Yoshimura history – 07

A group portrait taken in front of Akigawa factory in the early summer of 1971 shows one of the prototypes of the 4-into-1 exhaust. Standing behind the CB750 engine is Fujio, who at the time was the test rider for the project. Pop’s groundbreaking invention made its official debut at the final round of the AMA road racing series, the Ontario 250 Mile, in October of the same year.

Inventing the World’s First 4-into-1 Motorcycle Exhaust System.

1969-1972: Four-Into-One Revolution

It was in 1968 at the 15th Tokyo Motor Show (held at now closed Tokyo International Trade Fairgrounds in Harumi from October 26 to November 11) when Honda unveiled the world’s first mass-produced transverse inline-4-cylinder (SOHC 8-valve) bike.

The bike was called the Dream CB750 Four and was received as an epoch-making machine by both Yoshimura as well as the international motorcycle scene. Targeting the biggest market in the world, Honda subsequently introduced the U.S. version of the bike at a dealers’ meeting in Las Vegas in January 1969.

With a bore and stroke of 61x63mm for a capacity of 736cc, the CB750’s engine developed 67ps at 8000rpm, the highest output of any production motorcycle at the time, which provided an impressive performance to allow a claimed standing quarter-mile time of 12.4 seconds and a maximum speed of 200kph. Exportation to the North America began in April, while in Japan the official launch date was July 18 and the retail sales date was set as August 10. Fujio Yoshimura recalls, “I broke my right arm in a bike race and couldn’t make it to the Tokyo Motor Show, but my dad (Pop) must’ve placed an order right after he saw one at the show.”

The 4-cylinder 750 Honda was an overnight global sensation.Due to the popularity, it took a while to be delivered to Yoshimura. “But what we got was not a Sandcast. It was a Diecast K0,” Fujio adds. The first batch of the early K0 model had sandcast engines, and then it was switched to diecast engines in September 1969 to improve productivity. One day while waiting for their CB750 to be delivered, Koji Ota of the Meiwa Racing (Honda Saitama Factory’s in-house racing team) rode a CB750 over to Akigawa factory to ask Pop to work on its engine, starting with cam modification. Ota had been an admirer of Pop’s workmanship and personality ever since he was blown away with what Pop had done to Ota’s CB125 race bike. Pop thus got his skilled hands on the CB750.

This archive photo shows a set of crankshaft and connecting rods for CB750 that underwent a mirror polishing process aimed to enhance strength and reduce friction. Polishing had been Yoshimura’s regular tuning menu since the CB72 days. All done by hand using electric buffers, the process was an incredibly time-consuming task.

On August 17, just a week after Japan sales began, two CB750’s of Honda’s in-house racing team Blue Helmet MSC took a 1-2 finish at the Suzuka 10-Hour endurance race (Morio Sumiya / Tetsuya Hishiki in 1st place, Yoichi Oguma / Minoru Sato in 2nd), and on the following September 13 and 14, another CB750 (ridden by Michel Rougerie and Daniel Urdich) won the 33rd Bol d’Or 24-Hour. Then in 1970, a CB750 factory race machine ridden by Dick Mann scored a historic victory at the Daytona 200.

1970 happened to be the year when the AMA adjusted the road race regulations. As they did with flat-track and speedway racing the previous year, the 500cc limitation for OHV/SOHC/DOHC engines –––– which has been providing a competitive advantage for the side-valve Harley-Davidson KR750 –––– was eliminated to allow full 750cc displacement for all engines regardless of valve location or number of cylinders. The internationally celebrated Daytona 200 then became a battleground between multi-cylinder machines such as Triumph/BSA triple, Kawasaki 2-stroke triple and Honda four.

Although Mann’s was the only one out of the four CB750’s that finished the race, the victory of the new 4-stroke 4-cylinder bike created an international sensation. The U.S. military veterans and retirees who had become Yoshimura customers while stationed in Japan subsequently began to enquire about Yoshimura’s performance parts for the CB750.

At Akigawa factory, Yoshimura began to explore the tuning potential of the CB750 using their K0 as a test mule, and to develop performance parts such as high-lift cams and big-bore piston kits (initial release was the 810/812cc 64mm piston kit and then the 823cc 64.5mm piston kit was added; both based on the stock pistons for the CB350 twin). CR carbs and close-ratio transmission were supplied by Honda RSC.

As the year 1970 was coming to an end, Yoshimura received an inquiry from an American Honda dealer to tune CB750 engines for the following year’s Daytona 200. The name of the dealer was Krause Honda. Their boss, Ronald Krause, who happened to know that Honda factory team is not returning to Daytona, had decided to challenge the race by themselves in order to keep the good reputation of the CB750. Race results back then, especially that of the Daytona 200, were an extremely important factor affecting motorcycle sales.

