caution range for engine speed?

[WillJr]

Can anyone shed some light on the caution range for engine speed? Why is it there? Is there a time limit? What happens when you disregard it?

[cessna170bdriver]

If you’re asking about the C-145/O-300, it’s my understanding that it is rated to run continuously at 2700 rpm. I’ve been running mine at 2600 for years with no ill effects. At 600+ hours since overhaul I’m still getting about 12 hours to the quart of oil and the lowest compression at the annual last month was 77/80.

[Bruce Fenstermacher]

With a C-145/0-300, in a 170, there is no caution range. The TCDS for the C-145-/0300 says max continues RPM and the Take-Off 5 min RPM at full throttle is 2700 RPM at sea level. At higher altitudes you won't be able to reach 2700 RPM in a 170 with a legal propeller. The 170 TCDS say for all operations 2700 RPM.

Car 3 which the 170 was certified says the following:

Screen Shot 2025-04-13 at 5.54.55 PM.png

As there is no caution range for the 170 with a C-145/0-300 according to TCDS, you might conclude the tachometers as they were marked installed by Cessna are incorrect. The green arc should extend right up to 2700 RPM

Anyone see it differently than I do?

[n2582d]

Based on Will’s other entry, I think he’s referring to the caution range found on Lycoming engines with the Hartzell prop.

[GAHorn]

Can anyone shed some light on the caution range for engine speed? Why is it there? Is there a time limit? What happens when you disregard it?

If your airplane is modified in some manner which has operating limitations, you should be able to determine the limitations from the documentation which approved the mod.

In some aircraft installations, certain combinations of prop/engine have vibration in certain ranges which should be avoided. Your documentation should explain that limitation.

[WillJr]

With a C-145/0-300, in a 170, there is no caution range. The TCDS for the C-145-/0300 says max continues RPM and the Take-Off 5 min RPM at full throttle is 2700 RPM at sea level. At higher altitudes you won't be able to reach 2700 RPM in a 170 with a legal propeller. The 170 TCDS say for all operations 2700 RPM.

Car 3 which the 170 was certified says the following:
Screen Shot 2025-04-13 at 5.54.55 PM.png

As there is no caution range for the 170 with a C-145/0-300 according to TCDS, you might conclude the tachometers as they were marked installed by Cessna are incorrect. The green arc should extend right up to 2700 RPM

Anyone see it differently than I do?

Wow, Thank you so much for making me see the light at the end of the tunnel. You really make a whole lot of senses, Preciate you.

[Bruce Fenstermacher]

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