.....theories look good on paper....and the MAN .60's
shoot-out was good reading....but it's beginning to look
like you perhaps, have not owned a K&B .61, or flown with
one for period of time.

I've been flying them for over 30 years....I have five or six
of them. My original one has been rebuilt 5 times, and is on
the third crankcase. I could say, that probably, I'm
as experienced as just about anyone (except Clarance).

flianbrian ....(who started this thread) said....
Quote: "I have a used old K&B 61"....

The fact of the matter is....the 6550 series perform very well
with the 11-7 prop. The very best prop for a nice crisp one
is the 11-71/2 MAS. The last ones....with the smaller muffler
Kicked off at a slight angle....calls for one prop to be used.
The 11-8. And they pull 'em really good. The older ones won't
pull the 11-8.

The K&B .61's don't like the 12-6....too much lumber, and
not enough pitch.....they're just "doggy" with the 12-6, and
don't perform well. I've flown the 6550's at sea level for
18 years, and at 3000 feet for 12 years.

Trust me, if you don't have an 11-71/2 MAS in the box....
the 11-7 is THE PROP....the motor will dog-out with the
12-6, or the 11-8....all my running is with 15% nitro, and
I've had them in all types of planes. I've flogged every oz.
of power from those motors since I started flying.

Here's one from about 20 years ago....

Dave.