Second riders #12 Tom Kipp Jr. and #45 Matt Blair head out onto the track. The 1991 event was the last Suzuka 8 Hours for the oil-cooled GSX-R750. The race is known for scorching temperatures that cause engines to suffer from heat buildup, but that year's final was held in the rain at a relatively cool 25.4°C. Both Yoshimura bikes crashed, but #12 Aoki/Kipp finished 9th and #45 Martin/Blair finished 28th. Photo courtesy of Osamu Kidachi.
1990-1991 The End of the Air/Oil-Cooled GSX-R Era
The 1989 season was a great one for Yoshimura. Doug Polen won the TT-F1 and TT-F3 titles in the All-Japan Championship, while Jamie James won the Superbike and 750 Supersport titles in the AMA Championship, giving both riders double championships. Meanwhile, rival water-cooled machines were beginning to show their strength. By 1990, its sixth year, the oil-cooled GSX-R750 was clearly showing its age, and its power disadvantage could no longer be denied.
On top of that, beginning in 1990, the AMA Superbike series homologated not only mass-produced bikes but also limited-edition models. This allowed high-performance machines such as the Honda VFR750R (RC30), Yamaha FZR750R (OW01), and Suzuki GSX-R750R (known as the "Double R") to compete. Unlike TT-F1, which required only the engine to be based on a production model, AMA Superbike required the chassis and bodywork, including the fairings, to be based on standard specifications. Limited-edition TT-F1 replicas styled after works racers therefore became ideal base machines for AMA Superbike racing.
Miguel Duhamel rides a 1990 Superbike through Daytona's Turn 1. The 1990 AMA-spec machine featured two air ducts running through the fuel tank—the silver corrugated hoses visible between the upper-cowl ducts and the tank—and a side-mounted oil-cooling system. A small oil cooler was dedicated to cylinder-head cooling, while the larger lubrication oil cooler remained in its usual position in front of the engine. Polen was involved in an accident while testing this specification. Duhamel later crashed and fractured his hand, forcing him to withdraw from the Daytona 200 final. Photo courtesy of Takao Isobe.
Amid all this, Yoshimura was suddenly struck by misfortune. On February 15, 1990, an accident occurred while the team was testing a Superbike for the Daytona 200 at Willow Springs Raceway in Rosamond, California—about a 90-minute drive from Los Angeles and two hours from Yoshimura R&D in Chino.
Polen was descending the short straight from Turn 4—the highest point on the track, a right-hand corner with the Budweiser Balcony on the outside—toward Turn 5, a 90-degree left, when the throttle stuck open. He repeatedly worked the throttle grip, but the engine remained at full revs. As he tried to bail off the bike, the toes of his left foot became caught between the chain and sprocket and were severed through his racing boot. He lost four toes, leaving only the little toe.
About a month and a half later, Polen appeared in Daytona. It was his first outing after about half a month in the hospital, followed by home recovery and rehabilitation. His left foot was wrapped in bandages, over which he wore a sneaker converted into a sandal with the tip cut off. “It's a new model from Nike, the Air Polen. I'll be back in three or four weeks,” Polen told reporters in both English and Japanese.
Polen met up with his racing friends and breathed in the racetrack's distinctive mixture of racing machines, engine oil, and fuel. The familiar atmosphere made him even more determined to return.
In the 1990 season, Doug Polen raced with the #1 plate in the All-Japan series. He returned just over two months after his February accident and proved that he could still ride at full speed. He took pole position at Suzuka in April and won an AMA Superbike race in May. Photo courtesy of Takao Isobe.
True to his word, Polen returned at the third round of the All-Japan Championship at Suzuka—the second of eight TT-F1 races—with the final held on April 22. Instead of his modified "Air Polen" sneaker, he wore a regular racing boot padded to compensate for his missing toes. Remarkably, he took pole position and finished 4th in the final.
"The accident was a mechanical failure, not my fault. The machine just needs to be fixed, so I'm not afraid. When riding, I move my left foot forward slightly and step on the peg with my heel to put my weight on it. That's all the adjustment I've made. In fact, my riding is better than before," explained Polen.
Polen then made a one-off appearance in Round 2 of the AMA Superbike Championship at Road Atlanta on May 6 and won. Not even three months had passed since his accident.
Polen then returned to Japan for the opening round of the TT-F1 FIM Cup Series at Sugo, where he took pole position with an all-time lap record. His Yoshimura teammate Katsuro Takayoshi also entered, but a practice crash prevented him from racing. The final was a grueling 50-lap race requiring one pit stop. Yoshimura suffered an uncharacteristic delay during the stop, and Polen also developed raw skin around the surgical site. He finished 7th.
