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CHT Thoughts?

kklewin

Well Known Member
Always had high CHT's in my 6A....run the majority of mods to lower the CHTs.....here is what I have...chime in....

RV-6A, O-360-A2A (Aerosport / Solid Crank / Carb); Sensenich fixed pitch, cruise prop

Flying for 10 years, 300 TT, frequent flights, 100LL only, AS 100W Oil after break in in 2004.

RMI Micromonitor for individual CHT/EGT's....

Tested the CHT probes several times (ice water / boiling water)
Re-sealed baffles and re-adjusted baffles several times
Opened up bottom of cowl slightly (air exit over pipes)
Started with some small extra luvers...upgraded to larger luvers in the bottom of the cowl
Filed down the cylinders twice and clean extensively
Checked Mag Timing at each annual....
All compressions good at annual...

#2 runs the hottest. But #3 used to be my hot one for several years.

- To keep temps down, I usually reduce power at 1000 ft to 2300 rpm, mixture leaned before takeoff, and I climb no slower than 100 KIAS...

- Temps very high in the climb, but settle down to mid 350's in cruise...

Today:


15C OAT / Field Elevation: 4500 climb to 12,000 at 120 KIAS / 2300 RPM
(Taken in climb / 120 KIAS / 8000 MSL)
CHT/EGT #1: 390 / 1429
CHT/EGT #2: 470/ 1428
CHT/EGT #3 : 353/1406
CHT/EGT #4 : 401/1436

Observations: It runs hot. Not sure why (hence the question). Strangly the EGT's are closer than I would have imagined for a carb set up.

The CHT grouping seems too far apart (and hot), I've thought of the probes themselves, but they seem to test ok and when swapped give similar results.

OK....thoughts...???
 
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Temps very high in the climb, but settle down to mid 350's in cruise...

Today:

15C OAT / Field Elevation: 4500 climb to 12,000 at 120 KIAS / 2300 RPM
(Taken in climb / 120 KIAS / 8000 MSL)
CHT/EGT #1: 390 / 1429
CHT/EGT #2: 470/ 1428
CHT/EGT #3 : 353/1406
CHT/EGT #4 : 401/1436

470f is very, very hot for a CHT on a Lycoming O360 engine. If it is only this hot during a climb and stays around 350 the rest of your flight then a "step climb" is something you can do.

"Step Climb": When the CHTs climb above a comfortable temperature, lower the nose, and stay at that altitude. This increases your airspeed and cooling air flow for a while. When the CHTs drop to a reasonable level resume your climb. Repeat as necessary until you are at your cruising altitude.

:cool:
 
Leaning for takeoff? At what density altitude? I don't think Boseman is that high.

Test probes closer to 400 F using cooking oil and a good candy thermometer.
 
Walt....I've tried increasing fuel flow, via mixture in climb but hard to do much w our altitude here....

Bob.....Bozeman is 4500 ft.....with density altitudes over 6000+ in the summer....leaning before takeoff gives me an additional 80 to 100 rpm......

I don't know the carb model number (M. Schebler M-5) comes to mind....I'll have to check at the hangar.....

Love to hear how some carb adjustments work out.......

Kurt
 
I am in your same boat. From my deep research we need to re drill to a larger carb jet.

Drilled our O-320 from #42 to #40 ... dropped 20-30 degrees.

, mixture leaned before takeoff, and I climb no slower than 100 KIAS...

15C OAT / Field Elevation: 4500 climb to 12,000 at 120 KIAS / 2300 RPM
(Taken in climb / 120 KIAS / 8000 MSL)
CHT/EGT #1: 390 / 1429
CHT/EGT #2: 470/ 1428
CHT/EGT #3 : 353/1406
CHT/EGT #4 : 401/1436

OK....thoughts...???

