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The red box/fin - CHTs

C

Clipper1

Active Member
I have an IO-390 and I have noticed that when I'm cruising at 2500rpm at 2,000', 3,000', 4000', or 5000' my hottest CHT is never above 375 no matter where the mixture control is. Does this effective mean that the red box does not exist for this engine/cooling setup?

I've read in a few Busch articles that a good practice is the keep the CHTs on Lycomings above 350 yet below 400 to ensure that lead doesn't deposit on the exhaust valves and cause build-up.

Can anyone provide more clarity?
TIA.
 
No. That’s why I don’t like Mike Busch’s technique of leaning with respect to CHT. I prefer to lean with respect to EGT. Here’s what I do. Takeoff full forward everything. Notice your EGT on anyone of your cylinders, doesn’t matter which one. As you climb, lean to maintain THAT temperature. Once you get above 8000 or below 75% in cruise, you can now explore where your EGT peak is and lean with respect to that. The idea is that on takeoff, all of your EGT should be well rich of the red box and safe. Above 8000 feet, you should be under 75% and no matter what you do, it’s safe to explore the peak without fear of the nonexistent red box.
 
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I have often wondered the same thing as the OP. I would love to hear a reasoned explanation of how you can hurt the engine with CHTs all around 350. Clearly the engine is making less than 100% of rated power. Clearly no detonation is occurring or you would have higher CHTs. What is the concern?
 
Clipper1 said:
Does this effective mean that the red box does not exist for this engine/cooling setup?
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That'd be my take. Since the red box is all about making sure you don't overheat as defined by the CHT's, I'd agree that with your set-up, a "red box" doesn't exist.

Man, I wish my setup was that cool on climbout. I'm jealous. :)
 
Good for you that your cooling system is so effective, but I would suggest you lean with respect to peak EGT in cruise flight and note how CHT and fuel flow is affected at different settings. The fact that you didn’t refer to EGT in your post makes me think you don’t understand lean of peak operations
 
The "red box" is more than just temperature - it also considers ICP, which we have no way to measure in the cockpit. Temperature is only a single factor. It is best to apply the rules of thump in order to keep pressures reasonable. There is a ton of info at https://www.advancedpilot.com/. Gami has the facts, from research in their very well equipped lab/shop.
 
erich weaver said:
Good for you that your cooling system is so effective, but I would suggest you lean with respect to peak EGT in cruise flight and note how CHT and fuel flow is affected at different settings. The fact that you didn’t refer to EGT in your post makes me think you don’t understand lean of peak operations
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LOP is irrelevant to the question I'm asking. Sure, I could post numbers at 50 LOP, 50 ROP or 150 ROP but the question remains... my CHT's are still below 375 no matter what setting I use. For example, when I'm 50 LOP I see CHTs that are 280s. That's too cold from I've read.
 
Clipper1 said:
Does this effective mean that the red box does not exist for this engine/cooling setup?

TIA.
Click to expand...

No. The red box is more about detonation margin and cylinder pressure at high power settings. CHT is related, but even with low CHT's, you can push an engine into the red box.
 
What engine monitor / ignition system are you using? 280 seems a little low to be believable at least with electronic ignition….
 
Kyle Boatright said:
No. The red box is more about detonation margin and cylinder pressure at high power settings. CHT is related, but even with low CHT's, you can push an engine into the red box.
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I can only base what I'm asking on what I'm reading. Mike Busch says 'Peak ICP and peak CHT occur at almost exactly the same mixture setting'
 
I can see 280 CHT on my 180HP IO-360 if I lean well past peak. I usually find the power is so low that it is not a practical setting. I find that my options are usually WOT and limiting to 30 past peak on the richest cylinder; doing so gives 310 to 340 CHT's.
 
almost exactly

occur at almost exactly the same mixture setting'
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I can't answer your question but I never see any of my CHTs over 375 unless I deliberately lean with high power settings and low RPMs and that would be accidentally only.
After several years of chasing the LOP obsession I finally realized that I own an RV to go fast. I now lean for best speed, usually just a touch LOP.
MY CHTs are generally in the low 300s and seem to be happy at 700plus hours.

Your ignition timing may have more to do with your temps than your red knob.
 
Clipper1 said:
I have an IO-390 and I have noticed that when I'm cruising at 2500rpm at 2,000', 3,000', 4000', or 5000' my hottest CHT is never above 375 no matter where the mixture control is. Does this effective mean that the red box does not exist for this engine/cooling setup?
Click to expand...

I'm not a big fan of the red box concept for RV owners. It may be valid for turbo installations, but I think it's overly conservative (read "excessive caution") for a non-turbo Lycoming run at reasonable CHT.

That said, the big picture problem for a tech writer is offering guidance with a large enough envelope to encompass everyone who might read the material.

BTW, I share Tom's operation approach (post #2).
 
DanH said:
I'm not a big fan of the red box concept for RV owners. It may be valid for turbo installations, but I think it's overly conservative (read "excessive caution") for a non-turbo Lycoming run at reasonable CHT.

That said, the big picture problem for a tech writer is offering guidance with a large enough envelope to encompass everyone who might read the material.

BTW, I share Tom's operation approach (post #2).
Click to expand...

For standard compression and 100LL fuel, yes I would agree with you.

When I was setting up my aircraft for 91E10 operation, it was invaluable.
 
airguy said:
When I was setting up my aircraft for 91E10 operation, it was invaluable.
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Greg, did you actually tickle the dragon (refer to engine instrumentation to find margins using temperature indication), or simply observe the tales of the elders ("In that place there be dragons!")?
 
DanH said:
Greg, did you actually tickle the dragon (refer to engine instrumentation to find margins using temperature indication), or simply observe the tales of the elders ("In that place there be dragons!")?
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I went off the edge of map and found the dragons. I did it carefully, under controlled conditions, with full engine monitoring and a testing plan, but yes I went there. I wanted to know, not just guess.

Then I modified my timing map to stay away from the dragons.

The dragons are real, by the way. They do live out there, and they like to snack on pistons.
 
3bythelake said:
I have often wondered the same thing as the OP. I would love to hear a reasoned explanation of how you can hurt the engine with CHTs all around 350. Clearly the engine is making less than 100% of rated power. Clearly no detonation is occurring or you would have higher CHTs. What is the concern?
Click to expand...

While detonation is more easilly achieved with heat, you can get detonation with relatively low heat. Yes, detonation will rapidly raise heat levels, but light detonation doesn't raise it that much. Even still, the rapidly rising heat is an indicator that the detonation is already occurring; not an early warning. FYI, you can easily make 100% power without the CHTs at 350. This happens pretty much every time you go WOT at take off.

Larry
 
airguy said:
I went off the edge of map and found the dragons. I did it carefully, under controlled conditions, with full engine monitoring and a testing plan, but yes I went there. I wanted to know, not just guess.

Then I modified my timing map to stay away from the dragons.

The dragons are real, by the way. They do live out there, and they like to snack on pistons.
Click to expand...

What instrumentation did you end up using to monitor detonation? I've heard it's hard to find a knock sensor that works well on these motors.
 
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