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Is a 4 gallon fuel cell (Fc-140) big enough for most tracks or should I spend then extra 20 and put the 5 gallon FC-150 in if I have room?

A 4 Gallon cell like the FC-140, will easily run a 1200cc VW engine for any SCCA National or Regional of maximum length. (It will actually run a 1600 for ALMOST that long. But try not to win, since you won't complete the checkered flag parade lap. <g> )