:29","slug":"thunderbug-curtiss-1921-1922-gn-specials","status":"publish","type":"post","link":"https:\/\/www.supercars.net\/blog\/thunderbug-curtiss-1921-1922-gn-specials\/","title":{"rendered":"Thunderbug & Curtiss\u20141921\u20131922 GN Specials"},"content":{"rendered":"

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\u2026<\/p><\/blockquote>\n

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\u2019s no one else\u2019s fault is the unique realm of the special builder.<\/p>\n

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Photo: Joseph Mark Bates<\/figcaption><\/figure>\n

Mark Walker and Duncan Pittaway are both, if you\u2019ll 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\u2026<\/p>\n

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\u2019s cars have taken the principle of \u201cbig engine in a small car\u201d to new extremes: Mark\u2019s yellow and black \u201cThunderbug\u201d features a unique 4.2 liter V-twin up front, while Duncan\u2019s orange and purple GN boasts an 8.2 liter Curtiss X5 V8 aeroplane engine squeezed between the chassis rails.<\/p>\n

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Mark Walker\u2019s GN Thunderbug<\/h2>\n

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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.<\/p>\n

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Thunderbug’s 4.2 liter V-Twin engine is derived from a 9-cylinder Samson aero-engine. Photo: Joseph Mark Bates<\/figcaption><\/figure>\n

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.<\/p>\n

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.<\/p>\n

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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\u2019t 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!<\/p>\n