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High CHTs During Takeoff


G-SLOT

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I'm trying to troubleshoot what I think are too high CHTs on takeoff and wondered if anyone had similar experiences and solutions. Here are the relevant points:

  • Ovation II with TCM IO-550-G
  • 310hp STC with Hartzell Prop
  • Electroair electronic ignition
  • Gamijectors (checked and clear)
  • Baffles all in good shape.
  • No issues during pre-flight checks and runups.
  • Engine uses Aeroshell W100+ oil (recently switched from W80+ with the seasons).

Especially on cylinder 2 (left rear), CHTs climb rapidly and will easily exceed 400 degrees during takeoff at full power if left alone (confirmed, everything forward). Takeoff fuel flows are 29.5 to 30 GPH.

When I reduce RPM to 2500 (from 2700) very shortly after takeoff, the CHTs come right back to more comfortable levels.

I try not to let the CHTs exceed 400 but it is sometimes unavoidable (410 was a recent maximum and only there for a very short period). I appreciate the TCM red line is 450 but I have a personal red line of 400.

I generally pitch for a 500fpm climb; i.e. fast and shallow. OAT does not seem to make a huge difference.

Normal CHTs in cruise are mid 300s ROP and low 300s/high 200s LOP. I'm happy with those.

The rapidly climbing CHTs at full power are alarming and I'm interested to hear the experiences of others, especially with the 310hp STC and/or with electronic ignition.

I think the next thing to try would be to adjust takeoff fuel flows closer to 31 GPH.

Any thoughts, similar experiences or suggestions gratefully received.

Edited by G-SLOT
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Those CHTs seem high to me. What were the temps before the electronic ignition. What are the outside air temps?

I use 380 as my limit on climbs (also an O2 with 310 HP STC). I recently hit 400 for a few seconds climbing out of an airport where it was 104 F on the surface.

Some tricks I’ve used (WARNING: Not an A&P and have poor mechanical skills and knowledge at baseline):

1. Takeoff at 2550 RPM (almost like an O2).

2. Low boost pump on in climb.

3. Rotate, accelerate to Vy immediately and climb out at 120-130 kts.

Maybe someone that actually has some mechanical knowledge can give you better advice.

 

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The Electroair system was installed long before I bought the plane so can't say what full power CHTs were before.

OATs in the region of 60-80 degrees - nothing terrible.

I do very shallow climbs for better cooling generally (130-140kts) and, although I have tried low boost, I did not get the sense that it increased takeoff fuel flows.

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I’ve never taken off with low boost on (and wouldn’t) but it did seem to help a little during climb. I may be imagining it, though.

Is the runway long enough to take off at 2550 RPM? It seems like on my engine the difference between 2550 and 2700 RPM is huge when it comes to CHTs. A stock M20S takes of at 2400 RPM so it doesn’t seem too crazy to use 2550 when the conditions allow.

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My Ovation IO-550G experience with 310 HP,  we set max fuel flow to max. setting per TCM manual.........32 GPH.  Take off 2700 RPM if you wish, or less..............120 KIAS cruise climb after reaching pattern altitude........CHT's well below 400 degrees.  My information is based on my sea level experience.

For any RPM setting less than 2700, I would simply adjust fuel flow based on CHT readings...........again, well below 400 degrees.

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I have an O2 with the IO-550G with 310hp and have the same fuel flow and use the same oil. I take-off with 2700 RPM, rotate, gear up, flaps up and within a minute I pull the prop back to 2500 RPM. I climb out at 700 fpm and that seems to keep my #5 cylinder (my hottest cylinder) CHT at or below 375 degrees, I try not to exceed 375. All that and I’m climbing out at 135+ kts.

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I have a screaming eagle. I've climbed out at 90knots doing and 2700rpm and the CHT's were still around 375F. GeeBees baffle make a substantial difference. Thats where I would start. My fuel flow on takeoff is 28.5 GPH.

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1 hour ago, philiplane said:

27.5 to 29.5 GPH is the correct range for this engine. It needs to be adjusted with pressure gauges IAW Continental SID97-3G, not the fuel flow meter in the panel. Anything over 30 is over rich and will be bad for the engine.

Hmmmm........ interesting!  We’ll recheck TCM book again next week.   I believe 32GPH was top of range for IO-550G, 310 HP, 2700RPM, adjusted w/gage.  Perhaps my memory is suffering...... lol.  Stand by. :blink:

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1) The STC for 310 gives a range... for FF... 

2’) It is given in PPH... and when converted to gph... the the max number is about 27.2...

3) 27.2 works well for getting off the ground... but, CHTs can be challenge for an extended full power climb...


So...

4) Many people get that adjusted upwards easily 28, some 29...a few go 30...

5) Use caution with the higher number... it is possible to be too rich, and not produce full power...
 

6) +1 on having your mechanic follow the proper procedure... lots of related variables can be affected accidentally...

