To answer the original question - I was under the impression the note concerning the trunk-mounted cell was an allowance to the "12 inch rule" for the BMW's, that is otherwise in effect for other IT cars. Isn't it true that if the cell is trunk mounted in the 318, it will not be within 12" of the stock tank location?

So... it was my understanding that this allows the cell to be further than a foot from the original tank, but it limits the size of the cell to minimize the weight distribution shift (to the rear) that it facilitates. BTW, this would be true on some other IT-classified cars as well. But no one has apparently noticed the discrepancies, or the cars are classified but not being raced. Or they are being raced with stock tanks.