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CHT's WHAT IS IDEAL??? Lycoming IO-360

rv969wf

Well Known Member
First of all I have mostly built water cooled race engines and a few air cooled engines. My question: Does anyone know the best / optimum CHT to operate a Lycoming IO-360A1B6 Angle Valve 200HP + with 10:5:1 compression. :rolleyes: Reason I'm asking this is because this engine has CHOKED / tapered cylinders up towards the combustion chamber. The reason for the CHOKE is to bring the cylinder bore up to the size of the rest of the cylinder at operating temp because the top of the cylinder expands more than the bottom. One example is a Cont..520 series cylinder which will expand 19 thousandths at 475F degrees CHT and at room temp the top of the barrel is 6 to 8 thousandths smaller than the bottom. I'm aware that the CHOKE is the first thing to wear out on a cylinder. Cold starts is a killer on the CHOKE area in the cylinder along with ring and piston ring groove wear issues. I have talked to Lycoming / engine builders XYand Z and I get every answer in the world. I've been told anywhere from 325, 375, 425F CHT is when the cylinder is true and straight. :p Jeeze that's a 100F spread and 100F variation can make a few thousandths difference, which is best?? I'd like to hear some feedback from Experienced engine builders / pilots that have good hard data and not a thread return like: (Well I run mine at 325F or I run mine 425F with no problems). If I split the 325F and 425F I'm at 375F. :confused: I'd just like to know what is Ideal for maximum cylinder, ring, piston ring groove life. Thanks AJ
 
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These docs...

Alan... Lycoming has some numbers in this document...

"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 350? F to 435? F."

http://www.lycoming.textron.com/mai...yReprints/operation/moreCylinderHeadTemp.html

... and this one talks about cruise operation....

8. For maximum service life, maintain the following recommended limits for continuous cruise operation:

(a) Engine power setting - 65% of rated or less.

(b) Cylinder head temperatures - 400oF. or below.

? Oil temperature - 165oF. - 220oF.


http://www.lycoming.textron.com/mai...ons/keyReprints/operation/leaningEngines.html

Hope these help... high CHTs seem to be a staple of the Grummans - with much newsgroup discussions... :)

gil in Tucson
 
CHT's are way COLD

"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 350? F to 435? F."
Hope these help... high CHTs seem to be a staple of the Grummans - with much newsgroup discussions... :)

gil in Tucson
[/QUOTE]

Hi Gil, thanks for the info. I have seen these attachments on Lycomings website and they're very helpful. I'm mostly wondering what is the minumum CHT to not aggravate cylinder wear as I'm running WAY COLD and have reduced my cowl inlets to tennis ball size holes to get the CHT temps up to 350F range. I was quite shocked when Lycoming publishes a min of 150F and max of 500F CHT's. They somehow or should know what the magic CHT number is and how did they know how much CHOKE to put in the cylinders to begin with. Some aircraft air cooled engines don't even have CHOKED cylinders while some do. :confused: I know you cannot have one ideal CHT at all times without some means to adjust the cooling air needs of the engine. Maybe I'm a little picky and should not worry about it and just keep it in the green range. If I use what Lycoming recommends: 350F - 435F then split this in the middle would be around 390F. Thanks again and I'll look for other comments. AJ
 
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The temperature at which the cylinder top would expand to the same diameter as the cylinder bottom would obviously be the "ideal" temperature - but this will change somewhat with airspeed and power output, so you're only going to have one "ideal" temperature for any given speed/power combination - resulting in a "band" of ideal temperatures for normal operations across 20% or 30% of the top portion of the power range centered on cruise power, and dozens of knots of airspeed variations around cruise speed. To make things a little more fun, you'll need to throw in the OAT effects on top of your airspeed variations to consider thermal transfer changes with altitude, as well as air density and mass flow. This all assumes perfect baffling, of course - which is about as common as the proverbial pot of gold at the end of the rainbow.

Got a headache yet?
 
