The 318's aren't as competitive because they don't have the power for the straights. In Spec E30 a 318 can weigh 2600 pounds at the minimum, and a 325 can weigh a minimum of 2750, so there really isn't a lot of weight difference. Also the only thing that NASA allows you to do to keep the 318 competitive is to use a stock BMW single mass flywheel. And honestly think about it and ask around, how many people are actually running an m42 in SpecE30, there aren't many.