Tuned mufflers are often louder than basic mufflers are. Car engines are hard to quiet down because of their robust timing and often higher nitro content fuels. The nitro makes the exhaust note significantly sharper.
Basic sport airplane engines such as the OS LA and FP series are very low timed engines - anyone on the planet can operate them, and they’re not suitable to compare against a truck engine as mentioned in the beginning of my post.
Granted, this is an old thread that has been discussed at great length in the past, but blocking the pipe with foreign material is asking for trouble. Not only in respect to the pressure wave resonance (tuning) of the pipe, but also the fact that small fragments can dislodge and get pushed into the engine. There are ways of quieting an engines noise down, but it often comes with adverse consequences.