 

7) IO550s typically have a CHT challenge with the #5 cylinder first...
 

8) 380°F is the usual, best not to exceed, temp... used around here...
 

9) Climbing out at 120kias... can exceed 2k’pm... depending on weather and loading... :)
 

Now...

10) About that electronic ignition... what do you have there? (Electroaire... do tell more if able)

11) Electronic mags with their strong spark are known for burning fuel more efficiently to a higher completion within the cylinder... more fuel burned leads to slightly higher CHTs... you may see a corresponding lower EGTs to match...

12) Good idea to confirm... Mag timing is supposed to be 22° BTDC?
 

13) The strong ignition spark of the electronic mag has a similar behavior as turning up the timing further BTDC...

14) Being too rich won’t be improved by the strong spark or advanced timing... the fuel ratio is what it is... too lean, it won’t burn, too rich, it won’t burn...

15) Use caution when adjusting fuel flow without any documentation to support it... POH, STC... (good)... magazine articles, some guy on the net, not as good...  :)

16) As far as adding fuel pump during the climb... read up on how your fuel pump works... at WOT, the fuel pump is Probably on already... somebody already automated that... 

17) hmmmmmm.... more on Being too rich... the extra unmated molecules have a tendency to get in the way of the other molecules that are meeting up to burn... so the more rich the mixture goes... the slower the burn occurs...

18) This becomes an interesting challenge of related rates... and chemistry dynamics...

  • The engine is behaving as if the fuel is burning quickly...
  • Going over rich will have the tendency of slowing things down...
  • If the STC says max FF at 2700rpm is 27.2 , more than that would be ‘over’ rich...
  • There is a magazine article that recommended 29gph in a screamin’ Eagle many years ago...
  • There doesn’t seem to be any official documentation to recommend a FF of 30-31...
  • Technically, it could work... it has reason to work...
  • You can always pull back on the red knob if for some reason you are getting too much fuel delivered...
  • Consult with your mechanic...
  • Also check the obvious... the baffling may be a bit aged, and may be ready for an update...

19) When discussing flame fronts and ignition timing... it is alway a good idea to bring Ross along... @Shadrach

PP thoughts only, not a mechanic... just supporting a what if type of conversation...
 

Best regards,

-a-

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5 hours ago, MooneyMitch said:

Hmmmm........ interesting!  We’ll recheck TCM book again next week.   I believe 32GPH was top of range for IO-550G, 310 HP, 2700RPM, adjusted w/gage.  Perhaps my memory is suffering...... lol.  Stand by. :blink:

Yep, memory fading...........I stand corrected, thank you for refreshing me.............. it's Unmetered Pump PSI 32 top of range [28-32], not 32GPH. 

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7 hours ago, carusoam said:

10) About that electronic ignition... what do you have there? (Electroaire... do tell more if able)

11) Electronic mags with their strong spark are known for burning fuel more efficiently to a higher completion within the cylinder... more fuel burned leads to slightly higher CHTs... you may see a corresponding lower EGTs to match...

13) The strong ignition spark of the electronic mag has a similar behavior as turning up the timing further BTDC...

I had a really helpful conversation with the folks at Electroair yesterday. As I understand it, the electronic ignition does not burn the mixture faster (I thought this may have been the case which would, as @carusoam suggested, have a similar effect as advancing the timing) but rather that it ensures all of the mixture is consumed. It does lead to higher CHTs but should not cause my CHTs to rise that high.

Everything runs well with my engine at 2500 RPM and below. It's something between 2500 RPM and 2700 RPM that is causing the temperatures to rapidly increase.

I'll be having the timing checked, among various other things.

 

Note, thanks to @Hyett6420 for the tip on linking to a person's name.

Edited by G-SLOT
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This is a problem with the tight cowled ovation. Limited time running in the ground, lightly loaded (pilot and half tanks- allow climbing into cooler air faster), 140 kts climb, 29 ish gph, etc all help. However, in the end you’ll find the only way to solve the problem on hot days is to reduce the rpm to 2500-2550 after you clear ground hazards. It can be added back in at ~6k to increase climb rate. That is our SOP but I’m the odd man around here because I climb at 25 squared in my C as well. 

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“9) Climbing out at 120kias... can exceed 2k’pm... depending on weather and loading... :)

Really Anthony?  Man, I don’t remember that happening during my Ovation era. 

My memory (and there’s the rub maybe.... lol ), 120 gave me good CHT levels in combination with good horizontal forward view. 

I used 120 MPH in my former F, currently in the C, and used 120KTS in the R (2500 RPM, full throttle, light to medium loading, standard sea level, good weather). 

I sure don’t recall 2K ft p/min.

Your R can achieve that?  Is that at 2700RPM possibly?  I would use the 2700 for initial take off, then once reaching pattern altitude, reduce back to 2500 ( and adjust FF accordingly).  That sure was fun!! 

