The C18 has between 400 and 700 HP so the fuel flow can be between 72 GPH and 126 GPH.
The choice of TFP80 from the other manufactures is too small. Based on its size the residence time of the fuel is less than 20 seconds. Our 5×8 because of the design will give 30 seconds of residence time. Meaning the longer the fuel is in the purifier the longer gravity works on the water to drop it out.
Using the largest possible purifier for longer residence time aka cleaning power and the more capacity to hold water before draining is recommended.
Based on 72 GPH the 5×12 has a residence time of 45 seconds where the 5×16 has a full 60 seconds.
The water sensor will trigger when water reaches the probe at approx 0.400 of 1 gallon. The system will continue to work unit the volume reached .750 of one gallon then you run the risk of passing water on into the engine. Drain it when the alarm sound or within 3 days not 3 months and the problem will no longer exist.
By using the competitions suggestion the customer will not be happy. His knowledge of how the separation works is lacking.