Jump to content

Surefly base timing 20 or 25?


PT20J

Recommended Posts

The IO-360 engines (except dual mag versions) are approved for timing of either 20 degrees or 25 degrees BTDC per the TCDS. Twenty degrees has been standard since 1975. Since the Surefly SIM can be set to either, I’m curious if anyone has tried 25 rather than 20.

Skip

Link to comment
Share on other sites

In the other SF thread there was someone who’s engine was timed at 25btdc and they said the SF caused bad issues (heat and lower performance) however it also seemed to me that there was more to it than just the timing or SF.

I’m guessing you already read that though...

Edited by Ragsf15e
  • Like 1
Link to comment
Share on other sites

40 minutes ago, Phil EF said:

The engine data plate is the approved data for non-experimentals. 


True, but...

There is one Mooney case where the engine was certified one way, then got recertified the other way...

In which case the data plate may need proper updating...

Allowing for the change from 20 back to 25BTDC...

That would be separate from the topic here... where the question is more specific to the electronic ignition...

Old fuzzy memories of a PP, not a mechanic...

Best regards,

-a-

Link to comment
Share on other sites

My IO360 (original to the plane) was at 25 deg (as it was on the data plate) so we set the Surefuly tp 25 degree as well. I had no issues but I had noticed some higher oil temps last summer, perhaps also summer before the install. CHT were always good. I think engine might have some other issues which led to ordering zero time factory reman. I wonder what timing will be on the new engine I'm to receive in a month or two...

Link to comment
Share on other sites

22 minutes ago, Igor_U said:

My IO360 (original to the plane) was at 25 deg (as it was on the data plate) so we set the Surefuly tp 25 degree as well. I had no issues but I had noticed some higher oil temps last summer, perhaps also summer before the install. CHT were always good. I think engine might have some other issues which led to ordering zero time factory reman. I wonder what timing will be on the new engine I'm to receive in a month or two...

It will be 20 deg. Lycoming has been setting them that way (except for the dual mag versions) since about 1975. From the Operator’s Manual:

httpswww.lycoming.comsitesdefaultfilesO-HO-IO-HIO-AIO202620TIO-36020Oper20Manual2060297-12_pdf.thumb.png.bef1db14e0294ec546e7d3be777e7b11.png

  • Like 1
Link to comment
Share on other sites

If you want 25 deg, you can just make a logbook entry when you install the Surefly and stamp the data plate accordingly.

I think that 20 might make more sense with a Surefly, however. This would get you the lower CHTs at full power on a hot day. At reduced (cruising) power, the Surefly will handle the advance. The Surefly is missing air/fuel ratio information -- all it knows is MAP and rpm. The optimum advance (MBT -maximum brake torque -timing) would be greater LOP than ROP due to the difference in combustion rates so the Surefly timing map has to be a compromise.

Skip

  • Like 1
Link to comment
Share on other sites

8 hours ago, Igor_U said:

My IO360 (original to the plane) was at 25 deg (as it was on the data plate) so we set the Surefuly tp 25 degree as well. I had no issues but I had noticed some higher oil temps last summer, perhaps also summer before the install. CHT were always good. I think engine might have some other issues which led to ordering zero time factory reman. I wonder what timing will be on the new engine I'm to receive in a month or two...

I noticed slightly higher chts with my SF initially, maybe 10-15 degrees more each cylinder at ~10,000’.  Maybe 350-380 in cruise ROP.  330 Lop.  Then last summer I noticed higher oil temps to go with the higher chts.  So at annual I sent my vernatherm and oil cooler to Pacific coast Oil Cooler.  It was so corroded that they said they couldn’t even overhaul it.  I now have a nice new $725 oil cooler and the oil temp is right at 185. Summer will be the real test.

  • Like 2
Link to comment
Share on other sites

10 hours ago, PT20J said:

It will be 20 deg. Lycoming has been setting them that way (except for the dual mag versions) since about 1975. From the Operator’s Manual:

httpswww.lycoming.comsitesdefaultfilesO-HO-IO-HIO-AIO202620TIO-36020Oper20Manual2060297-12_pdf.thumb.png.bef1db14e0294ec546e7d3be777e7b11.png

the 1E10 TCDS says the 20 degrees timing is optional. Thats the regulatory black letter.

  • Like 1
Link to comment
Share on other sites

46 minutes ago, jetdriven said:

the 1E10 TCDS says the 20 degrees timing is optional. Thats the regulatory black letter.

