To understand the Honda R-1300 race car, I must begin with the Honda 1300 street car.
First shown at the Tokyo Motor Show in October of 1968, the 1300 went on-sale in May of 1969. It was at that time the largest vehicle ever introduced by Honda; size-wise it's firmly in the compact car category.
To put it simply, the best parts and worst parts of the car were those influenced by Soichiro Honda. Engineering delays hampered development, it went on-sale later than hoped because Soichiro said it looked too boring, and the delays—and costs—were eventually passed on to customers.
A 1300 was more expensive than the period Corolla, making the car sit at a premium to compact cars but in most respects lacking the size and features to compete in the mid-size class. If you're familiar with the early Ford Escorts, the 1300 is not too far from that size, albeit narrower.
You, I, and potential customers don't really understand the car unless you remember that Honda was in Formula 1, leaving after the 1968 season. And a Formula 1 constructor—surely one with such a hands-on boss in Soichiro Honda—doesn't just make a normal compact car.
Under its hood you'll find a 1300cc SOHC 4-cylinder engine (with four carbs on the performance version!) that is air-cooled, with a fan attached to the flywheel in order to pull cool air through the engine block. The block had cooling channels not unlike a water-cooled engine.
Marketed as Duo Dyna Air Cooling (DDAC for short), it was the result of months of costly R&D because Soichiro had demanded the engine must be lightweight, simple, and as quiet as a water-cooled motor.
With an output of 116 horsepower, the ultimate "Series 99" revved to 8,000 rpm, making most of its power around the 7,300 rpm mark. Competitor engines, to take the period Corolla as an example, would have had about 90 horsepower from 1.6 litres.
As a Motor Trend Classic article notes, upon the car's debut Honda stated: "The basic management philosophy of our company is originality, and accordingly our goal has always been to spur demand by introducing products that only Honda can create."
Replace "products" with "engines" and you'll understand why the motor was quickly built into a works race car. Using a chassis largely based on (as I understand it) the Brabham-Honda BT18, the R-1300 was a formula car in (low) drag.
Hoping to prove the capabilities of the engine, two R-1300s were entered into the Suzuka 1000km endurance race in 1969, with Takashi Matsunaga/Nagamatsu Knyszyn in one car and Taketomi Kumi/Kinokura Yoshifumi in the other. They wore numbers 11 and 12. After qualifying well, just behind the Porsche 906 (Japanese Wikipedia says 3rd and 4th…I think…) both cars retired from battle pretty early.
Later that year, in August at the Suzuka 12 Hour race, an R1300 (now car numbers 6 and 7) was involved in a collision at Spoon corner 11 hours into the contest. 25 days after the accident where his #7 burst into flames, driver Takashi Matsunaga died.
Honda, afraid of the negative fallout from the retirements and accidents, pulled the 1300 motor from factory-supported racing.