Can anyone direct me to a good source of information about the best way to look after your engine, in terms of operating limits for MAP and RPM when considered together?

The OM for the engine provides some guidance, but in many cases to get engine longevity there seem to be other rules applied, based on experience. I understand the old "squared" concept, but that's got no scientific basis and doesn't offer much help.

For instance, the Lycoming O-540 OM says that operations at high engine speed and low power settings should be avoided to prevent damage to the dynamic counterweights. The power curves show a minimum RPM limit of roughly (MAP - 6) x 100 i.e. at 24"Hg 1800RPM is the minimum. I have heard other use 4 as the maximum difference, which is about the limit if your take-off is 29"Hg and 2500rpm. Also some POHs specify limits on the prop-engine combination.

Obviously the other aspect of the equation is the mixture, as the power the engine develops is important to the question of what RPM can be safety run. What I'm not clear on, is what damage can be caused outside of limits - is this a prop vibration thing, or bearing damage, detonation?

What rules do you apply to keep your engine from being needlessly stressed?

I guess this is a theoretical exercise, because I can't think of many situations where you would want to run a long way "undersquare" or "oversquare" continuously.

I don't have the answer to your question, but a good read I found was by a John Deakin march 21 1999 at www.avweb.com. I don't know if he has all the answers or if their just opinions like everyone else. However it gives a guy something to think about.
Good luck in your quest.
46tcrft

Battson, I appreciate this post, because I often have the same question(s.) I've long been bothered by the "squared" rule of thumb, because it's not rooted in any engineering principle or even true physical relationship of manifold pressure to prop RPM. In fact, "squared" is even a misnomer applied to multiplying MAP x 100.

It just so happens that coincidentally, prop RPM being manifold pressure x 100 has treated a lot of engines okay over the last 70 years, and apparently hasn't steered anyone so badly that they blew something up. I was taught that being "undersquare" is bad (which is it? undersquare or oversquare? this term makes no sense), and that it's akin to lugging the engine in too high a gear.

I don't think any manufacturers ever reference this method. In fact, they give a table of values, or a graph, or some actual data collected from flight or bench tests on the engines and props that suggest best operating practice.

Here's an interesting article from Mike Busch, who I guess is a renowned aircraft engine guy: http://www.avweb.com/news/airman/184483-1.html

Thanks for your thoughts and those links guys,

There seems to me there's been increasing interest in running lower RPM. The question seems to be the same all round - will it wreck my engine.

The manufacturer's seem to be saying if you follow their charts / tables of tested values and you'll be fine. But you get the sense there's still some apprehension; which could be well founded - the tests probably didn't involve continuous running low speed and high power for hundreds of hours.

From one point of view, it seems like low RPM could be good. If you look at the friction curve, a good proxy for wearing force in the engine, it experiences less friction (wearing and heating) at lower RPM: