RE: OS 81FS-a
OK! finally got around to running the engine. Took out the baffle which didn't make much of a difference. The muffler is rather small, it looks like the muffler found on the 56a both of which I've sold recently.
I ran the saito 82a and the fs for a side by side comparision. The 63s would be off the power band so it wouldn't be fair.
Fuel used Wildcat15% as I don't want the engine to be addicted to more than 20% nitro.
Prop of choice 13x8apc.
Saito 82a------------9800rpm ( standard muffler with velocity stack)
OS fs81 alpha---- 10250/10290rpm less than one tank running!!
I have to admit that I didn't hold much promise for the 81.The readings surprised me.The muffler had a smaller bore than the saito and the engine looked ordinary.
The saito has had 15flights on it so it would be well broken in - almost.
I expect a bit better rpm increase with a bit more running
I'll see if I can substitute a ys muffler.
The os91 surpass II weighs in at 650gm with standard muffler and a few gms lighter with the older style version and you can fit the velocity stack onto it so it could be another alternative. Don't have time to tach it but I suppose figures should be better than the 81.
I would rationalize my purchase by the fact that its a few mm shorter than the OS91, has lower vibration than the 82 at full bore ,is quieter with the baffle and it doesn't leave the plane with an oily residue.
That said, the saito is lighter and the OS91 is almost of the same size, you can fit the velocity stack to save some fuel and improve response and would be more powerful.
I leave you to decide the choice.