I have an OS EX-502 Silencer Set for my FS 91S. Does anyone know if these silencers are capable of increasing the power on my OS FS 91 S?

I would be very surprised if you see an increase in power. Primarily it will be much quieter than a stock muffler.

Arch

The pipe is a 4-stoke silencer to reduce noise as Arch said.

You won't see much difference in RPM compared the stock muffler.

Want a little more power out of the OS 91 4-stroke?
I believe Super Tiger 90 2-stroke has the same bore and the headshim is thinner.
You might want to check it out. Dick Hanson told me about it long ago.

There is no such a thing.
Tuned silencers are only good for 2c engines, which exhaust valve is the piston.

For 4c engines, the best silencer is no silencer at all, which means zero restriction for exhaust gases.
The bigger and shorter the exhaust pipe, with proper noise attenuation, the better for the performance of a 4c.

The tuned pipes increase the power on a 2 stroke engine by supercharging it when it pushes the unburned fuel mixture back into the cylinder, so increasing the pressure inside the chamber of new fuel mixture and "unburned" fuel mixture from the exhaust.
Because the four stroke has valves, the engine can not use this method as the exhaust valve will close before that can happens and the intake valve will blow out the fresh mixture during the valve overlap.
As previously noted, on the four strokes they are only silencers, but they evolved enough to have no power reduction in the engine performance and still being able to keep it quiet, but this statement is applicable to Hatori and Asano pipes, I do not have experience with any other, but I do now the standard mufflers like the OS will reduce the engine power.

Regards