Thursday, October 14, 2010


After a successful career housing a supercharged 994cc JAP engine for several attempts at the Motorcycle Land Speed Record with Joe Wright aboard, the unusual OEC chassis built for the specific purpose of breaking records was host to another blown engine, this time from a car.

The Aug. 30, 1934 edition of 'The Motor Cycle' has a short article on a 'A New Racing "Four"', detailing the attempts by J. Granville Grenfell (what a name!) to create a 500cc machine, with the specific intention of being the first motorcycle to achieve 100mph for an hour, and win a 'Motor Cycle' sponsored trophy for a multi-cylinder/100mph/1hour British motorcycle.  While the OEC was being built, a New Imperial 500cc v-twin took the prize with 'Ginger' Wood the brave rider - it was said he was the only person willing to risk his life aboard the evil-handling New Imp, which spat him off with a tank-slapper at 100mph on the Brooklands banking, while practicing for the record. By comparison, the OEC was a far safer bet at over 100mph; the 'Duplex' steering system being almost too stable at speed, taking some effort to deflect from a straight-line course.  Of the two possibilities, I'd take stable over wobble.

As the raison d'etre of the revised OEC had passed, the Grenfell's (under the sponsorship of Minnie Grenfell, a trials rider) entered the machine in the 'Hutchison 100' race at Brooklands that August of '34.  F.W.S. Clarke was the pilot, although the machine 'failed to distinguish itself' in the races, as the engine was yet 'too tight'...although building a record-breaking engine to 'tight' clearances is dubious. The motor in question is an Austin 7 four-cylinder sidevalver, sleeved down from the original 742cc to 490cc, making a very long stroke indeed at 46mm bore x 76mm stroke.  The crankshaft, notoriously prone to breakage on 'sporting' Austin 7s, was an especially strong one-piece item, yet retained only one main bearing at either end (in '36, Austin introduced a '3-Bearing' crank).  The crank end held a sprocket, presumably between the main bearing and flywheel clutch, which drove a Centric supercharger, mounted above the Morgan 90degree bevel-drive box, which 'turned the corner' and made a rear drive chain possible.  There was no gearbox per se, making the OEC a supercharged four-cylinder single-speed racer!

More technical points on the modified engine: the Centric blower began puffing at 25rpm, giving maximum pressure of 20lbs/sq" at 6000rpm, producing an estimated 46hp 7000rpm...about twice the safe revs of the standard Austin engine, and nearly 3 times the original 17hp.  A sporting Watmough cylinder head gave a better combustion chamber shape, although the compression ratio remained a lowly 4.5:1...not an issue with a supercharger, which needs neither high compression nor radical valve timing to produce maximum power.  In fact, a very tame camshaft with little 'overlap' of valve openings produces the best results with a blower, as it's the job of the compressor is fill the cylinder completely with fuel/air mix, no fancy engine tuning is required, other than the ability to hold the engine bits together, after a dramatic increase in horsepower.

To keep the engine cool, a Scott radiator fits snugly at the front of the engine; hopefully a waterpump was used rather than the 'thermosiphon' system of the Scott, notoriously prone to boiling, although the OEC used glycol rather than water for cooling - a novelty at the time.  The OEC frame was modified to fit all this machinery, and it was estimated the bike would be good for 118mph, and although 'nothing like this speed' was achieved during the 'Hutchison 100', the 60" wheelbase chassis was reported to handle 'extraordinarily well'.

Of course, the OEC wasn't the only motorcycle using an 'Austin 7' engine, as Brough Superior used the engine as well - see the Road Test here.