I have owned two 61Fx's and put macs mufflers on both of them.

I have had nothing but good results. Both engines gained a few revs but the biggest change was a quieter engine with exceptional response.

I first went to this muffler when I lost the back half of the first engine. Then when this one died from too many flights and one last one into the asphalt ibought a new on and bought a macs muffler at the same time.

Try it you will like it