Nothing beats cubic inches!
When it came time to decide what was going to power my new Vigor CS, I was in a bit of a quandary as to whether to stick with the tried and tested OS 61 with Muscle Pipe, or try the new OS 91 with one of the mufflers. After listening to a lot of American pilots going on and on about how much power the 91 had and how smooth it was I decided to hell with it, get the 91!
These big engines require big exhaust systems. Some of the most popular exhausts
are provided by Hatori and KSJ. These things are huge! Word on the street is
that Curtis is designing a new Muscle Pipe for the big engines. Don't skimp
when it comes to exhausts for these things. A friend of mine put a 60 size muscle
pipe on his new OS 91 (one of the first in New Zealand) and it chewed it up
within 10 or so flights. When you think about it, it makes sense, after all,
the 91 is 50% bigger than the 61. I decided to go with the KSJ exhaust after
hearing lots of good things from pilots in the states. These big exhausts are
none too cheap in themselves. If you're thinking of putting a 91 in your machine,
remember that in addition to the engine cost itself, you're going to have to
stump up with another NZ$~300 (US$139) for the exhaust. Then you've got gearing
Externally, the carburettor looks much the same as the OS 50's although ofcourse it is much bigger. It has only two needles, a low and high speed needle which seem to work together very well in achieving nice smooth mid range power for hovering. The machine idles very well on the ground and once the needles have been set transition from idle to power is smooth and steady.
With the KSJ exhaust, the engine sounds quiet and smooth. On 10% nitro, the smoke content is more than I expected when the engine is running at what I believe is optimal settings, but I like the fact that it runs rich.
Stabbing the collective into a vertical climbout results in rocket like performance even at twelve (that's twelve) degrees pitch. I'm running MS 710 FAI blades on my CS and 12 degrees is no problem. I would dare to say that with my setup and my needle settings, 12 degrees is probably about it, but really, who wants more than that? There's no point. Looping with this setup is something else! However, I think I'll back the pitch off a little to maybe 10.5 or 11 degrees to keep the engine howling.
I'm really impressed with how smooth the engine runs. It's quite happy to sit there chugging away in the hover and then instantly transition to full power without any fuss.
If you like the OS 50, you're going to love the OS 91, tonnes of power and very smooth.