Magneto timing

denverpilot

denverpilot

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Haven't posted a complete story about this year's Annual + Maintenance other than some rambling in a few threads...

But... one item in our inspection has me curious and I'll admit I'm clueless and defer to the mechanics here for interesting and enlightening info, since honestly I forgot to ask the AI.

He says he found that our mags (which are coming off anyway for inspection, over 500 hours, and I suspect will need some TLC) were timed to 24 degrees instead of the called-for 22 degrees on our lovely and still well-behaved Continental O-470-S. :)

Just random dumb questions...

- Any reason a different mechanic might have done this?

- Any benefit to being timed further advanced at high altitude where we live?

- Are other variants of the O-470 further advanced and someone "did it from memory" even though its printed on the data plate (I think it is, anyway...?)

- Back to the "are there any reasons" question... here at home base at almost 6000' MSL, with the mixture pulled out slightly and things set right, we have wonderful easy starts in a blade, and idle is the usual loping O-470 "Harley" blub but nice and even, smooth, yadda yadda. She runs really nice at all power settings, and I'd hate to see a change mess with that. If the AI sets things to 22 and the darn thing gets cranky, would that be an indication that 24 was done for a reason? And if so... What would the reason be technically?

Other stuff we found this year with a new set of eyeballs on the airplane I can piece together in my head "how that got here"...

But this one is a bit of a mystery to me, and made me realize my understanding of aircraft mags and how they're handled, is a bit weak...

Oh, and of course, no mention of it in the logs. Which *is* annoying... or an indication of a mistake. Either way, annoying. :) So should be annoyed that something was done wrong... Or that it wasn't logged properly?

Take your pick. I dunno.

Thoughts from those who know more on this topic?

(I guess there's one more possibility in the matrix... The new AI is wrong, and it's supposed to be at 24. Hahaha. Crap...)

If you're now laughing because this is truly a "don't care" item, and I'm just clueless, that's okay too. Confirmation of cluelessness is often as useful as getting a clue. :)
 
Not an A&P so posting more for sharing perspective than anything.

First off, timing should be set to spec rather than for home field. No telling where an aircraft will go. The second part is the pilot sets mixture so there shouldn't be a need to play with timing to offset a mixture problem (like setting up an old-style fixed jet car carb). Of course the O-470 has a reputation as having poor fuel distribution.

Setting timing fast may result in a hotter engine with more complete combustion. Since it's a 182, temperature shouldn't be a problem. Of course the manufacturer had the opinion that 22* was correct...
 
According to the TCDS it's 22 deg and regardless of what "reason" may have been considered for setting it to 24 it's wrong and there is no way to justify it. Even if it runs better. Magnetos also have an internal timing called e-gap which the A&P will check when he opens them for the 500 hr service inspection.
 
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One thing that comes to mind is that the timing marks on many of these engines leaves a bit to be desired. Depending on how each mechanic set the timing I could see there being some variance between where one mechanic thinks 22 degrees is vs. where another mechanic thinks it is.
 
How do you know they were timed to 24 degrees if it isn't in the books? Sometimes the timing will drift a degree or two, especially if it wasn't checked at last annual or longer....

Just another possibility.


-VanDy
 
mondtster said:
One thing that comes to mind is that the timing marks on many of these engines leaves a bit to be desired. Depending on how each mechanic set the timing I could see there being some variance between where one mechanic thinks 22 degrees is vs. where another mechanic thinks it is.
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:yes::yes:
 
Clark1961 said:
Not an A&P so posting more for sharing perspective than anything.

First off, timing should be set to spec rather than for home field. No telling where an aircraft will go. The second part is the pilot sets mixture so there shouldn't be a need to play with timing to offset a mixture problem (like setting up an old-style fixed jet car carb). Of course the O-470 has a reputation as having poor fuel distribution.

Setting timing fast may result in a hotter engine with more complete combustion. Since it's a 182, temperature shouldn't be a problem. Of course the manufacturer had the opinion that 22* was correct...
Click to expand...


Actually a motor with advanced timing will run cooler.. not hotter :no: That is unless it is so advanced it is detonating......

As for the 22 -V - 24 degrees.... You could probably not even pick that out unless the motor was on a dyno with some pretty fancy data acquisition equipment.... IMHO
 
mondtster said:
One thing that comes to mind is that the timing marks on many of these engines leaves a bit to be desired. Depending on how each mechanic set the timing I could see there being some variance between where one mechanic thinks 22 degrees is vs. where another mechanic thinks it is.
Click to expand...

