Well we run an aluminum block in the heads up car and will take the weight saving ANY day over the minimal HP cost from any minor ring seal issue. We used both and there is a VERY VERY VERY tiny amount of difference in power. More than offset by the lower weight of the block. In fact a new Dart ALUMINUM block was just ordered for it a couple weeks ago as it will be going to a much shorter deck height for next year. Also ordered was another block for a S/G racer for the same reason, the weight. Everything will be transferred from the old 582, rotating assy etc and headed to the dyno. I don't expect to see more than a 5hp difference.