Hey guys, thanks for all the help. I thought I gave all the details, but I failed to mention that this is a brand new 120 2-stroke. Trust me, due to the close quarters of the cowl and a big engine, there is PLENTY of air getting out. I had to do alot of cutting. I have had problems in the past with Pitts style mufflers due to less back-pressure. I mounted the stock muffler in a Pitts style and I'm headed out to see the results. The 120 2-stroke comes with an adapter to do so. The reason I am running a APC 16x8 is because in the review by Jon Lowe on Great Planes' website, he used both a 16x8 and a 15x10 with pleasing results. I have always had satisfactory results with no greater than 15% nitro. I'll check back this evening with the results. Thanks for all your help.