Polen continued in the All-Japan series, finishing 4th in Round 7, the Suzuka 200km on June 10, and 2nd in Round 8 at Tsukuba on June 24. He retired from Round 9 at Sugo on July 8 with an igniter failure. The rival water-cooled V4 Honda RVF750 riders, Kenichiro Iwahashi and Shoji Miyazaki, proved formidable and consistently finished ahead of Yoshimura.
As the All-Japan series entered its summer break, attention turned to the 13th Suzuka 8 Hours, with the final held on July 29. In 1989 and 1990, the endurance series was held as the FIM Cup rather than the World Championship. Yoshimura fielded two bikes: #12 Doug Polen / Miguel Duhamel, under AMA Superbike contracts, and #45 Katsuro Takayoshi / Rick Kirk. At 7:15 p.m. during the final stint, white smoke began pouring from Polen's #12 machine while it was running 5th. Lap times slowed by 10 seconds, and the engine sounded wrong. The pit board read "12 GO +42 L4." The team decided not to call Polen in, believing the engine would not restart if stopped. Polen reached the same conclusion. Officials appeared ready to show him the black flag, ordering him into the pits, but Yoshimura's mechanics desperately persuaded them not to. The checkered flag fell, and #12 finished 6th.
The severely damaged No. 3 cylinder head from the #12 Yoshimura Suzuki GSX-R750 after the 1990 Suzuka 8 Hours. A dropped valve caused the damage: all four valves were completely gone, and a large hole had opened in the crown of the forged piston. It was a miracle that the engine kept running and the bike finished the race in this condition. Carrillo H-beam steel connecting rods had been adopted after Satoshi Tsujimoto's bike suffered an engine failure at Daytona in 1986 because its connecting rods lacked sufficient strength. Carrillo rods offered excellent strength and balance. Photo courtesy of Tomoya Ishibashi.
The white smoke was caused by a dropped valve in cylinder #3. The piston and cylinder head were destroyed, with a hole in the piston crown and all four intake and exhaust valves completely gone. Fortunately, all three piston rings remained intact, and both the small and big ends of the connecting rod still moved smoothly. In short, cylinder #3 was dead and the engine was running on three cylinders, but the piston continued to complete its stroke. Meanwhile, #45 Takayoshi/Kirk finished 12th.
After the Suzuka 8 Hours, Polen entered Round 8 of the Superbike World Championship at Sugo on August 26. He crashed heavily in Heat 1 after slipping on oil from his own bike. The race was red-flagged, and the results reverted to the order one lap before the crash, placing Polen 8th. He missed Heat 2 because of an injury to his left leg.
In the resumed All-Japan TT-F1, Polen finished 2nd in Round 13 at Suzuka (September 9), 4th in Round 15 at Sugo (October 7), and 6th in the final round at Tsukuba (Round 16, October 28), ending the All-Japan season without a victory but still finishing 3rd in the overall standings.
Meanwhile, Yoshimura's Masanao Aoki, a former Mirage Kanto rider, competed in the All-Japan TT-F3 series and finished 4th in the standings.
Flat-track king and American hero Jay Springsteen riding a Yoshimura Suzuki in 1990 was big news. Springsteen had strongly wanted the opportunity, and Yoshimura R&D provided him with its latest machine. His riding position was almost entirely "lean-with," but years of racing counterclockwise dirt ovals made him exceptionally fast through left-hand corners, even on pavement. He finished the Daytona 200 in 8th, the highest-placed Suzuki rider. Photo courtesy of Takao Isobe.
In the eight-round AMA Superbike series, Duhamel won Round 7 at Topeka but finished 10th in the standings. He also won one AMA 750 Supersport race and finished 7th overall. Meanwhile, American flat-track legend Jay Springsteen, the three-time AMA Grand National Champion from 1976 to 1978, rode a Yoshimura Suzuki to 8th place in the season-opening Daytona 200, the best result among the Suzuki riders.
In the 1991 season, #33 Masanao Aoki became Yoshimura's regular rider for TT-F1. He rode well, but the inferiority of the oil-cooled GSX-R750 was clear. Even so, in the rain-soaked final race of the final round, the MFJ Grand Prix, he beat #01 Niall Mackenzie aboard a Yamaha FZR750 in a close battle to finish 3rd on the podium. #3 Polen, who made a spot entry with a Ducati 888 Superbike, took pole position with an all-time lap record in qualifying, but finished 10th in the wet final. Photo courtesy of Takao Isobe.