That’s because the engines were originally certificated with 25 deg timing. When Lycoming came out with service instruction 1375 in 1975, it made 20 degrees available as an option and the wording of the TCDS is similar.

Nonetheless, Lycoming has been shipping engines with 20 deg timing starting with the serial numbers listed in the operator’s manual posted above. 

The final authority for any particular engine is the data plate. So timing must  be set per the data plate. Per the Lycoming rep I discussed this with, you are free to set either timing (that means changing a pre-1975 engine to 20 deg  or changing a post-1975 engine to 25 deg). The approved data for either case is the TCDS. A change can be done with a logbook entry and restamping the data plate.

Skip

Link to comment
Share on other sites

Skip,

I think you are more knowledgeable on this particular subject than most people on this forum! :) You should give the 25 timing a try and let us know. I don't have a Surefly but now I'm curious. I'm just still disappointed that you can't replace both mags with a modern solution. Not sure I see the point of variable timing with one magneto and not the other. I maintain a fleet of Austro diesel powered electronically controlled Diamond aircraft and they work just fine. Add a separate power source (the Diamonds have a dedicated back up battery for the ECU) and you could have a modern, reliable ignition system. 

  • Like 1
Link to comment
Share on other sites

21 hours ago, Igor_U said:

My IO360 (original to the plane) was at 25 deg (as it was on the data plate) so we set the Surefuly tp 25 degree as well. I had no issues but I had noticed some higher oil temps last summer, perhaps also summer before the install. CHT were always good. I think engine might have some other issues which led to ordering zero time factory reman. I wonder what timing will be on the new engine I'm to receive in a month or two...

It will be set at 20° as Skip @PT20Jmentions above. I think it’s unfortunate that Lycoming has made this policy. Only in the world of FAA regulation can a ignition advance be reduced by 20% on a certified engine with no mention of the effect on performance. In my anecdotal experience in similar airframes with both configurations, it does make a difference...most recognizably in climb, somewhat less recognizably in takeoff roll and much less recognizably in cruise though I think that a difference in cruise may become more apparent above DAs of 12K. 

I don’t know the exact reason for the change but I suspect it was to help lower CHT‘s. Given how cool my 25° engine runs it seems like it was catch all treatment for applications with poor cowl design.

  • Like 1
Link to comment
Share on other sites

I have attached two documents. The first is Lycoming SI1325 from 45 years ago wherein Lycoming states that the purpose of the change in timing was to improve cold weather starting, cruise CHT reduction and increased detonation margin at full throttle and extremely low temperatures.

SI 1325 Timing Change for IO-360 Series Engines.pdf

Perhaps a review of timing fundamentals might help understand the effect. There is an optimum timing called Maximum Brake Torque timing. It is the timing where the ratio of positive work done on the piston after TDC to the negative work done on the piston before TDC is maximized. It varies somewhat for each operating condition (MAP, rpm, mixture strength). For most engines, variation of timing +/-5 deg either side of MBT cause only a 1-2% decrease in power output. Sometimes engine manufactures purposely retard the timing from MBT to avoid detonation and reduce combustion chamber temperatures (in an airplane engine the temperature reduction would be to reduce CHTs due to limitations of air cooling and in auto engines the reduction is usually to reduce NOx formation). Heywood's, Internal Combustion Engine Fundamentals, has pretty good explanations of this. 

In order to understand how timing effects a particular engine it is necessary to do performance measurements while performing a "timing sweep." Fortunately, Nigel Speedy has done this for us.

Nigel Speedy - Ignition Advance .pdf

EDIT: I forgot to mention that Nigel’s experimental engine has 10:1 compression as opposed to stock 8.7:1. So, while his results indicate trends, they may not be exactly the same as the stock engine. 
 

Skip

 

  • Like 3
  • Thanks 1
Link to comment
Share on other sites

4 hours ago, PT20J said:

I have attached two documents. The first is Lycoming SI1325 from 45 years ago wherein Lycoming states that the purpose of the change in timing was to improve cold weather starting, cruise CHT reduction and increased detonation margin at full throttle and extremely low temperatures.

