rotor&wing wrote:
Newps;639635 Wrote:
This is also why owners with constant
speed props are told to always reduce manifold pressure first. If you

are at 800 psi with everything wide open and you reduce rpm you have
made the situation worse. The engine stays at the high pressures
longer, and at a different spot relative to top dead center, because
you just made the engine turn slower. All bad.

Welcome once again to "Amateur Hour".

Funny thing, most of the radial engines I've flown, not to mention
GTSIO-520's and TSIO-540's always operate at high MP versus low RPM.

Please show me in a POH where it specifically says "Do not reduce RPM
before MP."

That's not the point. You have to look at the example I gave. Every
engine is different. For a given power setting on any engine reducing
rpm makes the pressure peak last longer. Very simple since you just
slowed the engine down. Reducing rpm has no effect on the internal
cylinder pressure. Reducing manifold pressure does. If your engine is
already below the point at which detonation can occur then it is
irrelevant which you reduce first. On the IO-520 in my Bo on takeoff at
sea level standard day the engine is nibbling the edge of detonation.
You would be foolish to reduce rpm first just after takeoff. That's how
this engine is designed. You can design the engine to handle more or
less pressure, for example turbo'd engines that run 40-50 inches of
manifold pressure. Run that on my 520 and all kinds of things will go
flying out the cowl. It's not designed for it.
So it's irrelevant that some radial engine can run oversquare. So
can mine. At cruise power. But you're not going to run an IO-520 at 31
inches and 2300 rpm and get very many hours out of it.