wshall,

I guess you're refering to my Skybolt.

The muffler in the pics is a Turbo-Jett muffler from Jett Engineering. It's a tuned muffler in a compact in-cowl format.

The .75AX is a perfect match for the Skybolt. It spins an APC 14x6 prop at an even 10,000rpm which is a few hundred rpms more that the 4-stroke .91 with the same prop. It gives it very good performance and great vertical.

Now, about those mufflers.
The stock muffler works great with the 14x6 prop. It's very quiet. The only thing is that it wont clear the fuse with the ARF's horizontal engine mount. There's only about 1/4" in it so you could either use an exhaust extension or rotate the engine mount sligtly so that it clears.

The Turbo-Jett muffler gave me exactly the same rpm figures with the 14x6 prop, but it needed 3/4 turn richer on the high needle. It's really tuned for higher rpms and started to come into it's own with smaller props and rpms over 11,000.
On a model like the Skybolt it provided absolutely no benefit over the stock muffler other than being easier to mount.

The surprise for me was the Bisson Pitt's muffler for the .61FX. I tried it on the test bench and got 10,300rpms with the 14x6, but it didn't have enough exhaust pressure out of the box and would need one of the outlets plugged before i would consider flying the model with it. It's also considerably louder than the other two pipes.