I was always taught that WOT was a "renter's setting" and that you should
back off to 2300-2400 if you want to get good life out of your engine. I
keep running across the Advanced Pilot Seminar guys asking, "Why EVER be
partial throttle in cruise?

Of course, they are probably also talking about advanced engines, with GAMI,
analyzers, matched injectors, etc. What about those of us flying behind the
glorified lawn mower engines of simpler aircraft. I've asked the question
over at the CPA forum but I'd like some opinions from this group.

Should I take our 172 N above altitudes where the POH says max power is 75%,
put the throttle all the way in, lean till it gets a bit rough, and then
enrich until it's smooth? As long as CHT and oil temps remain in an
acceptable range, can I then cruise along confident that I'm getting there
fastest and getting the best life from the engine?

I've often suspected this was the case. Turning faster seems intuitively to
be the same as the engine working harder but the trade off is that it doesn'
t work as long. Speed of metal surface over metal surface (with oil film)
within normal RPM ranges doesn't seem as significant a wear factor as the
total number of firing cycles and revolutions. It seems like those should be
about the same whether you fly at 115 knots or 90.

Figuring the RPM's out from the POH:

At 4000 feet, WOT, 2500 RPM there will be 129,591 revolutions per 100 miles.
At 2400 RPM, 130,896. At 2300, 131,100. Going WOT instead of 2300 REDUCES
firing cycles 1.15%!

If the faster speed saves a bladder break, you'll get a huge savings in
engine wear avoiding a thermal cycle and restart.

On the other hand, marine engine factory reps, who I have more frequent
contact with, tell me that the only significant indicator of engine life (as
long as temperatures remain in normal range) is the total amount of fuel
that goes through it. Seems like that should be true for aviation engines as
well.

Slowing down from WOT to 2300 in my 172 N should reduce fuel consumption
14.5%. That's pretty significant as well as probably saving a fuel stop
somewhere on a long trip.

--
Roger Long