Automotive Engineering - April 2023 - 29

HONDA GP ENGINE LEARNINGS FEATURE
POWER THROUGH REVS: HONDA GP ENGINES 1959-67
BORE X STROKE
50cc
YEAR
1962
1964
1966
125cc
1959
1962
1964
1965
1966
250cc
1959
1964
1965
1966
1967
350cc
1962
1966
1967
500cc
1966
1967
TYPE
RC-110
RC-114
RC-116
RC-141
RC-145
2RC-146
RC-148
RC-149
RC-160
2RC-164
RC-165
RC-165
RC-166
RC-170
(285cc)
RC-173
(349.5cc)
RC-174
(297cc)
RC-180
(489.9cc)
RC-181
(499cc)
The engines listed in the table above show the Honda designers' trend toward higher rpm to win the GP war against the two-strokes.
Irimajiri's radical design changes were effective, enabling Honda rider
Ralph Bryans to win the 1965 50cc Grand Prix championship. His success
led to his next assignments: design an improved 125cc race engine (an
inline five-cylinder, replacing the previous four-cylinder) to parry Suzuki's
1965 125 championship, and design a cranktrain for the 250cc six-cylinder
3RC-164. Since its introduction in late 1964, the Honda " six " had failed to
stop Yamaha's 1964-'65 two-stroke dominance of the 250cc class.
These multi-cylinder cranks (save for the 50cc twins) continued the
use of a central gear driving the clutch by a jackshaft and the cams by
spur gears located behind the cylinders. Because the torque transmitted
through right and left crank halves increased as it approached the
central gear, shafts, crankpins and their bearings also increased in diameter.
In the six-cylinder cranks, three different crankpin diameters
were provided, and the same for main bearings and shafts. All were
supported by caged needle bearings.
It remains normal to this day to reduce crank bearing size to a
practical minimum because friction torque rises steeply with diameter.
It's likely that as smaller-diameter shafts were tested, fatigue
failures appeared. This presented a choice; either stop short of such
failures or upgrade crankshaft material. This " torque tailoring " of the
pressed joints also required that the crank webs be of minimum
mass, seeking to raise the torsional frequency of each half of the
crank above excitation by the firing frequency.
AUTOMOTIVE ENGINEERING
GP engine design
leader Shoichiro
Irimajiri went on to
head development
of Honda production
bikes then headed
Honda's Ohio
manufacturing
operations. He was
later an executive
with Sega.
NO. CYLS.
1
2
2
2
2
4
5
5
4
4
6
6
6
4
4
6
4
4
(MM)
49 x 39
33 x 29
35.5 x 25.14
44 x 41
44 x 41
35.26 x 32
34 x 27.5
35.5 x 25.14
44 x 41
44 x 41
39 x 34.8
39.34.8
41 x 31.5
44 x 41
50 x 44.5
41 x 37.5
57 x 48
57.56 X48
BORE/STROKE
RATIO
1.03
1.14
1.41
1.07
1.07
1.1
1.24
1.41
1.07
1.07
1.12
1.12
1.3
1.15
1.12
1.09
1.19
1.2
HP @ RPM
9.5 @ 14,000
12 @ 19,000
14 @ 21,500
15.3 @ 12,500
24 @ 14,000
28 @ 16,500
32 @ 20,000
34 @ 20,500
35 @ 13,000
48 @ 14,000
55 @ 18,000
60 @ 17,000
66 @ 18,000
49 @ 12,500
64 @ 13,000
65 @ 17,000
85 @ 12,000
85-93 @ 12,300
COMPRESSION
RATIO
10.5
11.5
12
10.5
10.5
10.5
12
12
10.5
12
12
12
10.5
11
11
11
11
VALVES/
CYLINDER
4
4
4
2
4
4
4
4
4
4
4
4
4
4
4
4
4
4
BMEP
(PSI)
176
164
145
130
182
181
170
177
143
183
163
188
195
179
183
167
188
197
Aerospace materials
Irimajiri explained in an interview that the 1950-53
Korean War opened a high level of industrial cooperation
between Japanese and U.S. companies.
" When we had trouble with materials we went to GE
or P&W in the USA (both are builders of aircraft turbine
engines). They asked us, 'what problems do you have?'
We explained and they introduced us to a very special
steel, a new material they were using for turbine shafts.
April 2023 29
FROM TOP: CAMERON RESEARCH; COURTESY S. IRIMAJIRI
Automotive Engineering - April 2023

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