hi flyer95. On my xls 52 I have the stock muffler, only with the baffle removed and I also have opened up the outlet a bit. the instructions for the mvvs tuned muffler says it going to increase engine power in the 13-15000 range. I thought the 12.25-3.75 prop would be in this range, might be just a tad to heavy, so I`ll try the 12-4. I`m waiting around for the light to be sufficient to do some tachreadings today