Yamaha OX66 engine

With the OX66, compactness was prioritized over even power delivery, hence the unconventional bank angle of 75 degrees. In order to reduce weight, the cylinder block is cast from aluminum. Cast iron wet cylinder liners are used to prevent the thermal deformation of the block from affecting the cylinders, and were designed to be replaced if needed, aiding in maintenance.

The OX66 uses a five-valve system derived from Yamaha's prior experience with five-valve technology, with three intake valves and two exhaust valves per cylinder. The use of five valves decreases the inertia of each individual intake valve (since they are smaller), but also offers more stable combustion under low-load conditions compared to Honda's four-valve RA260E.

A timing belt was chosen to drive the camshafts for weight reduction and maintenance reasons. Unlike with a gear drive system, where incorrectly tightened cylinder heads can introduce backlash, a tensioner can be used to ensure the belt works optimally.

In a normal belt drive, the crankshaft has a sprocket that directly meshes with the belt, which usually results in the camshaft sprockets being made twice as large. To decrease the diameter of the camshaft sprockets and make the engine more compact, a reduction gear train is used that indirectly connects the crankshaft to two separate timing belts, one per cylinder bank.

When the OX66 was first announced for the 1985 season, electronic control was adopted for the fuel and ignition systems. The fuel system used electronic fuel injection manufactured by Nippon Denso. Using an air temperature sensor and an engine speed sensor, it issued corrections based on the position of the throttle valves. Initially, the injectors were installed directly above the intakes, but were later repositioned to the sides of the intake stacks. The ignition was also electronically controlled. One igniter was installed on each bank, with six coils to distribute power. Timing was controlled by a pulse generator at the rear end of each intake camshaft.

Per the mass production specifications of 1986, the electronic injection was replaced with Bosch mechanical injection, which was easier to adjust. In addition, a Bosch system had one more control element than a Lucas system (accounting for volumetric efficiency varying at different engine speeds), and the injection amount could be controlled more accurately. Since the fuel injection pump is installed between the cylinder banks, the six coils that had been installed previously could not fit. Therefore, the number of coils was halved and a distributor was installed. The fuel injection pump is driven by a belt connected to the left bank's intake camshaft.

In 1985, the OX66 was tested in two cars under actual competition conditions, driven by Geoff Lees and Keiji Matsumoto. As the OX66's performance was on par with that of Honda's RA265E, it was greenlit for use by both works and customer teams in 1986.

In 1986, the OX66 saw use by 9 teams in the Grand Champion Series and 11 teams in F2. Alongside Ken Matsuura Racing Service, Tomei Engine and Ogawa Motor were also in charge of maintenance. Lees won the second round of the Grand Champion Series (later becoming series champion), and Matsumoto won the third round. In that year's F2 championship, Lees won a total of four races, but lost the overall title to Satoru Nakajima.

However, after it was found that the upkeep cost of the OX66 would be two to three times that of a BMW F2 engine, teams unsuccessfully negotiated with Yamaha to reduce maintenance costs. As a result, in 1987, the All-Japan Formula Two series made way for the All-Japan Formula 3000.

Aguri Suzuki and Kaoru Iida drove for Yamaha in the 1987 Grand Champion Series, which still allowed F2-engined cars. Their OX66-equipped cars weighed a minimum of 560 kg (1,230 lb). Although 10 kg heavier than the previous year's F2 cars, they were still at a weight advantage compared to the 610 kg (1,340 lb) F3000 cars. Suzuki decided to compete against the F3000-spec cars with his lighter-weight machine, but was ultimately outpaced by the F3000 cars' higher power. He did, however, place fourth in qualifying and third overall.