C172M 180hp AirPlains conversion runs HOT

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1 year 8 months ago #1207 by GARY PALMER
I have a 172M, the 180 HP Air Plains conversion, and hot #1 and #3 cylinders. I replaced the cylinders about 1.5 years ago because the prior owner ran it hot and my oil consumption just kept getting worse. Turns out my oil rings were shot. Now I have new steel cylinders (replaced the chrome things which I cannot remember right now). I also replaced the baffles using the new baffle kit from Air Plains, but that was before the cylinders.

My oil cooler was refurbished this year and that has helped a little.
My oil cooler is mounted on the firewall and the scat tube was replaced 2 months ago, the old one was worn and not forming smooth curves.
I spoke to Air Plains and they had a baffle mod which helped #1 and #3 by lowering the baffle height in front of #1.
Last oil change I notice a poor seal between the cooler and the scat hose (the flange) and plugged that.
I used to have plug base o-ring type temp sensors which are known to be hot, I replaced them with bayonet base screwed into the receptacle in the engine block.

I spoke to Lycoming and they said:
"The maximum cylinder head temperature for the O-360-A4M at the bayonet location is 500°F. For maximum service life, cylinder head temperatures should be maintained below 435°F during high performance cruise operation and below 400°F for economy cruise powers.

As aircraft speeds are typically lower and engine power settings are typically higher in a takeoff / climb configuration than a cruise configuration, higher temperatures may be encountered during takeoff / climb. Higher power settings generally equate to higher engine temperatures, and less cooling airflow will also contribute.

While I am not intimately familiar with the Airplains STC for the 180 HP conversion, the oil cooler would most likely have been stock equipment (although the STC might change it), even with the O-320 that the 172M model was originally equipped with. However, increasing oil cooler capacity or efficiency will most likely not have a large impact on the CHTs.

Some important aspects to consider are the fuel/air mixture being delivered and the amount of cooling airflow being delivered, as these will have the most effect. Ensuring that the carburetor is the correct setting for the application, and that there are no induction air leaks that will lean out the mixture even further are two items that can be evaluated. Airframe baffles and seals should be in the best condition possible, and the relative fit between these components and the cowling is extremely important as well. Even a number of seemingly small leaks or gaps may have a significant effect."

If I do more it will be to put the cooler back on the baffle to remove the scat tube, other than that I am watching this forum for suggestions.

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1 year 8 months ago #1206 by Martin Fass
I have a ‘75 C172M with the AirPlains 180hp STC and EDM830. The right side cylinders, #1 and #3, heat up very fast in climb. Climbing at 85KIAS, those cylinders are passing 420 degrees by 1000’ AGL even on a cool day. In order to keep them below 460, I need to lower the nose and reduce power. So far I’ve checked timing, replaced baffling (twice), flushed out the oil cooler, and hoses, even removed paint from the cylinders. Engine has only about 400 hours SMOH with new millennium cylinders. It cools off nicely at cruise and climbing at 100KIAS helps but isn’t very practical. I understand the M in particular has an airflow problem. Any ideas?

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