  Observations of type Ia supernovae include information about the
characteristic nucleosynthesis associated with these thermonuclear explosions.
We consider observational constraints from iron-group elemental and isotopic
ratios, to compare with various models obtained with the most-realistic recent
treatment of electron captures. The nucleosynthesis is sensitive to the highest
white-dwarf central densities. Hence, nucleosynthesis yields can distinguish
high-density Chandrasekhar-mass models from lower-density burning models such
as white-dwarf mergers. We discuss new results of post-processing
nucleosynthesis for two spherical models (deflagration and/or delayed
detonation models) based upon new electron capture rates. We also consider
cylindrical and 3D explosion models (including deflagration,
delayed-detonation, or a violent merger model). Although there are
uncertainties in the observational constraints, we identify some trends in
observations and the models. We make a new comparison of the models with
elemental and isotopic ratios from five observed supernovae and three supernova
remnants. We find that the models and data tend to fall into two groups. In one
group low-density cores such as in a 3D merger model are slightly more
consistent with the nucleosynthesis data, while the other group is slightly
better identified with higher-density cores such as in single-degenerate 1D-3D
deflagration models. Hence, we postulate that both types of environments appear
to contribute nearly equally to observed SNIa. We also note that observational
constraints on the yields of $^{54}$Cr and $^{54}$Fe, if available, might be
used as a means to clarify the degree of geometrical symmetry of SNIa
explosions.
