A split-cycle variable capacity rotary spark ignition engine system having at least a first rotary configuration (C1) including repetitively volume variable working chambers [60, 61, 62] for carrying out the combustion-expansion and exhaust phases and at least a second rotary configuration (C2) including repetitively volume variable working chambers (70, 71, 72) for carrying out the intake and compression phases of a four phase engine cycle. Dividing seal means (73, 74 of C1, 75, 76 of C2) for periodically dividing each of successive working chambers into a volume enlarging leading portion and a volume contracting trailing portion. Discharge valve means for varying compression chamber capacity through discharging fraction of trapped intake gas from compression chambers. A first phase altering arrangement is provided for varying the phase relation between the first rotary configuration (C1) and the second rotary configuration (C2). A second phase altering arrangement varies phase relation between the discharge valve means and corresponding compression chambers. The first rotary configuration (C1) having variable capacity combustion chambers operatively synchronize with the variable capacity compression chambers of the second rotary configuration (C2) so that accomplish nearly full-load-like combustion environment through a substantially wide engine operating range.