It moves the engine forward 4". I would love to have my engine 4" further aft. My W and B is very forward. As is three of my friends. We all have our batteries in the rear. These are 912, 914, and 915. With Constant speed props, these are getting into O-200 weights. Funnily enough. My friend with the 912 has the forward sweep because he took a Subaru off of his plane and put the 912 on. It still has a too forward C of G.