SI 1325 Timing Change for IO-360 Series Engines.pdf 470.94 kB · 1 download

Perhaps a review of timing fundamentals might help understand the effect. There is an optimum timing called Maximum Brake Torque timing. It is the timing where the ratio of positive work done on the piston after TDC to the negative work done on the piston before TDC is maximized. It varies somewhat for each operating condition (MAP, rpm, mixture strength). For most engines, variation of timing +/-5 deg either side of MBT cause only a 1-2% decrease in power output. Sometimes engine manufactures purposely retard the timing from MBT to avoid detonation and reduce combustion chamber temperatures (in an airplane engine the temperature reduction would be to reduce CHTs due to limitations of air cooling and in auto engines the reduction is usually to reduce NOx formation). Heywood's, Internal Combustion Engine Fundamentals, has pretty good explanations of this. 

In order to understand how timing effects a particular engine it is necessary to do performance measurements while performing a "timing sweep." Fortunately, Nigel Speedy has done this for us.

Nigel Speedy - Ignition Advance .pdf 441.94 kB · 4 downloads

EDIT: I forgot to mention that Nigel’s experimental engine has 10:1 compression as opposed to stock 8.7:1. So, while his results indicate trends, they may not be exactly the same as the stock engine. 
 

Skip

 

Mr. Speedy's work is admirable even if the methods are less than optimal.  A test cell is really the only way to derive consistent repeatable numbers.  A deep dive into his data reveals many inconstancies but it generally shows just what I would expect.  His conclusion that 25° is the optimal compromise matches my anecdotal experience. Better performance may sometimes be realized with additional advance but retarding the timing below 25° almost always translates to power loss. The differences in his cruise numbers suggest a lot more than the 1-2% power loss. "The greatest increase ROP was at 8000 feet, where speed increased from 186 to 190 TAS when going from 25 degrees to 28 degrees ignition advance (a 2% gain in speed)".  I'm not saying his numbers are reliable because a 4kt increase all other things being equal requires significantly more power than I would think could be gained with just 3° of advance certainly more than the 1-2% you referenced above.

Here's my anecdotal story.  Years ago after having my mags IRAN'd and installed by the local shop the plane felt a bit off. The test flight went fine but performance (take off and climb specifically) felt diminished to some degree. I likely would not have noticed at a strange airport but at one's home airport there are subtle cues that one picks up on that something has changed.  Did it feel like a different airplane? Not really, but not as strong as usual, like the DA was higher than reported.  I shared that with the shop while collecting my logs.  Later that evening after reviewing the engine log it became obvious that the mechanic had inadvertently set the timing to 20°.  I reset the timing to 25° the following day with an IA buddy and the artificial increase in DA went away. 

Edited by Shadrach
  • Like 1
  • Thanks 1
Link to comment
Share on other sites

I thought some more about Speedy's data. His higher compression ratio would have the effect of increasing the density and temperature of the fuel-air mixture which would increase the burn rate (higher density means the reactant molecules are closer together and higher temperature means they are moving around more rapidly. The molecules of fuel and air have to make contact to react, so the closer together they are and the more they are moving the more often they bump into each other). The increased burn rate would increase the peak pressure and move it closer to TDC similar to increasing the advance. I'm not sure by how much, but if we assume 5 deg as the equivalent advance to account for the increased compression, we would need to subtract 5 deg from his timing curves to make them correspond with the stock engine.

Skip

Link to comment
Share on other sites

On 2/19/2021 at 5:20 PM, PT20J said:

I thought some more about Speedy's data. His higher compression ratio would have the effect of increasing the density and temperature of the fuel-air mixture which would increase the burn rate (higher density means the reactant molecules are closer together and higher temperature means they are moving around more rapidly. The molecules of fuel and air have to make contact to react, so the closer together they are and the more they are moving the more often they bump into each other). The increased burn rate would increase the peak pressure and move it closer to TDC similar to increasing the advance. I'm not sure by how much, but if we assume 5 deg as the equivalent advance to account for the increased compression, we would need to subtract 5 deg from his timing curves to make them correspond with the stock engine.

Skip

Educate me here.  Wouldn't a higher C/R mean a faster flame front/combustion event? I thought the reason that NA race engine tuners advanced timing at high altitude was to compensate for the lower speed of the combustion event at higher DAs. Seems like an increase in C/R with all other things being equal, would warrant retarding the timing to achieve MBT.  This comports with my general understanding of tuning forced induction street cars. Yes, retarded timing increases detonation margins but in many cases, power starts to fall off.

