The JS29 should have made its debut at the 1987 Brazilian Grand Prix using the Alfa Romeo 890T V8 turbo until the 415T was ready. However, following Arnoux's unfavourable comments, and Fiat's withdrawal, this meant that the team had no engine with which to compete until the second race in San Marino. It was there that the JS29B with its Megatron engine would make its competition debut. The JS29B was updated following the Detroit Grand Prix to its final JS29C specifications. While the JS29 was originally designed for the 4-cylinder Alfa engine, it wasn't simply a case of slotting in the replacement Megatron 4-cylinder in its place. Major differences in the 'plumbing' of the two engines caused the team to have to redesign the car, most notably its rear suspension as tests showed that the Megatron was causing bad vibrations which was causing suspension failures.