Engine wear and fuel savings can be achieved by reducing rpm and increasing manifold pressure to maintain horsepower. Modern engines have approved power ranges for which this practice is approved. These ranges are in the POH but can also be found in the engine manufacturer's handbook.

On this page we talk about why running over-square is not harmful to the engine, think of it as switching to a higher gear as is done in a car. Obviously, this is for engines equipped with a controllable propeller.

Running oversquare

Charles Lindbergh visited the South Pacific in World War II as a civilian representative of an aviation manufacturer. He helped pilots lift higher bomb loads and fly greater distances. He was successful on both counts, achieving greater range mostly through over-square operation. For example, the P-38 was regarded as having an operational range of about 400 miles; when he left the pilots regarded it as having a 950 mile range, plus combat time over the target and a fuel reserve. However, the P-38 pilots took a lot of convincing that running their engines over-square would not cause them to fail early (thereby dropping them in the vastness of the Pacific with little change of survival).

Oversquare means having the MAP higher than RPM divided by 100. Thus a MAP of 25" and a RPM of 2300 is called over square. In case of turbocharged engines they are almost always run oversquare, for example: The Lockheed Super Constellation runs on a MAP of 51" at take-off with 2900 engine RPM and cruised with 44" and a much lower, 2600 RPM.

Examples
A turbo normalized engine has a lower MAP, usually 30-32 inch, which is kept constant until a certain altitude. RPM is 2700 at take-off and in the range of 2200-2500 RPM during cruise. Even in this situation you are running oversquare. Another example is a C-172 with an O-320 and a fixed pitch propeller: RPM during initial take-off is around 2300 (it will rise when airspeed increases) and MAP is 29-30 inch, also oversquare.

Another oversquare example is a C-172 (or any type aircraft with a fixed pitch prop) static RPM is around 2200 full throttle (max MAP). When taking off the MAP will be 29-30 inch and the RPM around 2300. On each and every take off this engine runs in a good oversquare condition but no harm is done.

Running your engine with a lower RPM has a number of advantages:

• less vibration, noise, heat, and wear
• longer engine life as a result of this
• much less noise emitted from the engine/propeller
• more efficient cylinder charging and better combustion, gives lower fuel consumption if leaned properly
• less power lost to friction inside the engine

For example, in one particular aircraft (like the Piper Turbo Arrow IV PA-28RT-201T with a TSIO-360FB engine), the recommended combinations for 65% power at sea level and higher are:

• 2200 + 32.8 inches (up to 6000ft) or
• 2300 + 31.1 inches (up to 10000ft) or
• 2400 + 30.0 inches (up to 14000ft) or
• 2500 + 29.2 inches (up to 18000ft)

As indicated above you can use a low RPM and a high MAP or a high RPM and a low MAP to achieve exactly the same power output from the engine. The low RPM / high MAP combination probably gives more efficient cylinder charging and better combustion plus less friction. The high MAP also acts as a cushion in the cylinders, reducing engine stress. MAP is usually measured in inches of mercury [inHg] rather than hectopascals. Standard sea level barometric pressure is 29.92 inches Hg or 1013.2 hPa.

But to enjoy all the advantages mentioned above you will need to select the lowest RPM and highest MAP. On take-off do the same: 30" MAP and 2700 RPM and after passing 500' AGL reduce RPM to 2500 but leave the throttle wide open, this will reduce engine noise. Do not worry about the MAP this will reduce too as air pressure drops with 1" per 1000 ft, thus engine power reduces as well. This will not work with turbocharged engines.

Now we know that oversquare is not so bad, lets not over do this as even your car will not start driving from standstill properly in fourth gear.

More background information
For readers wanting more information on this subject we refer them to the next series of articles (opens in new window):

• Recreational Aviation Australia Inc -- Engines and propellers
• AVweb: Mike Busch -- Why oversquare is good (free registration required)
• AVweb: John Deakin -- Engine related articles (free registration required)

The above articles are a must read for anyone with an engine and controllable propeller interested in keeping the engine in good shape for years (and flying hours) to come.