On SLP's SF-1020 SuperFlow flow bench, the AFRs, stock 990s, and ported 291s went head-to-head in a flow comparison test. The iron and aluminum heads were flowed with a realistic bore size fixture of 4.280-inch (typical 427 or 454, .030 over). Most BBC heads are flowed using a 4.600 bore. A larger bore unshrouds the valves to help show better flow numbers. Surprisingly, our flow bench results (with the smaller bore) were similar at the lower (.200-.400-inch) lifts but not as much at high lifts (.500-.700-inch). Still, the AFRs flowed so much more cfm at all lift values (see the charts) than the old castings that we couldn't wait to see the difference on the dyno.