I have the MVVS 91, currently in a Seagull Harrier. It is fitted with a 16 x 4 wideblade and is revving towards 9000. The engine has had almost one gallon of 20% castor, 5% nitro, through it so it is still quite tight and the revs vary a little. Pe Reiver reckons these engines need "a season" to loosen up (whatever a season is...)

Anyway, this engine pulls like a tractor and spins that big prop up very fast, so is responsive for hovering. Another bonus for 3D is that it is light, especially if you get the Weston throttle pipe from Just Engines. With that setup you would be pushed to find a better power weight ratio with such a tractable engine. (<600g)

Do not use the box silencer that the engine comes with as it restricts the power significantly. Thes engines apparently perform very well on a full-length tuned pipe, although I have no experience with that setup. I do know, however, that a large volume silencer is preferable.

Just a point to note: This is the second of these engines I have had. The first leaked a lot of oil from the front bearing, which got inside some models. I have had many discussions about this with various authorities, including Dar and Pe with various diagnoses, but nothing conclusive. I think the engine had a loose fitting shaft. The new engine has a sealed front bearing fitted by me and leaks no oil. Whether it would otherwise I don't know, but it is an easy and cheap enough mod to make. If you look very carefully at the piccie, you can just make it out.

Regardless of the above, I thaink the MVVS91 is an excellent power unit, both for 3D and for pattern. (Goes well in a Swallow EX and pulls a 13 x huge pitch)). It is reliable, powerful and starts well; it does not seems to mind running upside down. This engine is not, however, a revver. It seems to like to pull a big prop. I am impressed with it. I think it out-fourstokes the fourstrokes...