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Low CHTs during break-in ??

prkaye

Well Known Member
Today I did my first flight after a major IRAN. All 4 cylinders were replaced. It all seemed to run normally and smoothly. One thing that surprised me, though. I was expecting to have MUCH hotter CHTs. I kept the power up to about 90% for most of the flight, MAP about 26, and my CHTs never got above about 375. My front CHTs were actually down around 320 for a while - after about an hour running at about 85% power my CHTs were 358,336,379,378. EGTs were typically in the 1150-1350 range, depending on how much i leaned (I didn't lean at all for the first hour). Oil temp consistently about 185.
Are these low CHTs during break-in a cause for concern?
 
Data points

Definitely cooler than my repaired cylinders, which both ran well over 400 for about the first 40 minutes at high power, and then started trending down to normal.

Was there a lengthy ground run at low power prior to the first flight?

What was the oil consumption?

Today I did my first flight after a major IRAN. All 4 cylinders were replaced. It all seemed to run normally and smoothly. One thing that surprised me, though. I was expecting to have MUCH hotter CHTs. I kept the power up to about 90% for most of the flight, MAP about 26, and my CHTs never got above about 375. My front CHTs were actually down around 320 for a while - after about an hour running at about 85% power my CHTs were 358,336,379,378. EGTs were typically in the 1150-1350 range, depending on how much i leaned (I didn't lean at all for the first hour). Oil temp consistently about 185.
Are these low CHTs during break-in a cause for concern?
 
Did your shop run it on the test stand for an hour or two? Most of the heat is gone after an hour or two and this would happen on the stand. Also, when I researched, one of the big three does a plateau hone (400 grit) and the others an old school 220 grit hone. The former will produce quite a bit less heat during the aforementioned hour or two, as there is significantly less wear and therefore heat, going on during ring seating.

I would NOT be concerned with the lower than expected CHTs.

Larry
 
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Yes it was run for 2 hours in Aerosport's test cell. I just looked at the report from Aerosport, and the CHTs i saw in flight were actually fairly consistent with what is in the test report.
I won't know oil consumption until next time i go out and dip.
Ground run - maybe 10 min... just long enough to get the oil up to operating temp. I actually shut down at the end of the runway for about 5 minutes to let the CHTs cool back down to under 300F before taking off.
 
Yes it was run for 2 hours in Aerosport's test cell. I just looked at the report from Aerosport, and the CHTs i saw in flight were actually fairly consistent with what is in the test report.
I won't know oil consumption until next time i go out and dip.
Ground run - maybe 10 min... just long enough to get the oil up to operating temp. I actually shut down at the end of the runway for about 5 minutes to let the CHTs cool back down to under 300F before taking off.

CHTs are often lower on the test stand, as they use a big ole fan that moves more air than your cowl does. Each stand is different so that is not universal.
 
If it ran for 2 hours on the test stand, guaranteed the cylinders are already broken in. What you are seeing are normal operating temps now.
 
If it ran for 2 hours on the test stand, guaranteed the cylinders are already broken in. What you are seeing are normal operating temps now.
How can this be? Everything i've ever read or been told about break-in is that it typically takes 20+ hours. Aerosport also sends very specific break-in instructions for break-in operation for the first 25 hours.
 
For what it's worth, I just pulled up a picture of my EFIS at 2.0 hours, which was near the end of the plane's second flight. CHTs were all in the 330 degrees F range, oil temp was 185. That was at 26" MAP and 2500 RPM at cruise.

I kept my ground runs to an absolute minimum and ran at high power settings for the first five flight hours. The engine builder ran about 1.5 hours at their facility. Oil consumption now is maybe a quart every twelve hours on an IO-360.
 
How can this be? Everything i've ever read or been told about break-in is that it typically takes 20+ hours. Aerosport also sends very specific break-in instructions for break-in operation for the first 25 hours.

break in is far more involved than just initial ring seating. It is the initial, compression ring seating with the cyl wall that creates all of the heat as the rings wear off the tops of the peaks in the cross hatch scratches and turns them into plateaus, and it only takes an hour or two to be 80% complete. After that, the engine is still breaking in, including the rings, but the excessive heat is no longer present. HIgh CHTs are part of initial ring seating but not necessarily the full break in. There is a bunch of polishing and light wear that is still happening even though it is not creating a lot of extra heat. This is why you should run non additive oil for the FULL break in period, NOT just the initial high heat period. AD is an acceptable additive during breakin. It is the anti-friction additives that are to be avoided. Friction during break in is a friend not an enemy.
 
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How can this be? Everything i've ever read or been told about break-in is that it typically takes 20+ hours. Aerosport also sends very specific break-in instructions for break-in operation for the first 25 hours.

That's time for the overall engine break-in process. If done properly (high power, heat kept within limits), the cylinders themselves break-in (rings seat) within the first hour unless the cylinder walls glaze. After the cylinders break-in and the danger of excessive heat screwing that process up is over, you can pretty much run the engine like you normally would post break-in.
 
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How can this be? Everything i've ever read or been told about break-in is that it typically takes 20+ hours. Aerosport also sends very specific break-in instructions for break-in operation for the first 25 hours.

Break-in is not at all linear. Most of it happens in the first hour or two; a little more over the next 20.
 
So many different perspectives on break-in. Aerosport told me:
Typical break in is done between 65 and 75% power, and run the engine “over-square”. This means the manifold pressure should be higher than the rpm/100, so if you are running at 26” your rpm should be lower than 2600. This is easy to accomplish with a constant speed prop, but not obtainable with a fixed pitch prop.
When they run the engine in in a test cell, do they put a prop on it that allows them to run it over-square?
 
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So many different perspectives on break-in. Aerosport told me:

When they run the engine in in a test cell, do they put a prop on it that allows them to run it over-square?

They use a test club typically and yes it applies a good amount of load. Some use a dyno which has infinitely variable load. Break in should always be done under the heaviest load possible. High MAP creates more combustion pressure in the chamber and this, in turn, puts more pressure behind the rings, which applies more force against the cyl walls. More pressure = more wear/friction = better break in. dialing down the RPM for any given throttle setting further adds load and therefore cyl pressure. Just don't go so far that it is lugging. More load often means more heat and must watch for that also, as excessive heat can create glazing during the break in. It's a bit of a balancing act.
 
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