Automotive Engineering - April 2023 - 27

HONDA GP ENGINE LEARNINGS FEATURE
RC165 cylinder head showing combustion chamber geometry. At
top is tunnel for the camshaft gear drive.
Design engineer Shoichiro Irimajiri's masterpiece, the 50cc twin-cylinder, 21,500rpm
RC-115 put Honda in the ICE vanguard.
shafts cut the 'vibrating length' of the crankshaft in half.
Honda won its first FIM Grand Prix championship in 1961 - the first
of many. In 1962, the FIM added a 50cc class, won by Suzuki twostroke
single-cylinder racers for three years straight. Honda, its fourstroke
reputation on the line, rose to the challenge.
BMEP, or brake mean-effective pressure, is an IC engine's strokeaveraged
net combustion pressure - a figure of merit for cylinder
filling and combustion efficiency. In best-practice naturally aspirated
four-strokes, peak power BMEP had by 1960 reached a practical
maximum of about 200 psi (1379 kPa). This compelled Honda to seek
power by raising rpm capability through the shorter strokes of many
small cylinders.
Two-stroke rpm is limited by the difficulty of fitting the intake,
compression, power and exhaust functions into only two piston
strokes. As Honda increased rpm capability with more cylinders and
shorter strokes, the two-stroke builders replied with increased trapping
efficiency - learning to retain in the cylinders more of the
charge blown through them during scavenging.
The RM64 Suzuki that took the 1964 50cc world title needed
14,000 rpm to achieve 12.5 hp (9.3 kW), while Honda's RC-112 twin,
revving to 17,500 rpm, had replaced the company's 50cc single in late
1962. But four-stroke power was somehow not " scaling " properly;
Honda's 50cc BMEP was stuck in the 140s - only 70% of what other
racing engines had achieved. What was missing?
Tackling the variables
Honda's early race engines had been designed by Tadashi Kume and
Kimio Shinmura. The problem of beating the two-strokes in the 50cc
class was given to 24-year-old Shoichiro Irimajiri, who had joined
Honda after graduating from Tokyo University with an aeronautical
engineering degree. " Iri's " 1964 refinements to the existing 50cc twin
AUTOMOTIVE ENGINEERING
helped beat Suzuki at the Japanese Grand Prix, while
Suzuki retained the championship.
Irimajiri faced these problems:
* Small cylinders lose a lot of heat from their unfavorable
ratio of surface area to volume
* The added surface area of traditional deep combustion
chambers and tall piston domes further increased
heat loss
* Seeking power from ever-higher rpm drove up friction
losses
Compression ratio in most of Honda's racing engines
had reached 12:1 by 1966. The easy way to raise
compression was by raising piston dome heights. This
increased the heat-gathering area exposed to combustion
and transformed the chamber into a slowburning
thin lamina - the peel of half an orange. That
required very early ignition timing that further increased
heat loss. Yet by 1964, Honda's experiments
with research engines had shown that combustion did
not systematically fall behind even at engine speeds as
high as 27,000 rpm.
Irimajiri explored all variables, seeking any trend of
performance improvement. He increased bore/stroke
ratio from Honda's usual 1.07 to 1.41 for his 1966 RC-149
five-cylinder 125cc engine, to make room for adequate
valve sizes. He designed shallower combustion chambers
and piston domes by radically reducing valve included
angle. The early 250cc four-cylinder had a
76-degree included angle; the 1964 RC-113 50cc twin, 72
degrees. He now chose a much narrower 56 degrees.
Because reduced valve included angle left less
room for cooling fins between the cam boxes, more
of the cooling task was transferred to the oil. In 1966,
Honda's four-, five- and six-cylinder race bikes were
given oil coolers.
April 2023 27
FROM LEFT: MICK WOOLLETT ARCHIVE; USED BY PERMISSION; JOHN OWENS
Automotive Engineering - April 2023

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