The BruLine filters obviously cause a slight restriction but I've never tached an engine with and without to find the difference. I suspect it's quite small. The filters (coarse/fine) are sometimes used as a tuning aid with CL stunt engines because the fine filter has about the same affect as reducing the venturi size by .020" and the coarse by about .010" on .60 size engines. Basically this means you get better fuel draw without having any spray coming out of the carb to mess up the outside of the engine. It's a struggle getting them on a Rossi 45 carb though