The expansion tank catches vapours, and maybe fuel if the tank is really full. Say you filled the tank full on a cool night, and the next day the temperature ges to 40 degrees centigrade, as it can do in a summers day in australia. As the fuels expands, it has to go somwhere, as does the air in the tank too. the 240z just dumps the extra fuel out through the overlow pipe behind the back wheel, not very enviromentally friendly, and not too safe I suppose either. The 260z had the expansion tank. when thats full, then I guess there is an overflow pipe too. Dont know that the 280z turbo tank had. I guess you could use the interal expansion tank as a surge tank, as long as it never leaked, but then you wouldnt have an expansion tank.