I have done the homework I should have done in the first place. The cars are pictured at the 1931 Belgian Grand Prix, driven by Charles and Francois Montier. The "straight-eight" car was not built until 1932, and raced without success in 1933. So I'm fairly sure these are Model-A powered.
PS the other car pictured is Jean Pesato in an Alfa-Romeo 6C1750. Grand Prix racing was in a mess in those days, with little manufacturer interest, and just about anything on 4 wheels could enter. Rather like Formule Libre.