Krause Honda, the oldest Honda dealership in Pennsylvania, contacted Yoshimura through an ex-GI Yoshimura customer who had returned to the States. For the Krause Honda CB750’s, Yoshimura managed to increase the engine power from the stock 67ps to 97ps by providing modifications such as high-lift camshaft, heavy-duty valve spring, lightened crankshaft, head porting, head shaving (1mm off the head for higher compression ratio), CR carbs and close-ratio transmission.

At some point in its development phase, the Krause Honda CB750 was fitted with the early Yoshimura 4-into-1 exhaust pipe with a long megaphone end. It was replaced before debut race by the short straight pipe which was lighter in weight and had no difference in performance. The short seat cowling was also replaced by the elongated type before the Ontario 250 Mile.

As Ronald Krause expected, Honda factory team didn’t show up at the 1971 Daytona 200. Last year’s winner Dick Mann had left the factory Honda team and joined the BSA factory riding a 3-cylinder Rocket 3 along with Mike “The Bike” Hailwood.

Early in the 1971 Daytona 200 road race, the Yoshimura-tuned Krause Honda CB750 ridden by rookie Gary Fisher was leading the pack including GP veterans and AMA greats. It was definitely the fastest bike at the 31-degree banking as well as on the back straight until it was retired from the race on the lap 10 with a snapped cam-chain. It didn’t make headlines, but enough to let the world know of “Yoshimura Magic”. Satisfied with the engine’s performance, Ronald Krause proposed Yoshimura to continue the collaboration for the rest of the AMA road racing season. Pop likewise thought it was a good start, and he also had something special in mind.

Fujio looks back, “The 750 was still rare at the time. I remember one time when I parked my 750 roadside at a freeway bus stop, a motorcycle cop showed up to check it out. And then the cop noticed that it had a set of CR carbs.” Modifications like that were not road legal in Japan in those days. “There’s no way you can fool them. I knew I couldn’t get away with it, so I told him that I had been testing the engine setup for so-and-so reason. And guess what, he let me go. Motorcycle cops love motorcycles after all, just like us. As a matter of fact, they regularly dropped by our factory to see what’s going on.”

One early summer day, a strange set of black exhaust pipes was fitted to Fujio’s CB750 K0.

“The first thing I noticed was the sound,” he remembers, “the sound you’ve never heard before. And then the power! The engine revs up so quick! I thought, this is it. Just like the headers we made for the S8 (Honda S800), it was light in weight, which would eventually contribute to better handling and better overall performance. Since 750 and S8 shared nearly the same displacement per cylinder, Pop must’ve had a rough idea about pipe diameters and such.”

It was the birth of Shugokan: 4-into-1 exhaust for motorcycles.

“At the time we couldn’t find suitable steel pipes with the right diameter and gauge, so we had to fabricate them by rolling and welding sheets of metal into tubes, pouring sand into them and bending them with our hands. We tried various combinations of pipe diameter, pipe length and collector position.”

The 4-into-1 not only made the exhaust system much lighter, but also boosted low-to-mid range torque as well as high-rpm power. The only issue for motorcycle application was the bulkiness which might interfere with ground clearance, lean angle or the front tire.

Fujio repeated the road test using his own beloved K0.

The world’s first 4-into-1 motorcycle exhaust system made its official debut at the Ontario 250 Mile in October 1971. The Krause Honda CB750, looking very stylish with the distinctive, long aluminum seat cowl and yellow color scheme, was ridden by then-rookie Gary Fisher and was powered by a Yoshimura-tuned engine.

As the development of 4-into-1 exhaust progressed, Pop set its debut at the final (7th) round of the AMA road racing series, the 250 Mile National Championship Road Race, at Ontario Motor Speedway in California on October 17.

Located on the outskirts of Los Angeles and blessed with dry West Coast climate, Ontario was one of the American racing meccas of the era. In June, after completing the early stage of the development of the 4-into-1 exhaust, Fujio flew to the United States to support the maintenance of the Krause Honda CB750.

With the CB750 prepared by Fujio, Krause Honda team entered the 5th round of the AMA road racing series at Pocono International Raceway in Mt. Pocono, Pennsylvania (Krause Honda’s home state) and then the 6th round at Alabama International Motor Speedway (now known as Talladega Superspeedway) in Talladega, Alabama.

Fujio was furtively testing the early version of 4-into-1 during the races, however, the results of these races were not what they were hoping for and the invention didn’t get the attention it deserved.

Mamoru Moriwaki flew to the U.S. in August to research the local race scene and to enter races. After visiting Larry Shively, a local Yoshimura racing equipment dealer in Los Angeles, Moriwaki joined Fujio in Alabama in September.

At Talladega, Moriwaki entered AMA’s 76 Mile Junior Road Race with a Honda CB350 (twin), but the bike broke down, so he re-entered the race with a spare Krause Honda CB750 and took the 7th place (then-rookie Kenneth “Kenny” Roberts took the 3rd podium spot). The two then traveled to Pennsylvania to prepare the bikes before heading to Ontario, and on September 29, they were joined by Pop who brought the secret device –––– the definitive version of 4-into-1 exhaust –––– all the way from Japan.

