I had the same experience with the MACS stubby pipe on my Supertigre .61 equipped Kaos. It lost about 500 rpm over the stock muffler. Several "experts" at my field told me I did something wrong. "Ya jus gotta know how ta tune 'em dude!"[:@]

It looks like the manufacturer sweat soldered a piece of tube in the end of mine so theres some wall thickness to work with. Since its a POS anyway I wonder if enlarging the exit nozzle a tad would improve anything?