This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency- Domain Equalization). We consider iterative frequency-domain receivers that jointly perform equalization, multi-packet separation and channel decoding operations, for up to Qmax mobile terminals transmitting in one slot. In this paper we propose an accurate analytical model for a p-persistent random access MAC protocol, designed to handle a total number of mobile terminals J above Qmax. It evaluates the uplink non-saturated packet delay and calculates the optimal configuration for the packet transmission probability associated to each mobile terminal that minimizes the delay. Our analytical results are validated using physical and MAC layer simulations.