CHT question

[spiderweb]

With most of the engines in the airplanes I've flown, the recommendation has been to keep CHTs below about 380 degrees. The redline is, of course, higher.

Does this rule of thumb apply generally, or does it vary with other piston engines?

JOOC

 

[poadeleted21]

With most of the engines in the airplanes I've flown, the recommendation has been to keep CHTs below about 380 degrees. The redline is, of course, higher.

Does this rule of thumb apply generally, or does it vary with other piston engines?

JOOC

My A&P Preaches 380. I'be been trying to obey while breaking in a new engine (IO-470N). I think Lycoming has a higher temp. I'll defer to more expertise.

 

[SCCutler]

The idea, Ben, is that while the redline is higher, the metallurgy starts to get very bad when you operate an engine above 380 for extended periods.

 

[spiderweb]

The idea, Ben, is that while the redline is higher, the metallurgy starts to get very bad when you operate an engine above 380 for extended periods.

That's what I was told. With digital displays, it is really interesting to see what slight changes in power, mixture, or AOA do to the CHT.

 

[Sac Arrow]

Certainly below 400 degrees, anyway. But you also don't want to run CHT's TOO low either (e.g. below 350) or the condensation won't be driven out of oil properly.

 

[poadeleted20]

Make sure you know the limits for your engine.

Most Lycomings are redlined at 500F, and Lycoming's recommendation are established accordingly. First, the Operator's Manual on the O-360-A engine says the max CHT at the bayonet location is 500F in all flight phases. However, the note to that says "for maximum service life of the engine maintain CHT between 150F and 400F during continuous operation." The same verbiage appears in the O-320 and O-235 manuals. Second, the Lycoming Flyer Reprints book (1996 edition) says "the engines have benefitted during continuous operation by keeping CHT below 400F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350F to 435F."

OTOH, many Continentals are redlined at 420F, and those you want to keep down around 350-360 in cruise. Running them at 400F would not be a good idea at all. See your engine/aircraft manuals for more details.

 

[poadeleted20]

Certainly below 400 degrees, anyway. But you also don't want to run CHT's TOO low either (e.g. below 350) or the condensation won't be driven out of oil properly.

The issue there is oil temp, not CHT. Lycoming says you need to have the oil temp above 160-170F for at least 30 minutes to do that, without regard for CHT.

 

[Sac Arrow]

The issue there is oil temp, not CHT. Lycoming says you need to have the oil temp above 160-170F for at least 30 minutes to do that, without regard for CHT.

Yes, but oil temp is affected by CHT's.

 

[poadeleted20]

Yes, but oil temp is affected by CHT's.

Agreed, but you can get to 170F oil temp without getting to 350F CHT, you can get to 350F CHT without getting to 170F oil temp, and it's oil temp alone that tells you whether or not you are burning out the moisture.

 

[Witmo]

I seem to recall ROTAX CHT limits are significantly lower than your Lyc/Cont engines.

 

[taters]

Help a Cyl +400 out with less pitch , more mix or a or a "low"fuel pump if an option

 

[spiderweb]

Thanks for all the responses!

 

[BurtM]

With most of the engines in the airplanes I've flown, the recommendation has been to keep CHTs below about 380 degrees. The redline is, of course, higher.

Does this rule of thumb apply generally, or does it vary with other piston engines?

JOOC

What kind of piston engines are you talking about?

With accurate instrumentation, I sure wouldn't exceed 380F CHT in an air-cooled typical GA engine if I had an ownership obligation. Or if I flew it regularly over inhospitable terrain.

For a radial? Henning or someone else with experience would have to comment.

Liquid-cooled ground-bound engines are a different ballgame than air-cooled aircraft engines. You'll never see a car at a dragstrip with a CHT gauge. You'll see turboed cars and diesels with TIT gauges, though.

 

[Henning]

With most of the engines in the airplanes I've flown, the recommendation has been to keep CHTs below about 380 degrees. The redline is, of course, higher.

Does this rule of thumb apply generally, or does it vary with other piston engines?

JOOC

Because a couple of reasons, primary concern with high power ROP operations which are typical your exposure to detonation increase starts ramping up around 400 to the point of high risk above 430 or so. There is also possibly the factor of cylinder dwell. When you machine a cylinder, typically you don't machine the bore square, you put a slight taper (progressive if you're really fine tuning the dwell, but that's not typical for production machine work, it's more representative of 'black art' racing cylinder prep) due to thermal differences between the top and bottom of the cylinder causing expansion differences. The trick is to have the bore be square at operating temp. This typically isn't highly critical so it's a secondary factor.

