Retarding Ignition Timing

Pablo Canales

Pablo Canales

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Pablo
Hey guys! I’ve been doing everything to lower my CHT’s on my Continental IO-550 below 400°F

Yesterday my mechanic suggested retarding the ignition timing to 20 instead of 22

What do you guys think?
Are there any risks?


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That would result in lower BMEP and slight power loss.What method is being used to determined 22 degrees? On some engines there is a tolerance of 1 or 2 degrees so you may be legal.
 
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I believe that timing is stated on the TCDS. The TCDS is part of the type design. The definition of airworthy is that it conforms to its type design and is in a condition for safe operation.

So would setting it at something other than what the TCDS states be in conformance with the type design and therefore still airworthy?

Just a thought exercise.
 
Pablo Canales said:
Hey guys! I’ve been doing everything to lower my CHT’s on my Continental IO-550 below 400°F

Yesterday my mechanic suggested retarding the ignition timing to 20 instead of 22

What do you guys think?
Are there any risks?


Sent from my iPhone using Tapatalk
Click to expand...

I think that if the book value is 20 degrees, it should be set at 20 degrees. Note I don't know that it should be 20 deg, I'm just going from what you said.

I've read articles where the CHTs were high, where they tracked it down to a seemingly insignificant problem with the baffling. I remember one where there was some kind of a diverter plate that was installed backward - but still had the same basic shape (kind of like an "S" rotated 180 degrees is still an "S"), so it looked and fit just fine - but the slight difference caused significant higher CHTs.

So, yeah - check the baffling, carefully, and make sure it is exactly how it's supposed to be.
 
Also, This is what I’m getting at full power 25.7gph

Mechanics say its fine, but i feel like its not enough to keep the temps down
On a IO-550 with 300hp


1ae75a1edb3b03d9979289885f17aaaf.jpg



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So are your high CHTs on climb or in cruise? Your first post wasn't specific.

From your pictures above, I'd say the fuel flow is too low. What type of airplane? For Bonanzas with the IO-550 there are whole pages of articles and discussions at the ABS web site. The all generally recommend a higher fuel flow than you're showing for full-power climb.
 
make sure your fuel flow is at least 30 gph
 
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My point with “ method” is that it’s possible you could actually be firing at something like 25 degrees.

It’s unclear based on pix but it looks like full power rpm is 2500 and redline is 2700?

With most engines a slight leaning at full power should show about a 20 rpm increase. Easy to check on fixed pitch but on constant speed it must be checked before the Governor kicks in and takes prop off the low pitch stop.

Obviously, if fuel flow is too low you may not make enough power to make rated rpm.
 
Pablo Canales said:
Hey guys! I’ve been doing everything to lower my CHT’s on my Continental IO-550 below 400°F

Yesterday my mechanic suggested retarding the ignition timing to 20 instead of 22

What do you guys think?
Are there any risks?

Sent from my iPhone using Tapatalk
Click to expand...

I know you have tried everything, but what shape are the baffles in?
 
The GAMI guys do that....I've set mine higher and it helps. The 520/550 engines shoot for hp/10 gph for sea level takeoffs, i.e. about 28 gph for the 520.
Grum.Man said:
Where in the world did you get that information from???
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Pablo Canales said:
Also, This is what I’m getting at full power 25.7gph

Mechanics say its fine, but i feel like its not enough to keep the temps down
On a IO-550 with 300hp


1ae75a1edb3b03d9979289885f17aaaf.jpg



Sent from my iPhone using Tapatalk
Click to expand...
According to SID 97-3E that fuel flow looks about right but it wouldn't hurt to have the fuel set procedure done to be sure. The timing should be set to the TCDS. Are all the cylinders running hot or just a select few. You can have your injectors cleaned, increase climb speed, check baffling, reduce power futher.
 
Yup....but keep in mind temperature and pressure changes that....so a safety margin is good.
Grum.Man said:
Considering the set procedure is supposed to be done at every annual I would hope an A&P would follow the SID and not go rogue but to each their own.
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Pablo Canales said:
Yesterday my mechanic suggested retarding the ignition timing to 20 instead of 22
Click to expand...

Hmm. Be sure he plans on documenting the engine log book for this corrective action. I seriously doubt you’ll see it get signed off as such. Therefore not legal for the owner to fly the airplane. If he declines signing it off (as setting the timing to a different value) ask him why he can’t put that in the book.
 
