Yoshimura history – 06

1966-1970: The Rise of Yoshimura-tuned Race Cars

Since Pop had moved from Zasshonokuma, Fukuoka to Fussa, Tokyo, the demand for the engine tuning and modification, not just of motorcycles such as CB72/77 but also of cars, was growing exponentially. One of the most popular cars to tune and modify back then was the Honda S600 and S800 sports cars which were equipped with race-bred twin-cam 4-cylinder engines.

In 1959, to compete with 250 cc two-stroke racing motorcycles, Honda constructed the RC160 factory racer with a twin-cam 16-valve 4-cylinder engine designed for high RPM performance. Honda then introduced twin-cam production racers such as CR110 (49 cc single-cylinder) and CR72/77 (247/305 cc twin-cylinder) by applying their Grand Prix technology.

In 1964, Honda launched RA271 Formula One car with a 1495 cc twin-cam 48-valve V12 engine and won the following year’s Grand Prix with its successor RA272. In the realm of production automobiles, Honda consecutively launched the small but high-performance water-cooled twin-cam 8-valve 4-cylinder models, starting with the 354 cc micro truck T360 in 1963, followed by the original S-series sports cars S500 and S600 in 1964, and the S800 in 1966.

Tuning the twin-cam engines of the S-series was an easy task for Pop. Although they had twice the number of cylinders compared to the motorcycles he had been working on (such as BSA and CB72/77), they shared the same mechanisms and by then he had developed a strong interest in twin-cam engines through working on CR72/77s and CR110s.

Pop began tuning cars after moving to Fussa in 1965. In the meantime, Yoshimura-bred race rider/driver Kuniomi Nagamatsu, who had been racing a S600 since the second Japan Grand Prix in 1964, was proving the superiority of Pop’s handmade camshafts. Many Japanese race car drivers of the era, including legends like him, Kunimitsu Takahashi (former Honda GP rider), Tetsu Ikuzawa (raced a Tohatsu at Gannosu), Masahiro Hasemi, Kazuyoshi Hoshino and Motoharu Kurosawa (the latter three were all ex-motocross racers) were converts from motorcycle racing.

In 1965, another Yoshimura-bred rider Fukumi Kotake began car racing, in addition to racing a Yoshimura-tuned CB72 with Masahiro Wada for the race team Technical Sports (Honda-supported team based in Suzuka). In the same year he won the GT-2 class in the Suzuka 300 km race on a Honda S600.

After moving from Fussa to Akigawa, the car-tuning became a growing, profitable business for Yoshimura. In 1966, the Honda S800 made its debut amid the excitement of the opening of the two new race tracks in the vicinity of Tokyo; the Funabashi Circuit (1965) and the Fuji Speedway (1966).

Since the Japanese Grand Prix inaugurated in 1963, domestic car manufacturers such as Toyota and Nissan (merged with Prince Motor Company after the 2nd Japan GP) began to pour their resources into racing as a branding campaign. The car racing scene was further intensified by the numerous privateers driving modified sports cars. One of the most popular racing sports cars of that time period was the Honda S-series, which was usually equipped with the performance parts developed by tuners like Pop.

Hiroshi Fushida on a Yoshimura Honda S600 won the Junior Continental class in the 7th National Stock Car race, held at the Funabashi Circuit (1.8 km course) in 1965. It was the first official victory for Yoshimura-tuned sports car.

From left: Hiroshi Kazato, Pop, unknown and Mamoru Moriwaki. Future car racing legends such as Kazato often came around to hang out with Pop. The fame of the tuning god extended beyond the two-wheel racing world.

The 1970 JAF Grand Prix FJ class winner Otokichi Hori and his Formula Junior 360 machine, Otokichi Special, which had a chassis fabricated from scratch by Hori and a Honda N360 engine fully tuned by Pop.

Yoshimura’s new factory at Akigawa attracted many future racing legends, including Hiroshi Fushida (after winning the 1965 National Stock Car Junior Continental class on a Yoshimura-tuned S600, who became a Toyota factory driver and a regular competitor in the Fuji Grand Championship series), Yojiro Terada (before becoming a Mazda factory driver renowned for numerous successes at the Le Mans, was racing a S600 since 1965), Kiyoshi Misaki (debuted driving a S600 in 1965, prior to becoming a Toyota factory driver), Hiroshi Kazato (debuted driving a S800 in 1967 and soon established his name as a Porsche 908 & 910 driver, before entering the Can-Am series and the European Formula 2 Championship), Noriko Aoki (one of the pioneer female race drivers who once raced a S600), Otokichi Hori (raced a Pop-tuned CR110 to win the 1966 MCFAJ 50 cc class race and later designed Formula Junior 360 / Formula Libre 500 cars Otokichi Special and Otokichi Arrow) and Akira Mitsui (raced Honda N360 & Z360s as well as motorcycles before establishing a parts business) who all later prove their professional competences as well as the advantage of Yoshimura tuned engines.

Future legends were also among Pop’s own employees, notably Ken Matsuura and Mamoru Moriwaki.

Matsuura and his little Honda S600, with a set of futon mattresses tucked into it, came all the way from Ehime on the Isle of Shikoku in 1966. During his two years as a Yoshimura apprentice, he modified his S600 to compete against RSC-tuned Hondas and had achieved some impressive results as a racing driver, including a victory in the 1967 Japanese Grand Prix GP-1 class.

He later returned to Ehime and found the now internationally acclaimed engine tuning firm Ken Matsuura Racing. The company is specialized in high precision engine components – such as forged pistons, titanium connecting rods and hollow titanium valves – and has for decades been satisfying various requests from both motorcycle and automobile factory teams.

