The path from McLaren’s 1987 MP4/3 to the all-conquering MP4/4 did not jump straight from drawing board to domination. It passed through a small run of development cars built to answer a precise question: how to integrate Honda’s new 1.5‑litre turbo V6 into McLaren’s existing architecture under the most restrictive turbo regulations Formula One had seen. The MP4‑3B was that tool, a short‑lived but pivotal test mule that never took a race start yet underpinned one of the most effective seasons in Grand Prix history.

For 1988 the FIA cut maximum boost to 2.5 bar and capped fuel at 150 litres, forcing turbo teams to chase efficiency and drivability rather than headline power. At the same moment, McLaren ended its long collaboration with TAG‑Porsche, announced a new works partnership with Honda at the 1987 Italian Grand Prix, and confirmed Ayrton Senna alongside Alain Prost. The competitive risk in arriving with an all‑new car, all‑new engine and a transformed driver lineup was obvious. The MP4‑3B was conceived to de‑risk that transition while the clean‑sheet MP4/4 was still being finalised.

Structurally, the MP4‑3B sat between eras. It retained the MP4/3’s carbon‑fibre/aluminium‑honeycomb monocoque built with Hercules, but the rear of the car was re‑engineered to accept Honda’s RA168E 80‑degree V6, revised cooling and ancillaries, and a different gearbox and oil‑tank installation. The car’s purpose was not to chase lap records in period but to prove out packaging, cooling, fuel consumption and pop‑off‑valve behaviour in a representative chassis long before the MP4/4 was ready.

The development program began soon after Monza. The first MP4‑3B running came at Silverstone in late 1987, with Alain Prost conducting the initial shakedown in cold conditions. Honda then required a dedicated test platform in Japan, and Emanuele Pirro was brought in as test driver. Over the winter of 1987–88 he completed “thousands of kilometres” at Suzuka, working through fuel‑economy maps and drivability for the new 2.5‑bar, 150‑litre regime. By early 1988 the MP4‑3B travelled to Jacarepaguá in Rio, where Senna and Prost used the Honda‑powered mule in pre‑season running while the final MP4/4 was still in build; period photography confirms the MP4‑3B configuration at that test.

The timeline underlines how compressed the program was. The definitive MP4/4 did not appear until the last day of the final pre‑season test at Imola, only about a week before the Brazilian Grand Prix. It then went straight to Rio and dominated the 1988 season from the opening round. McLaren and Honda could afford so little MP4/4 mileage in part because the MP4‑3B had already done the integration work: validating the RA168E’s installation, fuel strategy and cooling in a known chassis, and giving the engine partner a vast bank of data before the new car ever turned a wheel.

Museo Juan Manuel Fangio’s documentation states that McLaren built two MP4‑3B chassis: one converted from a 1987 MP4/3 race car and one assembled around an unused monocoque, internal reference “SSC/7,” to allow adaptation during build. One of these cars survives today in the Fangio Museum’s permanent collection in Balcarce, Argentina, where it is displayed and occasionally demonstrated at events such as Autoclásica.

The example now presented by Mouse Motors is the other surviving MP4‑3B development car. According to the current owner, this chassis remained in McLaren’s possession until 2019, after which it entered private hands. The same account notes that McLaren’s Heritage Partner, Paul Lanzante Ltd, was commissioned to restore the car to its original race‑ready condition, with work beginning in 2020 and concluding in mid‑2022.

McLaren records, as reported by the owner, provide a detailed picture of this chassis’s working life. Notably, no testing is recorded in TAG‑Porsche MP4/3 specification; all documented running was as an MP4‑3B with the Honda engine. The test log lists mileage at Estoril in late 1987 with Prost, followed by an intensive 1988 program: Suzuka with Prost, further Suzuka running with Senna, and multiple Suzuka and Fuji test weeks with Pirro. Complementary narrative sources also reference a Silverstone shakedown by Prost and extensive Suzuka endurance work by Pirro to tune fuel usage, transient response and pop‑off‑valve behaviour across a range of conditions. This positions the car squarely within the key development loops that shaped the 1988 package.

Technically, the car remains a representative artefact of late‑turbo‑era Formula One. The carbon‑fibre monocoque supports a double‑wishbone, pushrod suspension with Showa gas/oil dampers, a Japan‑centric choice distinct from the Bilstein hardware seen on race MP4/3s. Braking is via Carbone Industrie carbon discs, with twin master cylinders, and the car rides on Dymag magnesium wheels—13×11.75 in front and 13×16.5 in the rear—shod in Goodyear Eagle tyres. Quoted weight is around 540 kg, with a top‑speed envelope of approximately 330–340 km/h depending on circuit and wing level.

At its centre is Honda’s RA168E 1.5‑litre, 80‑degree V6 turbo, using a long‑stroke 79.0 × 50.8 mm layout, 32‑degree valve angle, PGM‑FI fuel injection and a pair of IHI RX6D turbochargers. Factory documentation and contemporary summaries describe output in the region of 685 hp at roughly 12,300–12,500 rpm in qualifying trim at the 2.5‑bar boost limit, with the engine mapped as carefully for fuel economy as for peak power to handle 150‑litre race distances. In the MP4‑3B, this power unit was as much a calibration subject as a performance weapon, giving Honda and McLaren a controlled environment in which to refine mixture, boost control and response.

In the current collector landscape, this MP4‑3B occupies an unusual space. It is not a Grand Prix winner, and it never appeared on a starting grid. Instead, its importance lies in what it allowed other cars to achieve. As the research dossier notes, the MP4‑3B functioned as a bridge between the TAG‑Porsche era and the MP4/4, enabling McLaren and Honda to arrive at Rio in 1988 with an all‑new car that was competitive from its debut. For collectors interested in development history and engineering process rather than only trophies, the car represents a tangible link to the preparation work behind an extraordinarily successful season.

That context is reinforced by the small production run, the presence of a sister car in a major museum collection, and this chassis’s documented association with Prost, Senna and Pirro in period testing. The subsequent restoration by McLaren’s own heritage partner brings the car back to period presentation and mechanical specification as described in contemporary records.

This MP4‑3B can be read as a piece of applied engineering history: a carbon‑fibre bridge between two engine programs, built for a narrow regulatory window and used intensively before being retired once its job was complete. Today it stands as a concise expression of how top‑level teams manage risk and development under time pressure. Now offered through Mouse Motors, it is presented as a carefully documented, thoroughly researched example of a car whose influence was felt most where cameras rarely pointed.