Flying my Lycoming O-360-A4M powered Cessna 172-N, I learned through experience something that should be in the POH, but is not. It’s about fuel efficiency during cruise.
These engines are conservatively rated (180 HP from 360 cubic inch displacement), designed to operate continuously at or near full power, and their 2700 RPM redline is a prop limitation, not an engine limitation. Also, at cruise altitude full power is a smaller fraction of their rated power; for example, at 10,000′ the air is so thin, a normally aspirated engine can only produce about 70% power. All this is to say, when cruising above 5,000′ MSL, you can run the engine at WOT. This is considered normal operation; the POH cruise tables support this configuration. It is not hard on the engine, in fact, these engines are designed to run best at or near WOT properly leaned.
However, you shouldn’t run it at WOT, but slightly less. Here’s why, based on 2 reasons:
The carburetor for this engine (and for the O-320-D2J it had before) has an enrichment circuit that mechanically engages at WOT. This adds a margin of safety against detonation, with a minimal loss of power since the power vs. mixture curve is asymmetric, tapering slowly on the rich side, steeply on the lean side.
The intake manifold for this engine is immediately downstream of (fed from) the carburetor. On any engine having a single carb upstream from an intake manifold, the A/F ratio to each of the cylinders will never be exactly the same. There will always be one cylinder that is slightly richer or another slightly leaner, than the others. This A/F balance across the cylinders is “mostly even” but exactly how even varies depending on the throttle position & mixture.
These 2 factors combine to form an important aspect of engine operation that should be (but isn’t in) in the POH. The carburetor’s WOT enrichment circuit impairs the distribution of mixture to the cylinders. When it engages, it increases the difference between the richest & leanest cylinder. This means, in order to avoid roughness (the leanest cylinder running rough), you must set the mixture richer than it otherwise would be when this circuit is not engaged.
You might wonder, why not apply WOT, then lean the mixture to compensate for the enrichment circuit? You certainly can do this. The problem is, the mixture setting will be richer than it would be, if the enrichment circuit weren’t engaged.
Put differently: when you pull the throttle back from WOT just enough to disengage the enrichment circuit, the mixture distribution across the cylinder is more even. There is less of a difference between the richest and leanest cylinder. Thus, the mixture setting for any equivalent power level (peak, 50 RPM below peak, or whatever) is leaner.
The difference is significant: about 15%. That is, if you fly at WOT and lean the engine properly, you will burn about 15% more fuel than if you pull the throttle back from WOT just enough to disengage the enrichment circuit. The POH tables reflect non-WOT operation, so at WOT you will burn 15% more fuel than the POH indicates. That is, if the POH says 8 gph, you will actually burn about 9 gph.
Procedure: High Speed High Altitude Cruise
So what is the best procedure for high speed high altitude (above 5,000′) cruise?
- Apply WOT.
- Gradually lean until slightly rough.
- Very slowly pull the throttle back.
- Since the engine is already lean, when the enrichment circuit disengages the engine will suddenly get much leaner, and you will get a sudden drop in RPM and increase in roughness.
- Leave the throttle in this position, then enrich mixture to desired setting, typically peak RPM, or 50 RPM below peak.
Typically, step (4) happens about 1/2″ back from WOT.