Looks lean :eek:
 
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Also had my carb re drilled for the same reason

Have a o-320 in my 6a
Think they drilled from 41 to 38
GPH at takeoff went from 12 to 14
#3 cylinder dropped 40 degrees
 
Gentlemen,

A few quick things to remember, for a Standard compression 180HP at sea level and ISA the fuel flow should be around 17.5GPH. Parade Rest.

For a 160HP about 15.5GPH. Parade Rest here too!

15C OAT / Field Elevation: 4500 climb to 12,000 at 120 KIAS / 2300 RPM
(Taken in climb / 120 KIAS / 8000 MSL)
CHT/EGT #1: 390 / 1429
CHT/EGT #2: 470/ 1428
CHT/EGT #3 : 353/1406
CHT/EGT #4 : 401/1436

Provided you lean to an appropriate amount, not what the Lycoming book says, because its advice is pretty lacking, the EGT should be approximately 1270-1320. Push that red knob in until your EGT is down there.

You may still have some cooling issues, but remember the equation is "heat in" - "heat out" = CHT. Get the heat in bit sorted (fuel flow right / Target EGT) and then work on the heat out.

If you have an EI of some sort, it may also need retiming to knock a few degrees off it. If it is a magneto, then set the standard timing up. No advance.

If you want a back of beer box calculation, for the RPM on takeoff with the fixed pitch, and the altitude I would expect around 13.6 GPH


MauiLvrs
Looks lean :eek:

No, this is a Rich mixture. It is just not Rich enough! ;)
 
A few quick things to remember, for a Standard compression 180HP at sea level and ISA the fuel flow should be around 17.5GPH. Parade Rest.

For a 160HP about 15.5GPH. Parade Rest here too!

David, perhaps stating a firm GPH vs HP figure isn't the best approach.

To establish the above, it would appear you've simply assumed 0.58 BFSC as a perfect value. I can live with 0.58 (see below), but applying it to label horsepower is not at all practical.

(17.5 x 6)/180=0.5833
(15.5 x 6)/160=0.5812

The 160 and 180 values are rated HP. Rated HP is little more than a name, a label loosely based on corrected HP, when the actual lbs of fuel per hour required is based on brake HP....the conditions of the moment, not the conditions corrected for the theoretical sea level standard day. Climate conditions aside, actual engines in the field perform better or worse than their "identical" brother, and would have different absolute fuel quantity requirements. And fixed pitch installations never make rated power in climb. In truth, they rarely make rated power, unless installed on an airboat, or making a low pass near sea level on a cool day.

What we really want is fuel guidance for our particular engine. A BSFC meter would be a nice addition to our panels, but until one of the avionics manufacturers figures out how to put a Wheatstone on the prop shaft to get live torque, the next best thing is EGT.

If a pilot suspects inadequate fuel flow, I'd suggest the first thing to do is record an EGT spread. For each cylinder, what is the spread, in degrees, between full rich and peak?

You and I have previously debated the "desirable" spread. I'm of the opinion that the cooling system should be able to handle the maximum power available, which will be found around 150 ROP. You like 250 ROP. I'm not paying your fuel bill and you don't need my blessing, so sure, let's use 250 and study an example.

You've seen this chart before, a Lycoming test cell run as part of a detonation survey, WOT and 2700 on a IO-360 rated at 200 HP. The detonation survey conditions include hot day intake air temperature, among other things. As a result, the actual brake HP is nowhere near 200....nor is the required fuel flow. Max flow recorded was 105 lbs per hour, roughly 17.5, which put brake HP about 172, and BFSC over 0.6. Max power is between 80 and 85 lbs per hour, and a "conservative rich" .58 BFSC is about 97 lbs per hour, roughly 16 GPH for a rated 200 HP engine. BTW, that is about 250 ROP.

Anyway, forget GPH. Use EGT. If all cylinders ballpark 250 between peak and full rich, look elsewhere.

