I have a 181mm and love it. As mentioned, you cant get one new now. If I were to wear it out(??) or break it, I would likely go with a 178mm. I chose the 181 because I didnt want to push quite as hard on the engine as the 185, but wanted more than the 178. I hit max boost occaisionally with the 181, which I think messes me up on the top end at the track(???). It all comes down to choosing what you think will be best for your use and then enjoy the power and smiles per gallon.