Mamoru Moriwaki was from the city of Kobe and had been riding a Honda CB72. Right after he made his first visit to Yoshimura, along with his father in 1966, and by riding 17 hours all the way from Kobe, he brought his CB72 to be tuned. Then, after some regular visits to the Akigawa factory, he stopped returning home.

His CB72 went through full tuning – from engine modifications to chassis reinforcements – by Pop himself and his assistant Ken Matsuura. As Moriwaki recalls, the outcome was impressive as “it’s top speed was wildly improved from 130 kph to more than 200 kph!” It was like a magic to him. He later married Pop’s daughter, Namiko, and in 1973 established his own company, Moriwaki Engineering in Suzuka.

And there was Pop’s son Fujio Yoshimura who had reunited with his family after graduating high school. Pop made Moriwaki and Fujio join Yoshimura’s own racing team Nishitama Speed Club. The team mainly consisted of customers, many of them naturally talented riders.

In Fujio’s eyes, Moriwaki (born in 1944, making him four years older than Fujio) was a fascinating rider who is a very gentle person with a birdwatching hobby, and yet, once on the race track, he somehow always grabs a podium finish effortlessly.

Crashes and injuries made it difficult for Fujio to continue motorcycle racing. While not initially his kind of thing, he began racing on four wheels.

Fujio, on the other hand, was an all-or-nothing racing rider who would either be the fastest one on the track or the one to crash. So finally, one day when Fujio broke his arm right after setting a pole position time in a qualifying at Fuji, Pop told him to convert from rider to driver. The hot-blooded Fujio thus became a car racer, and by teaming up with the cool-headed Moriwaki, won the GT-1 class in the 1970 Fuji 1000 km race on a S800.

Namiko had been playing a significant part in the family business – from procurement to administration – as her father’s right hand since they moved to Tokyo, and as she had been painfully watching her younger brother’s risky riding style, she was overjoyed by the victory.

The #45 Yoshimura Honda S800 driven by teammates Mamoru Moriwaki (the starting driver) and Fujio Yoshimura on the way to win the 1970 Fuji 1000 km GT-1 class. Moriwaki had achieved outstanding results both on four and two wheels, including the 2nd place in 1970 Singapore Grand Prix Motorcycle Race.

Working on cars gave Pop some new ideas. One of them was the valvetrain upgrade for the Honda CB72/77 (with single overhead cam and rockers), which is done by replacing its original valve spring retainers and shims with those of the Honda S800 (with twin direct-acting overhead cams).

It was a practical and effective solution to optimize the performance of Yoshimura high cam, by averting the lateral pressure on the valve stem ends. When used along with special valves, valve guides and high cam produced by Yoshimura, the genuine steel (but lightweight) retainers, tappet adjusting shims (also lightweight) and cotters from S800 reduced friction at high rpms and thus increased engine power. The valvetrain upgrade significantly improved the lap times at the high-speed Fuji Speedway by approximately 5 seconds from 2:25.

Another idea was the high performance 4-into-1 exhaust system for race cars, which later revealed to be an invention. The idea came out when Pop was trying to install a S800 engine on a Brabham BT15 Formula 3 (or possibly a BT16 Formula 2) chassis at the request of racing driver Isamu Kasuya (a legend Formula Libre 500 driver and the winner of 1969 Fuji 300 km Golden Formula). While working on its exhaust configuration, the four separate exhaust pipes were tentatively welded into a collector in order to reduce weight and for better appearance. The outcome was surprising, as the devised exhaust system has led to a dramatic increase in engine power and mid-range torque, making the Brabham-Honda S800 faster than Toyota 2000GT.

Although at that time Akigawa factory had its own motorcycle dyno room housing a chassis dynamometer, the equipment was outdated and fairly inaccurate. Yoshimura consequently purchased an engine dynamometer and installed it in a new dyno room (both facilities were hand-built by Moriwaki who is also a skilled carpenter) so as to test various exhaust configurations, and after trying countless combinations of pipe diameters, pipe lengths and collector positions, developed the 4-into-1 exhaust system that works amazingly on the S800’s straight-4 engine. Later, after the launch of the Honda CB750 Four, the system proved to be also effective on motorcycles.

Yoshimura Honda Z360s were frequent podium finishers in the Mini Touring Car class, including this one driven by the late Akira Mitsui. Pop himself did all the tuning of the air-cooled 2-cylinder SOHC engines on his customers’ N360s and Z360s (launched in 1967 and 1970 respectively).

A Honda H1300 Coupe with a RSC-tuned chassis won the GT-1 class in the 1970 Fuji 1000 km, driven by Fukumi Kotake and Yoshimasa Sugawara (Guinness world record holder of 36 consecutive starts in Dakar Rally, in which he had participated since 1983 in motorcycle, car and truck category). Sugawara is second from right in the front row and to his left is Kotake. Pop is third from right in the back row and Ken Matsuura is in the back row wearing a cap. The H1300 was launched in 1969 and had an unique engine cooling system called Duo Dyna Air Cooling in which cooling air is forced through a double-walled engine block. Pop once ground down the outer wall of the engine block to eliminate the power loss caused by the cooling fan. The relationship between Yoshimura and Honda was going back to normal.

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Published on May 29, 2019

Stories and photos supplied by Yoshimura Japan / Namiko Moriwaki / Road Rider Archives

Written by Tomoya Ishibashi / Edited by Bike Bros Magazines

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