102tc0o.jpg
 
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CHT High

Kurt,

My opinion but I would not be comfortable flying a CHT of 470. That is way beyond normal. A very wise mechanic told me if I see anything north of 370 I should be concerned and anything near 425 I should figure out what's causing it. I had a cooler than normal CHT on #1 and ended up putting a small air dam in front of it. #1 got up to 420 so I trimmed about 3/4 of the dam off to get it in the right range. In cruise all my CHTs now rum 330-350.

Do you have anything blocking airflow through the front of the intakes? Another suggestion someone on the forum had was to attach a small lip on the bottom aft edge of the lower engine cowl to create a slightly turbulent airflow and pressure out the bottom of the airplane. I am not an engineer but there are some real smart guys here. I couldn't see how it would work, in fact I thought it make the situation worse. But I'll be darned if it didn't work. Don't notice any change in performance and it dropped the Oil temp 20 degrees and the CHTs about 10. I can send a pic if anyone wants one.

Oly
 
Dan
David, perhaps stating a firm GPH vs HP figure isn't the best approach.

To establish the above, it would appear you've simply assumed 0.58 BFSC as a perfect value. I can live with 0.58 (see below), but applying it to label horsepower is not at all practical.

(17.5 x 6)/180=0.5833
(15.5 x 6)/160=0.5812

Thanks for taking the time. I agree, better to be scientific about it, problem is 98% of the folk here are looking for a cookbook recipe or a rough number they can use in the cockpit.

I hate cookbook approaches. But most folk asking the question hate a complicated answer.

The APS approach has been to offer suggested fuel flows as the difference is usually tenths or hundredths of a GPH anyway. You have just confirmed this.

Thanks for helping out :)
 
OP said 4500 ft field elevation (plus density altitude) plus FP prop (2300 rpm in climb), but I never saw fuel flow for his plane.

Wouldn't that be ~12% off for altitude, ?% off for density altitude, and ~25% off for 2300 rpm?

I wouldn't expect fuel flow to be anywhere near max for a 180hp engine.

What about timing? Advanced timing would drive CHT up, and EGT down.

Charlie
 
No........

Well it sure looks like it, you were happy at 0.58?.then look at the numbers you printed. :roll eyes:

And did I not follow on with reference to EGT values with standard CR's?

People posting with varying levels of accurate data really can only cope with some indicative numbers.

But the data tells the story. It is not my opinion nor yours. Data has no opinion. But it sure can point the owner where to start looking.

You have always had an anti APS bug up your nose, and thats fine with me, but the folk needing help here need some education and with limited scope some rough rules of thumb are what is needed. As much as I hate the rough rules of thumb.

Despite Lycoming old charts we know what yields nice numbers on the CGMTC engine stand, and the GPH rules of thumb work, all the time. Of course depending on the type (CR/TC) the rule differs.

Please do not waste your time replying. I am not looking for a debate. Spend it helping folk on DP and cooling airflow. This is an area of the science most struggle with most, and you are most helpful.
 
New Product Available Soon!!!

.....We are currently building a new product that will solve the "High CHT" and "High Oil Temperature issues. Prototypes are done, installed, tested and production run will be available in a couple of weeks. I am currently working on the video that will show and demonstrate the product and should have it up in a week or so. I think you guys are going to like this one so give me a few days before you modify your setup trying to solve these temp problems. Thanks, Allan...:D
 
Have a o-320 in my 6a
Think they drilled from 41 to 38
GPH at takeoff went from 12 to 14
#3 cylinder dropped 40 degrees


So I did the wot egt test yesterday and only got a 100 deg spread. I will be redrilling to a larger jet and let you know my results.

Note - my fuel flow at full rich only showed 10.2 gph

Also my full rich egt was 1140 and my peak was close to 1240

I'm hopeful that I will be able to do a 120mph climb to altitude at 24 squared soon !
 