Oh boy, I new this would come up! Chuck N.....in Tulsa

az_gila said:
Then these guys say not to put any choke in the barrels.... :)

http://www.chuckneyent.com/cylinderoverhaul.html

... and apparently have some history behind them...

gil in Tucson

I new this might come up!!!! Chuck N in Tulsa boring the Cylinders straight. I checked into this several years ago and questioned this,,, :eek: but I went ahead and had the cylinders taken .010" oversize to Lycoming 1950 year old specs with the choke in the upper part of the cylinders. One reason I looked into this was the fact that after 250 hours on my Lycoming since Major OH from another source engine builder along time ago was the fact that the choke was gone. 250 hours running CHT's in the green and never getting the engine hot, the CHOKE was gone. :eek: The cylinder bore was good but not the choke. Reason I toredown a 250 hour engine down was because of one stinking lifter problem and it ate a cam lobe. I myself have only seen a few engines /// mostly aircraft that have choked / tapered cylinders. How come VW air cooled, motorcycle air cooled, what ever air cooled engine don't have this CHOKE / Taper in the upper last 1" to 1 1/2" in the top of the cylinder. I'm not going to list all of the engines that do or do not have this done. I listed only examples. Is the expansion rate on the angle valve cylinder Lycs.. that great???? Or does it truely need the choke because of maybe the casting design / thermal expansion rates etc. I hope BPA / Allen will chime in and help me/us out here. On my newest / 3rd engine I'm testing, I have been running the CHT's at the MIN or BELOW MIN and the oil samples with PPM IRON are VERY VERY low and below normal average. I'm seeing Iron levels in the oil samples at 5 PPM and the universal average is 25 PPM. Camhaft, gears, rings, cylinders / any steel parts that are iron will show up on an oil sample. I'm seeing good numbers along with Leakdown tests at 79/80 on all cylinders hot or cold at TDC and at piston travel 1/3 down where the rings seal is important in the bore. I'd like to know if running on the min CHT is OK for cylinder / ring life. So far what I've seen is good. My thought is HEAT can kill and engine, but it can make horsepower if given in the right areas. But to cold can kill an engine as well. BPA / Allen can you give some feedback on any of this. :D Thanks AJ
 
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AJ, the reason for so many different answers is that nobody REALLY knows what the optimum operating temp is. I DO know that guides will literally fall out of the cylinder head at 475 degrees F. I have done this to reject heads intentionally on a coupple of occasions. Everyone knows that the barrel choke, when heated 'straightens' itself. If the barrel were straight when cold, then it would actually get bigger in the choke end when at operating temps, creating blo-by and insufficient ring seating, and dirty oil.

Too little choke, blo-by. Too much choke, premature wear in the choke area (high iron in oil analysis).

Operating temps: we like 380 optimum for best engine performance and health.
 
Several things:

1) Lycoming says mininum operating CHT is 150dF. TCM does not list a value, but reading beteween the lines they think 200dF is a reasonable minimum CHT.

2) The aluminum alloy used in the heads has lost 50% of it's room temperature strength at 400dF.

3) As a general rule the choke is gone and the cylinder straight by 150dF.

So, based on the science, a cooler cylinder being stronger, the fact that it puts out more HP (it's more volumetrically efficient than a hot one), that you can't run it too cool unless you're operating out of Barrow, Alaska, in February <g>, and the cooler a cylinder, the wider the detonation margin, I would never do anything to make a cylinder run warmer. EVERYTHING I do with baffling, etc, it to get them cooler. The cooler the better. As an upper limit, we recommend 380dF as an operating upper limit, 400 as a "time to do something" to lower CHTs and 420 as the DO SOMETHING NOW limit. Cylinders that run cool will definitely be more durable.

People who have advocated running cylinders hot for some unidentified reason are usually the same people who have an installation they can't get cool! There is no known data supporting the notion that cylinders need to be at 350, or 375, or even, God forbid, as I have seen recommended, run at 400dF as a matter of choice.