In other words: either a mistake was made or a mistake is about to be made.
 
Norman said:
May have been stamped at the factory but changed by the MSB.
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The only change authorized by the MSB is the "eligibility" of O-200's with specific qualifying cylinders installed to be re-stamped for 28 deg advance, it's an option. There are no engines with timing marks stamped on data plates by the manufacturer that do not match what is in the TCDS and even in the case of the O-200, whether or not it was re-stamped the data plate is correct.
 
It couldn't possibly damage an O-470-S engine at 24° BTDC, on 100 octane gas. The very same engine, an O-470-U, not only IS timed at 24°, but it has higher compression and turns slower. There is much more stress on the "U" engine, yet it runs just fine.
 
N801BH said:
Actually a motor with advanced timing will run cooler.. not hotter :no: That is unless it is so advanced it is detonating......
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???? Timing causing detonation? No. Flame-front impingement? Yes. Same thing? No.

Disagree with the other statement also. Oh well.
 
Silvaire said:
The only change authorized by the MSB is the "eligibility" of O-200's with specific qualifying cylinders installed to be re-stamped for 28 deg advance, it's an option. There are no engines with timing marks stamped on data plates by the manufacturer that do not match what is in the TCDS and even in the case of the O-200, whether or not it was re-stamped the data plate is correct.
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Not gonna debate the point.:mad2:
 
Clark1961 said:
???? Timing causing detonation? No. Flame-front impingement? Yes. Same thing? No.

Disagree with the other statement also. Oh well.
Click to expand...

Hmmmm... Get your buddy Henning as the test pilot and set your timing on your turbo charged plane too far advanced and let him fly it at full boost.... The results ain't gonna be purty...:no:
 
N801BH said:
Hmmmm... Get your buddy Henning as the test pilot and set your timing on your turbo charged plane too far advanced and let him fly it at full boost.... The results ain't gonna be purty...:no:
Click to expand...

???? I didn't post a single thing about timing on the Frankenkota. Are you okay?
 
Clark1961 said:
???? I didn't post a single thing about timing on the Frankenkota. Are you okay?
Click to expand...

Just fine sir...................

For you to say advanced timing will NOT contribute to detonation is laughable...


Fly safe out there....
 
N801BH said:
Just fine sir...................

For you to say advanced timing will NOT contribute to detonation is laughable...


Fly safe out there....
Click to expand...

Detonation is not related to spark ignition, it is a fuel and mixture condition, but whatever Ben. Have a nice day.
 
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Okay so digging the info out of the never-ending arguments... (Heh)...

- It's hard to tell where it's set.
- Correct setting is 22.

Got it. Thanks. ;)
 
Different shop or different A&P/IA than you've been using? Who? I'm due in late August and think I need a new POV.
 
Timing can easily drift two degrees in 500 hours. If the contact points erode, the timing advances. If the points cam wears, the timing retards. I would stop blaming the mechanic who initially set the timing. The evidence is far too weak.

The timing should be checked every 100 hours. Within one degree is fine.

Dan
 
I got the impression this was via the timing marks, not measured timing electronically. You're discussing a measurement, correct?
 
Dan Thomas said:
Timing can easily drift two degrees in 500 hours. If the contact points erode, the timing advances. If the points cam wears, the timing retards. I would stop blaming the mechanic who initially set the timing. The evidence is far too weak.

The timing should be checked every 100 hours. Within one degree is fine.

Dan
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It's doubtful that timing drift due to internal conditions would cause both magnetos to be off exactly the same amount and it's unlikely that the timing has not been checked and adjusted for 500 hours. On the O-470's the timing marks are on the crank and visible through an inspection plug on the left case half at the nose. There are usually two stamped numbers such as 20 and 28 with hash marks in between that are 2 degree increments. So you need to look inside with a flashlight and rock the prop back and forth to see what's what and figure out which mark is yours and then set it right in the center of that inspection hole.

Needless to say there are different ways of interpreting this, there's the possibility of parallax error or just mistaking your mark. The other choice is to use a spinner protractor which is sometimes called a Popes Hat to find TDC and well, there are all sorts of ways to screw that up too. So an error of two degrees as we are discussing here in this thread - it could have been the previous mechanic or it could be the current mechanic. It's even feasible that each mechanic is off by one degree in the opposite direction and they are both wrong.

