Ya know, I hope that quotes like this don't surprise too many people, we are dealing with a RACER here. The SHOESTRING is one of the few planes that you can expect to come out nose heavy. Stick a big .91 fourstroker in it, and it's going to be even more nose heavy. All of this means, that you are going to need to add more weight to the tail to make it balance !

Result - You made the little plane overly heavy, and it's going to result in SURPRISE, higher landing speeds. Put a lightweight 2 stroke .60 engine in. It will require no extra tail weight to balance, and it will land as slow as a butterfly with sore feet. [/B][/QUOTE]

Actually, if you check Tower's web site you'll find the OS 61fx and the OS 91fs with mufflers installed weigh almost the same. The 91fs actually weighs slightly less by a tiny amount.
Rather surprised me!! Two stroke mufflers weigh more than 4 stroke mufflers. :surprised