Hello!
As has been said earlier if you intend to compete in a certain pylonracing class get the rules first and find out what engine is allowed. In the US the foremost national Q-500 pylonracing class is the AMA 428 class where 6,5cc Nelson and Jett reign supreme.
For club racing many other sport 6,5cc engines are used like the TT .40.

Over here in Sweden we race Q-500 pylon using standard 6,5cc engines running ordinary plastic glassfilled 10x6 props and no nitro (80/20) fuel.
Webra .40 GT is and has been for the last 15 years the most popular engine used.
It easily outperforms any other engine, including the Rossi (which is sooo heavy) and which has a very poorly made silencer.
The Webra turns a 10x6 a black Graupner Cam-prop at around 14000rpm on the ground and easily rews up some more in the air.
So if you want the most powerful standard engine with a silencer, running a 10x6 prop (unmodified) on 80/20 fuel; Get a Webra .40 GT!

Regards!
Jan K
Sweden