Walter
 
Walter Atkinson said:
Several things:

So, based on the science, a cooler cylinder being stronger, the fact that it puts out more HP (it's more volumetrically efficient than a hot one), that you can't run it too cool unless you're operating out of Barrow, Alaska, in February <g>, and the cooler a cylinder, the wider the detonation margin, I would never do anything to make a cylinder run warmer. EVERYTHING I do with baffling, etc, it to get them cooler. The cooler the better. Cylinders that run cool will definitely be more durable.

Walter

Question Walter, or anyone. I see you have done some racing at Reno in the past and I'm curious. So do you think running 260 F CHT's on my coldest cyl is alright in 70 F degree outside air. This is at full throttle 27.1" , 4,000ft MSL pushing 17.5 gph turning 2,680 rpm indicating 205-208 mph with a N-E-S-W GPS ground average of 221.25 mph? If I slow down to 23" x 2,450 and lean to 100 Rich of peak I'm seeing 278F degree on coldest cylinder. I did some playing around to see if speed would pick up by reducing the cowl inlet sizes in 1/4" increments. Started out today with 3.125" round diffusers at 15.95 square inches total and worked my way down to 2.125" round inlets on each side. NO this is not a miss print, 2.125" and I'm up to 346F on my coldest cylinder at 100F ROP. Hottest cylinder was 368F. I did not pick up any speed and the cylinder temps did rise, but I think I hit a wall between making horsepower and cooling drag reduction. The inlet area to cool the engine with 2.125 inlets is only 7.09 square inches total. Cowl outlet is 25.14 square inches. Oil cooler air is supplied via NACA duct on lower right side of cowl and is separate from cowl inlets to cool the engine, and it has a controlable front door that is at 12.56 square inches wide open but is mostly shut down half way. So the total inlet area to cool the cyls and oil cooler with the 2.125" inlets is 19.65" Cowl Inlet / outlet spread is 22%. I quess what I'm seeing is horsepower versus drag reduction. Another thing I saw was that as the CHT's rose higher and higher from the small cowl inlets, I had to compensate and open the air flow to the oil cooler to maintain it at 190F degrees and that cost me cooling drag with the way it is designed. All of these tests were done today with outside air temps within 4-5F degrees outside and the barometric pressure stayed very close all day. One thing I forgot to mention was the fact that I'm running a TRUE 10:5.1 compresssion ratio and detonation is a VERY BIG CONCERN to me and no one has anyway of knowing without very $$$$$$ equipment. A few seconds of HARD detonation will destroy the engine very quickly and make for a very BAD DAY!!! Anyone have any other thoughts.
 
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Running CHT's that cool is good. If I could get mine that cool, I would. Since you are running the high compression pistons and since COOL CHTs are the best way to widen detonation margin, it seems to me that running them as cool as possible is a positive thing. I can think of zero downside.

Walter
 
Temps. and chokes....

Walter Atkinson said:
Several things:
........

3) As a general rule the choke is gone and the cylinder straight by 150dF.
........
Walter

OK Walter, I'm confused.... sometimes easily done.... :)

If the choke is gone (i.e. the cylinder walls are straight) at 150F, then shouldn't a negative choke appear when the cylinders get hotter than 150F - say to over double that 350F?

Why does the choke disappear at 150F and then things stay magically straight at much higher temperatures?

gil in Tucson
 
This thread has my head spinning, logic says if chock wasn't beneficial lycoming wouldn't waste their time doing it. I think I will just keep flying and not worry about it
 
**Why does the choke not become negative at higher CHTs?**


The exansion is not linear. Your concern would be worth worrying about if it were a linear issue.

Walter
 
What's non-linear?

Walter Atkinson said:
**Why does the choke not become negative at higher CHTs?**


The exansion is not linear. Your concern would be worth worrying about if it were a linear issue.

Walter

Walter, what is non linear? The steel expansion is certainly linear with temperature. What other weird effects are you referring too?

gil in Tucson
 
My guess..

