The K&B does weigh the same - the ST .51 weighs 12.7 oz without the muffler and 18.3 with. As I'm reading about the ST, I have found that they are designed for overseas where they fly with very liitle or no nitro content. This allows better performance with fuels that have lower nitro levels. At 10%nitro this engine might have great performance. could be a money saver also.