Help please !!!:mellow:

 

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Just now, Niko182 said:

Yeah give or take 110 to 120 knots. Thats with 30 gallons and a 160lbs pilot. It has a low empty weight too.

Excellent! Hmmm,  I probably did that too getting up to pattern altitude sometimes.  I always cringed at the fuel flow readings at that 2700RPM, so I'd adjust to 2500 after that, keeping the 120kts for my cruise climb.  A few times I'd dial up the 2700 in cruise and wow, that was fun too!!

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A couple weeks back i tried climbing out at 2700rpm all the way up instead of 2500rpm. The #5 cylinder didnt get above 370F. Now i just climb out at 2700rpm. Its a bit louder but you climb faster so im not complaining.

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My memory is fuzzy at best...

I brought Byron with me... and he took pictures of the VSI... :)

The FF seams pretty high... If you ran 30gph for an entire hour...

Keep in mind cruising at 12.5k’ only takes a few minutes to get there...

lightly loaded, B and I aren't giant people... we were heading to a fly-in not far away... (at AOPA)...

We probably only climbed to near 5k’...

This was part of the later break-in flights for the reman’d 310hp engine...
 

Fortunately, Byron’s memory is better than mine... :)
 

PP thoughts only...

Best regards,

-a-

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14 hours ago, G-SLOT said:

I had a really helpful conversation with the folks at Electroair yesterday. As I understand it, the electronic ignition does not burn the mixture faster (I thought this may have been the case which would, as @carusoam suggested, have a similar effect as advancing the timing) but rather that it ensures all of the mixture is consumed. It does lead to higher CHTs but should not cause my CHTs to rise that high.

Everything runs well with my engine at 2500 RPM and below. It's something between 2500 RPM and 2700 RPM that is causing the temperatures to rapidly increase.

I'll be having the timing checked, among various other things.

 

Note, thanks to @Hyett6420 for the tip on linking to a person's name.

Interesting...

The electroair people are saying it doesn’t burn faster... (which can be technically true)
 

And what is really going on is a challenge to describe....

But what we have is actual data to work with...

it appears that on a percentage basis... 

We could define what percent of the fuel is burned before the exhaust valve, and what percentage Continues to burn after... 

And follow that up with what percent partially burns Resulting in CO, and the unused fuel that just absorbs heat...
 

if we looked at a time line from spark, to ignition ramp up, to full burning, to the cylinder running out of fuel.... or the valve opens...

It appears that The strong spark initiates More fuel burning... (would like to get electroair’s input on that if they have it...)

Like a virus spreading, the exponential energy release spreads the flame faster.... (hmmmmm, they said it wasn’t burning faster...)

Looks like The stronger spark is initiating the flame front better...

There is a lot going on in those initial micro seconds...(Dynamics of a batch reactor)

Keeping track of it would be hard to do...
 

It is possible, the electroair people don’t even know what is going on after the spark... Or what happens next, or why...
 

It appears that we are only starting to see a difference... as the end users... we simply see a possible rise in CHT...

 

A higher % conversion is a chemical engineering dream come true...

Burning it faster is often good...

Too fast is called detonation... something to keep an eye out for...
 

We can always slow it down... go more rich, or more LOP... changing the ratio actually messes with the speed of the reaction... follow the random walk of molecules and see how often a 100LL molecule bumps into an O2 molecule.... change the ratio and you will see when one O2 bumps into another O2 nothing happens.... same for a pair of 100LLs... (fun stuff) :)

PP thoughts only, not a reactor design or combustion engineer...

Best regards,

-a-

 

Not only is the IO5550 a collection of six identical (Nearly independent) engines... It is also a collection of six different batch reactors..!
 

To improve % conversion, A reactor design guy Would recommend... increasing the temp, pressure, and add a mixer... :)

Holy cow!

:)

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  • 2 months later...

Brief update on this - I think I've partly figured out why my CHTs on cylinder 2 have been reading so much hotter than the others.

It turns out that the Mortiz gauge reads off cylinder 2 via a bayonet probe. In my JPI (EDM700) installation, cylinder 2 uses a spark plug ring temperature sensor and I gather that it is commonly known that these sensors indicate a temperature that is substantially (40-50 degrees) higher.

The CHT indication on the Moritz gauge, which I now know to show cylinder 2, is at a much more comfortable level. 

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Ive owned two ovations and two acclaims and all four had unique idosyncrasies, #5 being the common hottest in most phases, especially takeoff.

Three of them I struggled to keep CHT's under 380 on the climb, one never had any problem with only number 5 occasionally bumping up just over 380.  

What I did find with each one that if I leveled out or reduced the climb to 200fpm or so until the temps trended down below 380,  i was then good to resume max climb without the cht's climbing again and remaining below 380.  The only challenge was when i was too low to safely reduce the climb. 

In my current plane, the fuel flow (per the panel anwyay) actually goes down when the low boost pump is on so that isn't an option for me.

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