FCSetMetricSignature sets the signature of the Minkowski metric used when working with Cartesian objects, like CartesianPair, CartesianIndex, CartesianMomentum etc.
The default choice is (1,-1,-1,-1) which corresponds to FCSetMetricSignature[{1,-1}].
Overview, FCGetMetricSignature.
FCSetMetricSignature[{-1, 1}]
SPD[p, q] // LorentzToCartesianp\cdot q-p^0 q^0
FCSetMetricSignature[{1, -1}]
SPD[p, q] // LorentzToCartesianp^0 q^0-p\cdot q