For these particular engines I prefer to use the crankshaft marks, I figure they are the most accurate but on most of the planes these engines are on such as Bonanzas and 206's the front nose cowl is not removable so it takes some extra diligence and an inspection mirror to be able to view through the port at an angle that will not cause parallax error. It's the same way on Lycomings where the timing mark is on the back side of the starter ring gear and has to be lined up with the case split. Even with a clamp on laser pointer you need to use a mirror to get it aligned perfectly with the mark before tightening the clamp.

The bottom line is that the timing indicator marks on these engines aren't particularly sophisticated and I wouldn't be so quick to string up a mechanic for being 2 degrees off. Especially when you honestly don't know which mechanic is the one that's off based on anything other than what one is telling you.
 
A piston stop and an angle cube replaces the degree wheel and the pointer.
The data tag isn't the place to find the proper timing on any engine, it may have been changed by a number of ways, STC, SB, or AD.

know what you are working on and use the publications required by FAR 43.
 
denverpilot said:
I got the impression this was via the timing marks, not measured timing electronically. You're discussing a measurement, correct?
Click to expand...

I'm drawing a blank on how one would check timing this way.
 
Tom-D said:
A piston stop and an angle cube replaces the degree wheel and the pointer.
The data tag isn't the place to find the proper timing on any engine, it may have been changed by a number of ways, STC, SB, or AD.

know what you are working on and use the publications required by FAR 43.
Click to expand...

I don't know of any STC, SB or AD that has you alter the timing and leave a different number on the data plate. Certainly some data plates are unreadable in which case I would use the TCDS but there shouldn't be any need to guess.

How would you use a degree wheel and pointer without a piston stop?:dunno:
I've thought about making an adapter to replace the wheel and pointer with my digital protractor but haven't gotten around to it since I rarely need to do it that way.
 
Silvaire said:
I don't know of any STC, SB or AD that has you alter the timing and leave a different number on the data plate. Certainly some data plates are unreadable in which case I would use the TCDS but there shouldn't be any need to guess.

How would you use a degree wheel and pointer without a piston stop?:dunno:
I've thought about making an adapter to replace the wheel and pointer with my digital protractor but haven't gotten around to it since I rarely need to do it that way.
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0-200 has an AD that retards the timing fro 28 degrees to 24 degrees to stop cracking. and the after market cylinders are timed to 28.
It was retarded by AD and returned to 28 by STC. data tag remained at 28. the AD did not require re-stamping the data tag.
 
Clark1961 said:
Detonation is not related to spark ignition, it is a fuel and mixture condition, but whatever Ben. Have a nice day.
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Advancing spark can induce detonation. An earlier burn while the piston is still compressing raises the pressures in the cylinder, therefore the temperature of the unburned part of the mixture, and detonation is related to temps and pressures as well as fuel quality and mixture.

Shoot, even our cars have knock sensors to retard timing if detonation shows up. In the old days we manually reset the timing if the thing wanted to ping.

Dan
 
Tom-D said:
...the AD did not require re-stamping the data tag.
Click to expand...

The current revision of that AD is 96-12-06 and par (e) says re-stamp the dataplate. The original AD was 77-13-03 instructing you to comply with the Continental service bulletin which told you to re-stamp the dataplate. If there is an O-200 out there that is required by AD to be set at 24 and has a dataplate stamped 28 then somebody goofed up.
 
Dan Thomas said:
Advancing spark can induce detonation. An earlier burn while the piston is still compressing raises the pressures in the cylinder, therefore the temperature of the unburned part of the mixture, and detonation is related to temps and pressures as well as fuel quality and mixture.

Shoot, even our cars have knock sensors to retard timing if detonation shows up. In the old days we manually reset the timing if the thing wanted to ping.

Dan
Click to expand...

Agree totally that detonation is related to temps and pressures as well as fuel quality and mixture. Which travels faster, the pressure front or the flame front?
 
Silvaire said:
The current revision of that AD is 96-12-06 and par (e) says re-stamp the dataplate. The original AD was 77-13-03 instructing you to comply with the Continental service bulletin which told you to re-stamp the dataplate. If there is an O-200 out there that is required by AD to be set at 24 and has a dataplate stamped 28 then somebody goofed up.
Click to expand...

That really depends upon if you are complying with para - (e) or (f) of the new AD (96-12-06 )
most of the data tags on the 0-200-A have nothing but the S/N. they are a blank piece of stainless steel that have been thru the strip tank many times.
 
Regardless of whether you are complying per par (e) or (f) and regardless of the condition of the dataplate if the AD is signed off there should be timing specs stamped on it. I don't see any other way to read it.