Link to comment
Share on other sites

Since SF no longer publishes their advance curve, we don’t know for sure, but I suspect their compromise between advancing for ROP and LOP goes too far to really improve ROP ops, even from a 20 BTDC starting point.  Mine is at 20, but cruising ROP at 10,000’ where the advance is probably close to maximum, my CHTs are a little hotter than before but there’s no increase, or at least very little increase, in speed.

For LOP ops, there are a couple knots increase ~ 10,000’ that I can document after adding the SF.

Link to comment
Share on other sites

2 hours ago, Shadrach said:

Educate me here.  Wouldn't a higher C/R mean a faster flame front/combustion event? I thought the reason that NA race engine tuners advanced timing at high altitude was to compensate for the lower speed of the combustion event at higher DAs. Seems like an increase in C/R all other things being equal would warrant retarding the timing to achieve MBT.  This comports with my general understanding of tuning forced induction street cars. Yes, retarded timing increases detonation margins but in many cases, power starts to fall off well before detonation.

Ross, I believe that everything you state is correct. My point about the CR was merely that if one extrapolates Nigel’s data to a stock engine there is perhaps a 5 deg (my uneducated guess) correction factor on the timing to correct for the faster burn time of Nigel’s engine.

Both mixture and timing affect BMEP and therefore torque and power. Unfortunately, we don’t know at what mixture Lycoming tuned the timing for. I would guess that it was set for MBT at a fuel air equivalency of about 1.2 which would be what we call “best power” or around 100F ROP but I would love to get confirmation of that.

Skip

  • Thanks 1
Link to comment
Share on other sites

OK, we have....

1) 25° BTDC... produces more power, at the cost of higher CHTs, with less margin on hot days for detonation...

2) 20° BTDC... is a method of lowering CHTs to avoid detonation... at the cost of power desired for T/O and climb...

3) Running 200° ROP is also a method of avoiding detonation... blue/white box method on the EGT gauge...

4) Running higher compression ratios... elevates the risk of detonation... but, detonation is not that close...

5) 25° BTDC with a standard lame magneto spark... is not the same as a higher powered spark that comes with the electronic mag...

6) Simply, the strong spark is like adjusting the timing further away from TDC...  but, by how much?

So... if you use an electronic mag with a good clean spark... keep an eye on CHTs in the T/O and climb... if you decide to go with the 25° BTDC timing...

Keep an eye out for who else has may have this experience... (experimental...?)

 

People avoid detonation.... because it is difficult to detect... it doesn’t always come with odd vibrations... but it can melt piston tops in minutes...

The piston tops are protected from the heat of EGTs... using a very thin boundary layer.   The boundary layer is disrupted by detonation...  compare the melting point of aluminum and the EGTs... 1,220°f for aluminum...  EGT near 1,500°F?

See what other engines are using as well... the Continental uses a number between 20 and 25°F... 22°(?)

When discussing  compression ratios I often invite @Sandman993...

PP thoughts only, not a mechanic...

Best regards,

-a-

Link to comment
Share on other sites

15 minutes ago, PT20J said:

Ross, I believe that everything you state is correct. My point about the CR was merely that if one extrapolates Nigel’s data to a stock engine there is perhaps a 5 deg (my uneducated guess) correction factor on the timing to correct for the faster burn time of Nigel’s engine.

Both mixture and timing affect BMEP and therefore torque and power. Unfortunately, we don’t know at what mixture Lycoming tuned the timing for. I would guess that it was set for MBT at a fuel air equivalency of about 1.2 which would be what we call “best power” or around 100F ROP but I would love to get confirmation of that.

Skip

I misunderstood your comment as suggesting that the higher C/R engine would benefit from more advance than its stock C/R brethren. Misfire on my part.  I too would like to see the Lycoming's original data but my guess looking at Nigel's data is that 100ROP was their target mixture setting.  My next guess is that for certification, 25° was found to be the best combo with consideration to power, temperature and low RPM operations.  The SI you posted baffles me.  I have never found the IO360 to be a challenge to cold start whether timed to 20° or 25° BTDC. I'm sure I wouldn't be able to discern the difference if I was not told.

  • Like 2
Link to comment
Share on other sites

14 minutes ago, carusoam said:

Ross,


Hard to tell during the start up process...

Since the timing is set to TDC on the mag used for starting... for both 20 and 25°BTDC mags...

Yes/no?

:)

Best regards,

-a-

The impulse coupling lag angle is 5 deg less than the advance, so I believe it fires at 5 deg BTDC in either case. 

Skip

  • Like 2
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.