The highlight of the final round of the 1971 AMA National Championship road race series was the duel between two Grand National Champions, Dick Mann and Gene Romero.

Krause Honda entered two CB750’s in the race, ridden by Gary Fisher and Roger Reiman. Former Harley-Davidson factory rider Reiman was a 3-time Daytona winner.

“People swarmed around our paddock as the warmup began,” Fujio recalls.

This was understandable since they have never heard the sound of the 4-into-1 exhaust. The odd, short, black steel pipe that Pop finally came up with through trial and error was producing a deep roar at low rpm which gradually turned into clear, breathy high notes as the engine revved up. But more surprises were waiting on the track. The Krause Honda turned out to be incredibly quick. Fisher posted the fastest lap in the qualifying session.

The final race, however, didn’t work out as expected. One of the Krause Honda’s blew its engine from sucking up sand when it ran off the track, and the other one broke down again from a snapped cam-chain. Both retired from the race, and to add insult to injury, someone stole a spare engine (which was replaced due to poor performance, however 4-into-1 exhaust was untouched) from the team’s pit box.

Nevertheless, race fans and all those involved in the race were excited about the groundbreaking invention in the world of motorcycle. The 4-into-1 then became a standard equipment for all inline-4-cylinder bikes, for both street and track, while Pop as usual didn’t even bother to take out a patent for it.

Daytona 200, March 1972. Krause Honda CB750, in the skilled hands of former H-D factory rider Roger Reiman, takes the first corner heading for the infield. The bike was equipped with the evolved version of Yoshimura 4-into-1 exhaust (the pipe ending near the footpeg) which was shorter in length than the one used in the 1971 Ontario 250 Mile (the pipe ending near the rear axle). The terrific power and sound produced by the 4-into-1 took everyone at the racetrack by surprise.

Responding to the Krause Honda’s subsequent decision to enter the 1972 Daytona 200, Fujio remained in the U.S. to build racing machines. Pop began devoting himself more intensely to the perfection of 4-into-1 exhaust and engine tuning after returning to Japan. He knew he could extract more power from the engine.

There was no theory at the time to help determine the best diameter and length for the 4-into-1 exhaust pipes, but Pop somehow knew in his mind what is going on inside them. He was adjusting the dimensions by imagining how the exhaust gas draws out extra power from the engine, as if it is a living thing. Coeval engineers at Japanese motorcycle manufacturers on the other hand were for long skeptical about the 4-into-1 –––– saying there is no explanation for the mechanism –––– and sticking to the conventional, separate megaphone exhaust pipes. They had never taken the time to analyze the device, and so they had to wait for some time to learn about the characteristics and influence of exhaust gas pulsation.

For the 1972 Daytona 200, Krause Honda again entered two CB750’s, ridden by Gary Fisher and Roger Reiman. The race was on March 12. As Pop arrived at the Daytona International Speedway, he was greeted by a peculiar sight. The 4-into-1 exhaust pipe became so popular among competitors that it was everywhere on the track. Most of them were fabricated by small tuning shops. It was phenomenal. Pop was fascinated by how Americans reacted to the new concept, which, in contrast, had been rejected by Japanese engineers. What provoked the American interest in 4-into-1 was perhaps not some logical reasoning, but the sound it produced.

Krause Honda mechanics preparing their Yoshimura-tuned CB750 racer at the Daytona 200 in 1972. The year marked the debut of the Dunlop KR83 tire which was originally designed to handle the power (around 100ps) of the 750cc 2-stroke racers Kawasaki and Suzuki were entering into the high-speed Daytona. Needless to say, the Krause Honda had also switched from Goodyear to Dunlop. Also note the Koni rears.

Fisher and Reiman respectively scored the 5th and 8th spot in the qualifying session. In the race, Krause Honda CB750’s were strong from the opening lap. Fisher was once again in the lead, followed by top 2-stroke riders including Yvon DuHamel & Gary Nixon on Kawasaki KR750’s, Art Baumann on a Suzuki TR750 and Kenny Roberts on a Yamaha 350 TR3.

The high pitch hums of 2-strokes’ expansion chambers were drowning out by the roars of 4-strokes’ 4-into-1 exhaust pipes resonating on Daytona’s 31-degree high banks. At lap 27, Fisher came out first from the 18-degree banking by the grandstand but was immediately forced to retire due to a cracked oil tank. The same failure dropped Reiman from the race. Still, a portal to the new era of 4-stroke 4-cylinder bikes was indeed opened by the introduction of 4-into-1 exhaust and the creative –––– in a way, magical –––– engine tuning methods as the names of Pop and Yoshimura began to become international.

Front cover of the 1974 edition of Yoshimura Racing Parts Catalog shows two Krause Honda CB750’s, #5 ridden by Roger Reiman and #30 ridden by Gary Fisher, at the 1972 Daytona 200.


Published on August 9, 2019

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

Written by Tomoya Ishibashi / Edited by Bike Bros Magazines

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