I always thought that the engines were less efficient because the props were less efficient, not because the combustion was affected.....Interesting!

Same thing will happen to a car. If I drive to sea level, my car will notably increase it's performance.

These Tower mufflers are semi tuned in a way. They have greater internal volume and allows the engine to breathe out more freely. In a test I did the Tower .75 muffler held 222 grams of water and my stock 91 FX muffler held 185 grams of water (could be a little off here, it is a long time ago, but the Tower muffler is definetly bigger)

They are also built very light due to very thin materials, quite opposite of the stock OS muffler which feels like a brick. The stock muffler is very restricted in the outlet and before getting the Tower muffler I just drilled out the stinger to the max and removed the baffle inside, that helped alot in the power departement

Are standard high torque servos for this plane enough or should I get digital?

Where you are you will likly need more engine then the OS 91 2 stroke, on the****************** site we have some people who live in Denver and they have to use more motor.

I would guess that a Saito 115 would be the best choice Normally the Saito 100 is the perfect match for the Swanyshouse Mojo MoAss 60.

No the 60 requires more servo then standard, You may get away with them on ailerons but on the tail I would stepup a bit, Metal gear would be best IMHO

Just set up with what you have and have fun

Then you can go from there....

I've got theprimo 40 w/ saito 72 325 hitec all around with 6vt and having a blast with it...

The reason I want to use the 91FX is because I have one lying around. If that engine wont allow me to hover the MOJO 60 then I have to consider another option.

An OS 91 FX will definitely hover a Mojo 60, even at your altitude. But it won't have much reserve power to pull out of a hover, or have enough power to let you feel comfortable learning to hover.

I'd recommend choosing a more powerful engine.

I agree with the two stroke Master Dave 3D McDonald

Given that I've already got the engine, isn't there any other plane which will be correctly suited by the 91FX?

There's a profile bird called a Hyper Active that might be a good match for the 91 FX at your altitude. Should be able to come in right around 5 pounds, which is 8 to 12 ounces lighter than the MoJo 60, with a good bit more wing area than a MoJo 40. The problem is that kits are very hard to find. It's kitted by the guy who designed it and he does small production runs which are usually sold out as soon as he announces that he's doing a run. If you poke around a bit on the profile brotherhood site, I'm sure you can find out how soon they might be available.....or perhaps find someone will to part with a kit. Here's a couple of pics of one built by a legendary Bro.

Plugin, I wouldn't worry to much. Build the Mojo 60 ...try to keep it light and use the 91FX. If you are unhappy with it THEN go through the hassle of buying a new expensive saito 115 ($359). If you buy a larger engine than the 91 FX, I think you will start to run into weight issues. Maybe a .91FX + the tower muffler will give you the oomph you need

If you're dead set on using a 91FX, then go ahead and build the Mojo 60. The 91FX will still give you plenty of power for basic 3D flying such as harriers, hovers, and torque rolls. Later on it might be worth experimenting with a Tower muffler, Ultrathrust, or a Mousse Can Pipe, but I'd start out with the stock muffler to make sure the engine throttles correctly for learning the basics. When it comes to 3D flying, a good throttling engine is far more valuable than raw top end power.