It all depends upon the engine. Some engines won't tolerate anything added to the stock muffler. Other engines don't "care". If you wish to use a deflector, you may need a larger exhaust outlet on the muffler, which then takes a deflector with a larger inside diameter. It all adds up to the amount of back-pressure seen by the engine. An exhaust system that's as 'tight' as possible to help keep down noise, or one that's finely-tuned to the engine won't tolerate much added to it if there's the slightest bit of added back pressure.