You don't start into metalurgic issues until about 520* which will usually be about 50* above redline.

 

[BurtM]

Because a couple of reasons, primary concern with high power ROP operations which are typical your exposure to detonation increase starts ramping up around 400 to the point of high risk above 430 or so. There is also possibly the factor of cylinder dwell. When you machine a cylinder, typically you don't machine the bore square, you put a slight taper (progressive if you're really fine tuning the dwell, but that's not typical for production machine work, it's more representative of 'black art' racing cylinder prep) due to thermal differences between the top and bottom of the cylinder causing expansion differences. The trick is to have the bore be square at operating temp. This typically isn't highly critical so it's a secondary factor.

You don't start into metalurgic issues until about 520* which will usually be about 50* above redline.

Interesting. When you said, "dwell" that brought me back to the old days of trying to set dwell angle on old Ford dual-point ignition systems. Talk about trying to hit a moving target...

Don't all typical GA engines have some choke in the cylinders? That's not my area of expertise though.

None of the auto engines I've built over the years had any choke, though.

 

[Henning]

Interesting. When you said, "dwell" that brought me back to the old days of trying to set dwell angle on old Ford dual-point ignition systems. Talk about trying to hit a moving target...

Don't all typical GA engines have some choke in the cylinders? That's not my area of expertise though.

None of the auto engines I've built over the years had any choke, though.

That choke in engine machining parlance is called dwell because it's achieved by dwelling the hone at the bottom of the cylinder. Most gas powered water cooled engines it's not standard in production machining because it just isn't that critical, it's the stuff we do looking for hundredths of a second gains. When running nitro it's more prevalent since one usually uses a solid block, but even in my wet block 392 I had .007 with a progressive ramp.

 

[BurtM]

That choke in engine machining parlance is called dwell because it's achieved by dwelling the hone at the bottom of the cylinder. Most gas powered water cooled engines it's not standard in production machining because it just isn't that critical, it's the stuff we do looking for hundredths of a second gains. When running nitro it's more prevalent since one usually uses a solid block, but even in my wet block 392 I had .007 with a progressive ramp.

Interesting. I've built a few auto V-8s and never heard that term...

So if you held the hone down low in the bore to get some choke in the cylinder, do you pick a particular point to mock-up and measure ring gap and such?

 

[Henning]

Interesting. I've built a few auto V-8s and never heard that term...

So if you held the hone down low in the bore to get some choke in the cylinder, do you pick a particular point to mock-up and measure ring gap and such?

At the top, I just calculate for the expansion. If I have the expansion figured correctly the rest is simple math. On really experimental stuff where we're trying to get nuts on, we'll take a big propane rosebud across the deck and bring it to the designated operating temp to make sure we figured it correctly. I learned machine work from a bunch of old men who learned it during the war so I may have it in terms that are antiquated jargon. The guy I learned to do cranks from, old Clarence, he could look at a crank and tell you it was bent .002...

 

[BurtM]

At the top, I just calculate for the expansion. If I have the expansion figured correctly the rest is simple math. On really experimental stuff where we're trying to get nuts on, we'll take a big propane rosebud across the deck and bring it to the designated operating temp to make sure we figured it correctly. I learned machine work from a bunch of old men who learned it during the war so I may have it in terms that are antiquated jargon. The guy I learned to do cranks from, old Clarence, he could look at a crank and tell you it was bent .002...

Sounds like the guy in my hometown that was the best alignment guy ever. He worked at the Ford dealer and used a Rotunda alignment system that consisted of a pit to make the suspension adjustments, bubble levels that attached to the wheels, and a system of convex and concave mirrors with a light bulb. Best alignment ever. Much better than the contemporary Hunter and Bear laser alignment systems that read out to 3 digits past the decimal point but are way off.

'Ol Virgil would ask do you drive in the right or left lane on the interstate? Well, mostly left but sometimes right. Well, make your mind up because the interstates are crowned - if you drive in the right lane, they tilt to the right and if you drive in the left lane, they tilt to the left, so how do you want me to set your car up?