Automotive Engineering - April 2023 - 30

DESIGNING FOR
HONDA GP ENGINE LEARNINGS FEATURE
200 BMEP AND
22,000 RPM
Oval pistons, each with
a pair of conrods and octet of
valves, were designed to give V8
breathing at over 20,000 rpm to the
V4 NR500 racer. The components shown are
from the NR750 street bike, a limited-production 1992 model.
" We took samples home, went to Daido Steel or
Nippon Special Steel, who analyzed it and then made
the same for us, " he said.
In reading about Honda's GP engines, one will discover
that specialists who have made modern replacement
cranks for the revered 250cc six have said, " We
found in them materials unfamiliar to us. " The reason
may have been ad hoc changes to established materials
such as AISI 9310 to optimize them in vacuum-remelted
form. Vacuum-remelted materials display increased
fatigue resistance because the process greatly
reduces the population of oxide inclusions ( " stringers " )
which may nucleate fatigue cracking.
The aim of adopting such materials in highly
stressed parts such as crankshaft and connecting rods
was to allow further reductions of bearing diameters
without penalty of failure.
Piston innovations
Because all these engines were air-cooled and internal
oil flow was limited, piston cooling was accomplished
mainly by intimate contact with cylinder walls. Piston
axial length in Honda's early-1960s 250cc fours was
1.24 times bore diameter, giving them a " 1940 Chevy "
look. Two compression rings and a single one-piece
perforated oil scraper were provided.
At the close of the 1967 season, piston length had
dwindled to 65% of the bore with two rings - a single
gas ring and sometimes a single taper-faced scraper
much like those used in large U.S. radial aircraft engines.
Honda's 1962 race-piston ODs were undercut in the
ring belt and relieved over a large area of the sides. By
comparison, the pistons of 1967 appear engineered for
top-to-bottom contact, much as two-stroke pistons
were in the same timeframe.
Lighter pistons reduced bearing loads and fuller
contact with the cylinder wall (especially in the ring
belt) worked to reduce piston crown temperature and
prevent ring sticking.
30 April 2023
Honda's 500cc four-cylinder RC-181 in 1967 generated up to 93 hp but the
motorcycle lacked the handling to match its power.
Despite rumors of V8 wonders yet to come, Honda left motorcycle
GP racing at the end of 1967 after racking up multiple world championships
and helping to establish Honda as the leading motorcycle
OEM. (Listen to a 250cc six-cylinder RC166 restored by Honda technicians
run: https://www.youtube.com/watch?v=eaRop_ZMwo0.)
Racing had indeed made Honda a worldwide household name.
By 1975, two-strokes dominated every class in FIM GP racing.
Honda was determined to reestablish four-stroke superiority.
Because two-stroke BMEP was now approaching four-stroke levels,
this proved beyond even Honda's reach. Its solution was the bold,
oval-piston, 8-valves-per-cylinder V4 NR500, engineered to operate
at near-double the rpm of the two-strokes (then nearing
12,000). This subjected its pistons to crack-producing peak accelerations
30% greater than the best F1 technology of the time could
survive. A limited-production 750cc V4 street bike with oval pistons
was later developed; see SAE Technical Paper https://www.sae.org/
publications/technical-papers/content/930224/.
The NR500 never won a single Grand Prix point but did ultimately
reach 136 hp at 19,000 rpm. And having seen what was coming,
Honda shifted its GP bike racing R&D to two-strokes, winning three
Grands Prix in 1982 and making American rider Freddie Spencer
500cc world champion a year later on the three-cylinder NS3 twostroke.
In 2002, new FIM regulations began a process, completed in
2012, mandating four-stroke power in all motorcycle GP classes.
Four-stroke racing development programs of the 1960s - at Honda
in Japan and at Coventry-Climax and Cosworth in England - worked
independently toward the eventual combination of four narrow-included-angle
valves above an essentially flat-topped piston. What worked
in racing also turned out to provide efficient combustion for the majority
of today's production auto, light truck and motorcycle engines.
Kevin Cameron has been analyzing motorcycle technology for 50 years as a
columnist in Cycle and Cycle World magazines. He is the author of the books,
The Grand Prix Motorcycle and Classic Motorcycle Race Engines.
AUTOMOTIVE ENGINEERING
BOTH IMAGES: HONDA
https://www.sae.org/publications/technical-papers/content/930224/ https://www.sae.org/publications/technical-papers/content/930224/
Automotive Engineering - April 2023

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