On Sep 17, 5:36 pm, Roy Smith wrote:
wrote:
The old no-more-than-square thing was a rule of thumb for pilots
who flew engines that had little or no operating instructions,

Keep in mind that there's nothing magic about "square" operation. Square
means "the manifold pressure is that same as the prop speed". That's
hogwash; the numbers only work out the same because of an accident of what
units we use.

There's nothing that says we have to measure prop speed in RPM; we could
just as easily measure it in radians per second or Mega-degrees per
fortnight. There's also nothing that says we need to measure manifold
pressure in inches of mercury. It could be in mm/Hg, torr, atmospheres,
PSI, Pascals, etc.

No, there's nothing magic about it. Just that the old guys
often avoided oversquare operation unless they could find
manufacturer's data recommending it. Some of these old practices get
carried forward into newer engines where they make no sense. Old
engines often had to run on low-octane fuels that suffered detonation
at low RPM and high MP, and the accident of RPM vs. MP was a handy way
to avoid it. Detonation was a sure way to end up on foot miles from
anywhere hospitable, and since fuel was cheap and the boss was paying
for it anyway, it was safer to use more and get home.
I have the cylinder from an IO-520 here that had been
detonating. The head is blown clean off the cylinder; the aluminum
fractured at the top of the cylinder threads. Things would get very
noisy, shaky, smoky and scary if that happened. Cylinder pressures go
out of sight during detonation, as do CHTs.

Dan