In 1991, Polen, the team's undisputed ace, departed for Ducati. Yoshimura named Aoki its lead rider in the All-Japan TT-F1 series alongside Steve Martin. Mike Smith, Dominique Sarron, and Matt Blair also made one-off appearances as the team explored which riders best suited the Yoshimura Suzuki TT-F1 machine.
In the 1991 All-Japan TT-F1 series, Aoki finished 16th in the opening round at Suzuka 2&4 on March 3, 12th at Suzuka on April 21, 5th in Round 5 at Sugo on May 12, and retired from Round 7, the Suzuka 200km, on June 9. He then finished 5th in Round 8 at Tsukuba on June 23 and 21st in Round 9 at Sugo on July 7. After the Suzuka 8 Hours, he placed 4th in Round 14 at Suzuka on September 8, 31st in Round 16, the TBC Big Road Race at Sugo on October 6, and 3rd in the final round, the MFJ Grand Prix at Tsukuba on October 27. He finished 7th in the overall standings, while Martin finished 17th.
For the 14th Suzuka 8 Hours, Round 3 of the World Endurance Championship, Yoshimura entered #12 Aoki/Kipp and #45 Martin/Blair. Martin crashed the #45, which caught fire, but the bike returned to the race after major repairs in the pits. Kipp also crashed the #12 during the final stint, and it too caught fire, but he restarted. The #12 finished 9th and the #45 finished 28th, with both bikes reaching the finish.
#99 Steve Martin was essentially a regular rider for Yoshimura in the All-Japan TT-F1 series. He finished 10th at best and ranked 17th overall in the 1991 season, failing to demonstrate his true potential. Photo courtesy of Osamu Kidachi.
Yoshimura contested the 1991 AMA Superbike series with the young pairing of Mike Smith and Tommy Lynch. Neither rider won a race; Lynch finished 11th and Smith 13th in the standings.
Beginning in 1991, TT-F1 eliminated the rule allowing displacement to exceed the class limit by 1% through a larger bore. Although AMA Superbike still permitted a 1mm overbore, the TT-F1-spec GSX-R750 returned from a 70.3mm bore to the standard 70mm bore. The air/oil-cooled GSX-R was clearly at a power disadvantage against its water-cooled rivals.
#68 Mike Smith rides the GSX-R1100. The Formula USA champion joined Yoshimura R&D in 1991 after winning the unlimited-displacement class organized by WERA, the Western Eastern Roadracers Association. He crashed on Lap 10 of the season-opening Daytona 200 and also made a one-off appearance at the All-Japan Championship round at Suzuka on April 21, where he retired. Injuries troubled him throughout the season. Behind him is #91 Grand Prix rider Rob McElnea on a Yamaha FZR750R (OW01). Photo courtesy of Tomoya Ishibashi.
Heat buildup had challenged the oil-cooled GSX-R750 since its debut. In TT-F1 trim, it used two oil coolers: a large curved unit for cylinder-head cooling and a medium-sized unit for lubrication. Yoshimura continually refined their placement, stacking them front-to-back on the 1990 model and arranging them vertically on the 1991 model. On the 1990 AMA Superbike machine, the cylinder-head oil cooler was mounted on the right side of the chassis. This reduced the aerodynamic drag caused by larger or multiple front-mounted coolers, an important consideration at high-speed circuits such as Daytona.
The new water-cooled GSX-R750W entered All-Japan and FIM competition in 1992, but AMA homologation was delayed by a year, just as it had been for the original oil-cooled GSX-R750. Yoshimura R&D therefore had to contest the 1992 AMA season with the oil-cooled model. Its unconventional solution was to add air-cooled-style fins to the cylinders. Whether because of that modification or not, David Sadowski finished 4th in the Daytona 200, the longest race on the AMA Superbike calendar and one held at exceptionally high speeds. Despite the air/oil-cooled GSX-R750's power disadvantage, Sadowski finished 12th in the standings.
The acclaimed air/oil-cooled GSX-R750 thus enjoyed a long and distinguished career in the All-Japan and FIM championships from 1985 through 1991 and in AMA road racing from 1986 through 1992, securing its place in four-stroke racing history.
Stories and photos supplied by Yoshimura Japan / Osamu Kidachi / Takao Isobe / Tomoya Ishibashi
Written by Tomoya Ishibashi
Edited by Bike Bros Magazines
Published on October 11, 2024