I would not use the FMF spark arrestor model unless you absolutely have to.  The spark arrestor insert adds considerable back pressure  The FMF straight through silencer with the 1 3/8" OD mid pipe works good up to 60 to 65 hp then you need to cut off and replace the small short stinger that connects the tail cone with the mid pipe with 1 3/8" OD 18ga tubing.

Laddbg,
The PT pipe is a great all around pipe especially for the money.  Jerry is saying to use the regular FMF silencer with NO spark arrestor unless you have over 60hp then you'll need to modify the end of the PT expansion chamber where the little 1" diameter piece of pipe is that the silencer pipe (mid pipe) plugs in to.  above 60hp you'd replace it with a 18ga 1.375" pipe instead of the 1" pipe it has from the factory.  But up to 60hp the regular PT pipe with a standard FMF silencer will work good.


Hope this helps clarify it for you.