Eric, you should stop wasting your time getting that Lycoming right and just scrap it and install a Rotax or a Subaru:p
 
Carb mixture balance and too much info

I went through the same vexations with my similar equipment. Finally resigned myself to an unbalanced fuel distribution. So I decrease climb rate as temps get to 400, usually on #3, and continue to cruise alt. I still have a great climb rate, and like you, temps all come down at alt. it's just a matter of having too much info and nothing you can do about it with a carb. Some day I'll retro fit a fuel injection system, untill then I pretty much do like I did with the old pipers I rented. Rich climb, then lean to rough then a turn in till smooth, just like the Lycoming manual says.
 
So I did the wot egt test yesterday and only got a 100 deg spread. I will be redrilling to a larger jet and let you know my results.

Note - my fuel flow at full rich only showed 10.2 gph

Also my full rich egt was 1140 and my peak was close to 1240

I'm hopeful that I will be able to do a 120mph climb to altitude at 24 squared soon !

Eric, based on your EGT values the fuel flow is possibly correct. I am only doing mental maths here but your altitude (UT) and a likely DA this is possibly correct. The EGT supports this also.

I would not go drilling the jets at all on your machine just yet.

PM me if you need some guidance.
 
.....We are currently building a new product that will solve the "High CHT" and "High Oil Temperature issues. Prototypes are done, installed, tested and production run will be available in a couple of weeks. I am currently working on the video that will show and demonstrate the product and should have it up in a week or so. I think you guys are going to like this one so give me a few days before you modify your setup trying to solve these temp problems. Thanks, Allan...:D

OK Allen, what are you up to this time?

I find myself buying things from you partly just because I like supporting you and your efforts. Well and you make good stuff too!

I purchased a new to me RV6A that had some strange feeling vibrations on roll out. I stole your Nose job parts along with the front wheel with the sealed bearing etc off of my 7A project, problem completely gone now. The front wheel just spins up on contact and she rolls out smooth as can be.

So I am guessing here that you have worked the bugs out of a nice little water mister spray bar type setup, am I right? It would make sense as we mostly only need the extra cooling for relatively short time periods to get us up into the cooler air. Maybe the little misters could be placed right at the cooling air openings to provide cooling help for the cylinders and the oil cooler all with perhaps two small nozzles? It would require good control of just the right amount of water as you sure would not want to spray too much in there. Works in Reno so shy not for us?

Looking forward to your announcement to find out if I am right!

My new to me RV6A spent most of it's time up in the Northwest so it needs a little help to handle the hot country in AZ and NM where I am flying it now.

Randall Crothers
 
As We Speak!!!

....We shot some video today with our new product and are editing it now. Will have it up on our website in a day or two. Hint! First word "EASY" :D
 
Eric, based on your EGT values the fuel flow is possibly correct. I am only doing mental maths here but your altitude (UT) and a likely DA this is possibly correct. The EGT supports this also.

I would not go drilling the jets at all on your machine just yet.

PM me if you need some guidance.


I got the carb off today and pull the jet and I believe we are measuring .170 and I see the number 47 on the jet and then an 828. So is it a 47 ? That is already quite large compared to others results I have read.

I feel lost now. I thought this was my solution. Guess I'll be hacking into the cowl because unless I go injected I'm out of answers.
 
If the jet actually measures .107 it is quite large for an O-320. I drilled mine to a #40 which is 0.0995 if memory serves me correct. The 828 number is the part number of the jet which has little to no correlation to the size it is drilled.
 
Also I cannot get my idle rich enough either- this was a separate issue a long time ago- cannot get rpm rise when I go to idle cut off at 1000 rpm. Was hoping that was somehow related as well.
 
New A.S.A. Video is up!!!!

... The latest video is now up on our website detailing our newest product. Thanks, Allan...:D
 
Lean

.....We are currently building a new product that will solve the "High CHT" and "High Oil Temperature issues.
I do not see how, if you are lean, this is going to fix that :confused:

I see no means that this item will enrich the mixture...
 
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