The car was designed by Technical Director Tony Rudd around BRM's complicated 3 litre H-16 engine, known as the P75, which had first raced in France in 1966 in a Lotus 43.

The P115 was a significant refinement of the P83 it replaced, with the most obvious differences being a squarer appearance and the external cooling pipes seen on the P83 being moved within the chassis for better aerodynamics.

[3] At the next race at Mosport Park Stewart qualified in 9th, and made a good start in wet conditions getting past Bruce McLaren and Jack Brabham who were immediately in front of him on the grid.

Stewart performed exceptionally as the rain came down but slid off while battling for second place and while the car was undamaged, dirt had penetrated the throttle causing it to stick and forcing him to retire after 65 laps.

For the United States Grand Prix at Watkins Glen Stewart managed tenth on the grid, alongside McLaren who was using his M5A with the BRM V12 engine that would replace the H16 for 1968.