The Birmingham Blower: 1939 Velocette Roarer Replica

Building a replica of the Velocette Roarer racer from scratch was no easy feat.

| March/April 2018

1939 Velocette Roarer Replica
Engine: 498cc air-cooled supercharged SOHC parallel twin w/two counter-rotating crankshafts, 68mm x 68.25mm bore and stroke, 7.5:1 compression ratio, 54hp @ 4psi boost (est.)
Top speed: 140mph (est.)
Carburetion: Single Amal 10TT9 w/Shorrock-type supercharger
Transmission: 4-speed, shaft final drive
Electrics: Lucas AC2 magneto
Frame/wheelbase: Steel tube frame/55in (1,397mm)
Suspension: Girder fork front, dual shocks rear
Brakes: 8in SLS drums front and rear
Tires: 3 x 19in front, 90/90 x 18 in rear
Weight: 350lb (159kg) (est.)

It's been famously said that as far as internal combustion engines are concerned, there's no replacement for displacement. But actually, there is.

As well as displacement, the power output of an internal combustion engine depends on its volumetric efficiency; that is, the volume of air/fuel mixture that can be fed into the combustion chamber on the induction stroke. In a naturally aspirated engine, volumetric efficiency is sometimes limited by restrictive intake tracts and/or valve timing.

Just as important is the density of air the engine is breathing, which depends on temperature and atmospheric pressure. Cold air is more dense than warm air. And atmospheric pressure is affected by altitude — the higher you go, the less dense the air. So how can you improve volumetric efficiency, and therefore increase power, assuming timing and intake tracts are already optimized?

This was the challenge piston-engine designers wrestled with during the 1920s and 1930s — especially in the aircraft industry. As aircraft flew higher, the density of the air their engines breathed was reduced. That meant their volumetric efficiency fell, leading to a loss of power at altitude. At 10,000 feet, for example, air is less than two-thirds as dense as at sea level. The answer was to push more air/fuel mixture into the intake so its density could be maintained as the aircraft flew higher. Airplane engines soon featured some sort of compressor: either a supercharger (driven mechanically from the crankshaft), or a turbocharger (driven by exhaust gases).

Designers soon realized that a compressor could push volumetric efficiency even higher by squeezing the intake mixture above atmospheric pressure — called "boost." As an example, over the six years of World War II in Europe, the output of the Rolls-Royce Merlin engine was doubled from around 1,000 horsepower to over 2,000 horsepower almost exclusively by improvements and upgrades to the supercharger. The Merlin's displacement of 27 liters never changed, but using a 2-stage, 2-speed intercooled supercharger added more than 30mph to the Spitfire's top speed and 7,000 feet to its service ceiling — keeping its performance on par with Germany's fastest fighter, the Focke-Wulf 190.

4/19/2018 8:51:58 AM

I think that's the most challenging homemade replica I've ever seen.. and it looks terrific.. Words fail me.. What a fantastic job..! Well done!!

Ride 'Em, Don't Hide 'Em Getaway

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