Checkout_my_Six said:
Yup....but keep in mind temperature and pressure changes that....so a safety margin is good.
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No it doesn’t. The specification covers the operating spectrum. If Gami requires a different setting then there will be a supplement with the paperwork.
 
Jdm said:
Hmm. Be sure he plans on documenting the engine log book for this corrective action. I seriously doubt you’ll see it get signed off as such. Therefore not legal for the owner to fly the airplane. If he declines signing it off (as setting the timing to a different value) ask him why he can’t put that in the book.
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He will also need to change the data plate on the engine and convert it to experimental.
 
Grum.Man said:
According to SID 97-3E that fuel flow looks about right but it wouldn't hurt to have the fuel set procedure done to be sure. The timing should be set to the TCDS. Are all the cylinders running hot or just a select few. You can have your injectors cleaned, increase climb speed, check baffling, reduce power futher.
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That’s right, they just cleaned the injectors and now i’m getting 28gph
So it might have been that the injectors were clogged up


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Pablo Canales said:
That’s right, they just cleaned the injectors and now i’m getting 28gph
So it might have been that the injectors were clogged up


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Click to expand...
That's a little high but could also be due to gauge error.
 
Also guys, you have to consider that I live in Cancun, Mexico at sea level where temperatures are pretty much above 90°F all year round

So plenty of fuel flow is well appreciated


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Pablo Canales said:
Also guys, you have to consider that I live in Cancun, Mexico at sea level where temperatures are pretty much above 90°F all year round

So plenty of fuel flow is well appreciated


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Click to expand...
Ugh please don't follow this logic! Hot also means higher density (edit) altitudes which means less power which equals less required fuel. I assume this is an IO-550-B so at full throttle the fuel flow should be 24.9-26.6. If you go higher you risk fouled plugs, washed down cylinders, reduced performance (which could cause over heating). It sounds like your cylinder head temps are right in the range they should be.
 
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Man, I prefer a little higher than book. So shoot me!

One useful thing to try - when you’re taking off and getting really hot, turn on your boost pump - you’ll get a couple extra GPH.
 
SCCutler said:
Man, I prefer a little higher than book. So shoot me!

One useful thing to try - when you’re taking off and getting really hot, turn on your boost pump - you’ll get a couple extra GPH.
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I should let this go but seeing you have A&P listed in your credentials I am curious where you draw the line on not following specifications? Oil pressure, fuel pressure, stall speed? If someone floods their engine on takeoff because the fuel flows were too high I am curious what your explanation would be to the FAA when they review the maintenance logs and see that you set the fuel pressure higher than specified, you felt it was better? Don't mean to be a jerk but this laid back attitude to aircraft maintenance has caused a lot of crashes.
 
To answer your question, yes, a change of timing from 22 to 20 BTDC will reduce power just slightly and CHT by maybe 20*. Legal? What’s on the nameplate?
 
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Grum.Man said:
He will also need to change the data plate on the engine and convert it to experimental.
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Hang on. That might make it kinda legal looking. I was hoping to see the mechanic’s sign off.
That’s what really bugs me. Mechanics are quick to shoot from the hip when making recommendations and doing ghost work. Funny how things magically change when you request a log entry.
 
Have you verified the accuracy of your gauge? I check mine with an oil bath on a hotplate and a lab grade mercury thermometer. Stir the oil to make sure its temp is uniform. Science supply houses have accurate thermometers.

I climb my IO-520A with an EGT 200 degrees ROP and cruise LOP at 65% or less.
 
The TCDS for the IO-550-N shows timing as "Timing,°BTC 22"
(with no +0°/-2° as other engines have)
Check me on this. It's TCDS #E3SO Rev No 12 from the FAA RGL.

Cont Engine Std Prac Mx Manual (2017) Ch6 - (you will see people refer to this as SID97-3X, but those are a few years old now), entitled "M-0", for the IO-550, says 25.6-27.3gph at full power, (table 6-4) although I cannot remember the last time I heard anyone say these values were acceptable...and most shoot for higher, usually 2gph higher, to correct for what is seen as a problem either in modern cylinder design, metallurgy, or fit.
This 2017 manual "M-0" is available free, on CSOBeech.com under pdf downloads.
 
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