[toomanytoys2]

Actually, there are two different styles of spark arresters. If you take apart an FMF Turbine core, it has what almost looks like a multi-blade propeller. The out going gases hit this plate and are forced to spin through the rest of the silencer. This way any burning particles impact this plate and swirl in the muffler and cool down.

The other type is the screen option and Xbird is right. It is on the outlet side of the silencer. The screen type does add a little bit of back pressure and also reduces the decibels a little. However, with the screen type they also tend to clog up with carbine, so you want to make sure that it is mounted so that it can be cleaned frequently.

That being said, you are still going to be playing Russian roulette with the forest service folks. From my experience with dirt bikes, I have run the gamete of how they check for a spark arrestor. In some cases, if it doesn't say on the silencer that it is forest service approved, you are dead in the water. In other cases I have seen the rangers grab a stick and poke it in the silencer to make sure that it wasn't a straight through muffler.

The only way that you can be guaranteed safe is to run a silencer that states that is forest service approved.