Have I ever seen an O-200 or O-300 with a dataplate that has nothing but a S/N? Yes. In the case of the O-300 I'll use the TCDS, in the case of the O-200 I'll stamp either L24, R24 or L28, R28 on the plate because of AD 96-12-06. If there are no timing stamps on the dataplate the AD is not in compliance.
 
Here's what I did this year,

We installed freshly overhauled TCM magnetos from the factory and we installed them using eyeballs and the starter ring gear timing marks just to get them good enough to perform some low power ground runs checking to make sure the wiring and everything was good and for any leaks.

Then I checked it with the piston stop and digital protractor cube and found timing about 3 degrees off and not enough adjustment left in the right mag due to it having the "dog ear" type mounting flanges. So I pulled the mag out of the hole and loosened the retaining nut on the drive shaft and turned the gear 180 degrees and re-installed the nut.

Worked like a charm, the accessory case studs are nearly centered in the slot of the dog ears on both mags, timed via the piston stop & cube
 
Tom-D said:
That really depends upon if you are complying with para - (e) or (f) of the new AD (96-12-06 )
most of the data tags on the 0-200-A have nothing but the S/N. they are a blank piece of stainless steel that have been thru the strip tank many times.
Click to expand...


I replaced one on an O-200 at the tune of $200 once and TCM sent me a different serial number and a letter saying the serial we had was incorrect. I added the letter to the engine log...
 
Clark1961 said:
Agree totally that detonation is related to temps and pressures as well as fuel quality and mixture. Which travels faster, the pressure front or the flame front?
Click to expand...

Got to understand detonation. As the pressures rise, so do the temperatures. The flame front is usually about 100 feet per second; in a detonation condition it's around 5000 fps. The unburnt fuel ahead of the flame front essentially doesn't wait for the chain reaction of the flame front and spontaneously ignites all at once, simply because it's being squeezed to the point where its temperature causes a breakdown of the complex molecules into simpler, easily-ignitable ones. The pressure front can easily outrun the flame front, considering that the high temperature caused by compression increases the speed of sound enormously.

Advancing timing just gives it more time to do this as well as aggravating the pressure and temperature problem. Bigger cylinders at low RPM are more susceptible, too, since the flame front takes longer to travel across the chamber and the low RPM adds even more time. Leaner mixtures burn more slowly and add more time yet. Higher induction temps raise the overall charge temperature and increase the chances of detonation.

Automobiles have long had variable timing to eliminate detonation, and modern cars added the knock sensor circuitry to allow more advance to get better mileage while still controlling detonation. Basically, anytime the throttle is opened, the timing retards. Anytime the engine slows, the timing retards.

It's not just a mixture condition.

Dan
 
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Silvaire said:
Regardless of whether you are complying per par (e) or (f) and regardless of the condition of the dataplate if the AD is signed off there should be timing specs stamped on it. I don't see any other way to read it.

Have I ever seen an O-200 or O-300 with a dataplate that has nothing but a S/N? Yes. In the case of the O-300 I'll use the TCDS, in the case of the O-200 I'll stamp either L24, R24 or L28, R28 on the plate because of AD 96-12-06. If there are no timing stamps on the dataplate the AD is not in compliance.
Click to expand...

This AD only applies to certain cylinders, if it didn't apply to this engine there probably won't be any compliance noted and or change to a blank data tag.

And when new cylinders are installed there is no requirement to re-stamp the tag.

So getting back to the point, you must know the equipment and check ADs, STC, SBs and all other pertinent data on the engine.

Which I guess you have already agreed with.
 
Dan Thomas said:
Got to understand detonation. As the pressures rise, so do the temperatures. The flame front is usually about 100 feet per second; in a detonation condition it's around 5000 fps.
Click to expand...

You've neglected to address my point about the speed of travel of the pressure front which is at the heart of the matter. If the pressure front hasn't disturbed the unburnt fuel/air mixture then it won't detonate. Advanced timing leads to flame front impingement, not detonation.
 
Heh. The engineers are going at it again.

Get a hose. ;)

We learned some new things (Cessna changed the hinge design at least once for the Cessna components involved...), but no joy on RSTOL parts from salvage or any other options.

Oh well... Guess the magic $900 hinge halves will be here in a couple of weeks... :)

Custom mods is custom mods. Always more expensive than stock.

I still think she needs the wheel pants off and some fat tundra tires... :)
 
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