Automotive Engineering - April 2023 - 26

DESIGNING FOR
200 BMEP AND
22,000 RPM
Honda's mighty 18,000-rpm,
66-hp 250cc six-cylinder RC166
was unbeatable with world champion rider
Mike Hailwood, shown here in 1966 in the U.K.
How Honda's Grand Prix motorcycle program in the 1960s created the world's
most-advanced IC engines.
by Kevin Cameron
F
ire-bombing destroyed Japan's 66 largest cities
and their industry during World War II.
After the war, more than 200 makers of light
motorbikes sprang into being because such
transportation could be quickly tooled for production.
Only a few of the startups - those known for
reliable, maintainable products - survived. By the
late 1950s, saturation of the domestic market imposed
a choice: export or stagnate.
Only a few returning U.S. servicemen had ever heard
of Honda motorcycles. Who would buy them? Soichiro
Honda, a self-taught mechanic and former auto racer,
knew that great names had been made through innovation
and international racing success. On a 1950s
tool-buying trip to Europe, he bought two racing machines
- a German NSU and an Italian Mondial - for
study in Japan. He committed his company to fourstroke
engines and declared that it would compete in
the Isle of Man TT road races that had shaped motorcycle
development since 1907.
Honda's production motorcycles offered only a third
of the specific power of the high-rpm racers he'd
brought back from Europe. It was grist for Honda's
fast-growing and well-equipped R&D department.
Their target: beat the Yamaha two-stroke bikes that
dominated Japan's " Volcano Race " held annually on a
rough cinder track. For 1959, Honda engineers designed
the RC-160, a four-stroke DOHC 250cc inline
four-cylinder, generating 35 hp (26 kW) at 13,500 rpm
and mounted transversely in the bike's chassis.
26 April 2023
In Europe, the superior specific flow of a single intake valve in a
hemispherical combustion chamber was unquestioned. However, the
mass of that large single valve resulted in European race bike engines
having as few as 300 revs between peak power and valve float.
Every time the RC-160's rear wheel left the bumpy track surface, its
tach needle shot into the red, but the opposition's two-strokes were
unaffected. Honda's young engineers needed a wider safety zone to
avoid catastrophic valve failure. The solution they chose ignored the
dogma that the flow coefficient of a single intake valve in a hemi
chamber substantially exceeds that of a pair in a pentroof chamber.
The engineers revived the old four-valve concept not for its airflow,
but for its higher resistance to valve float. Years later, engineers
would refer to " Duckworth's Rule " named after Cosworth Engineering
co-founder Keith Duckworth, whose four-valve DFV V8 dominated
Formula 1 auto racing: with four valves in place of two, important parameters
are improved by a factor equal to the square root of two.
Battle for BMEP and rpm
Honda entered the 1960s era of GP racing with three times the overrev
margin of European competitors. If more airflow was needed,
larger bores and shorter strokes could accommodate bigger valves.
Honda's successful RC-160 of 1959 was followed by later 250cc
fours with cylinders inclined forward 30-35 degrees, increasing airflow
over cooling fins between the intake and exhaust cam boxes.
The RC-160's original (and torsionally springy) towershaft-and-bevels
cam drive was replaced by a central train of spur gears, located behind
rather than between the cylinders to reduce engine width.
Mercedes had used the same approach with its straight-eight F1 engine
of 1953. A centrally located gear drive to the gearbox and camAUTOMOTIVE
ENGINEERING
B.R. NICHOLLS/MORTONS ARCHIVE
Automotive Engineering - April 2023

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