hey settle down...not all of us use a webra 50GT.. it would be best answered by someone using this engine.. a 9 X 8 is a commonly recommended speed prop for an engine of this size..I use one on an OS50SX but my info wouldn't be much good to you..it all comes down to experimenting to see what best suits your engine airframe combo.. the 9 X 8 equipped Webra 50GT on a viper will certainly keep you on your toes.. I'm sure someone who has one will come along..patients is an admirable trait!!