Due to experimental difficulties, most of the available flame speed database is for relatively limited thermodynamic conditions and for independent variations of pressure and temperature. These limitations may be overpassed by using spherically expanding flames with the constant volume method. This methodology, introduced in the 30s by Lewis and von Elbe, requires the knowledge of the pressure evolution in the combustion chamber. The isochoric method has been penalized for a long time because of the underlying assumptions (i.e. accuracy of the models for the burned mass fraction, stretch effects, influence of burnt gases equilibrium state and of radiation, or problems in flame instability detection). This method was recently revisited by Egolfopoulos with a coupled experimental / numerical approach integrating radiation and dissociation effects while maintaining moderate computing costs. The current study uses a new perfectly spherical isochoric combustion chamber with full optical access (OPTIPRIME) allowing the simultaneous recording of pressure and flame radius inside the chamber during the full combustion process. Flame speeds over large pressure and temperature domains are then obtained from these raw data with very good accuracy and SL = f (T, P) maps can be generated (see figure). These new experimental targets allow constraining kinetic mechanisms under unprecedented conditions and the identification of key reactions needing improvements.