I don't have any direct knowledge here, but what I would guess happens:
The top of the cylinders have a higher temp. then the bottoms so they expand more quickly. At some point (temp.) the expansion would balance the built in taper to make them not taper and at higher temp. the taper could go the other way.

Is this right?

Kent
 
???

kentb said:
I don't have any direct knowledge here, but what I would guess happens:
The top of the cylinders have a higher temp. then the bottoms so they expand more quickly. At some point (temp.) the expansion would balance the built in taper to make them not taper and at higher temp. the taper could go the other way.

Is this right?

Kent

Kent ... that would be my guess too... but Walter used a low number of 150F to say the the choke was gone....

Unless he was implying a difference of 150F between top and bottom of the cylinder... but this is not what is shown on a CHT gauge...

gil in Tucson
 
The choke is designed to make the cylinder the same dimensions from top to bottom when running. Before start, the cylinder is the same temp at the bottom as at the top. As soon as it begins to run, it becomes hotter at the top than at the bottom. The DELTA expansions are what makes it end up round. As long as the delta remains fairly constant, the cylinder remains fairly round. That's what I meant by non-linear... top to bottom. I wasn't too clear. My appologies.

Walter
 
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Fun with numbers. You guys with engineering degrees check me.

The coeficient of thermal expansion of steel is generally considered to be about 0.0000065 in/in/degF. A 5 inch cylinder has a circumference of 15.7 inches. 0.0000065 x 15.7 equals 0.000102 increase in circumference for each degree F rise in temperature.

Assume 70F as measuring temperature and 400F as operating temperature at the top of the bore. Difference is 330F, so 330 x 0.000102 equals 0.03366 inches increase in circumference. (15.7 + 0.03366)/3.14 equals 5.0107. The bore diameter increased a fuzz less than 0.011 inch.

Determining how much choke it takes to have a straight bore at operating temps would require also knowing the temperature of the cylinder near the base. I'd guess 220F, so the bore increase would be (220 - 70) x 0.000102 equals 0.0153, and (15.7 + 0.0153)/3.14 is 5.0049, or a fuzz less than 0.005.

If the assumed temps are correct, 0.011 - 0.005 would suggest a desired choke of 0.006 inch. Play with your own assumed temps and see what you get. Right offhand I don't remember what Lycoming specifies for choke.

Dan Horton
 
DanH said:
Fun with numbers. You guys with engineering degrees check me.

The coeficient of thermal expansion of steel is generally considered to be about 0.0000065 in/in/degF. A 5 inch cylinder has a circumference of 15.7 inches. 0.0000065 x 15.7 equals 0.000102 increase in circumference for each degree F rise in temperature.

Assume 70F as measuring temperature and 400F as operating temperature at the top of the bore. Difference is 330F, so 330 x 0.000102 equals 0.03366 inches increase in circumference. (15.7 + 0.03366)/3.14 equals 5.0107. The bore diameter increased a fuzz less than 0.011 inch.

Determining how much choke it takes to have a straight bore at operating temps would require also knowing the temperature of the cylinder near the base. I'd guess 220F, so the bore increase would be (220 - 70) x 0.000102 equals 0.0153, and (15.7 + 0.0153)/3.14 is 5.0049, or a fuzz less than 0.005.

If the assumed temps are correct, 0.011 - 0.005 would suggest a desired choke of 0.006 inch. Play with your own assumed temps and see what you get. Right offhand I don't remember what Lycoming specifies for choke.

Dan Horton
The numbers are good enough for a simple steel barrel although you don't need to calculate circumference. just work the thermal expansion on the diameter. It doesn't make any difference that a hole is in it. The fly in the ointment is the interference fit of the top of the barrel in the aluminum head. You have roughly double the thermal expansion in the head as in the barrel, so the mechanical interference rapidly disappears as temperatures rise, causing even more "nonlinearities" as the squeeze caused by the aluminum head on the top of the barrel relaxes.

-mike
 
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