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Optimal ignition timing changing with altitude?

Kalibr

Well Known Member
Based on my search of the archives and my amateurish understanding of how our engines work, the optimal (in terms of power, efficiency and detonation safety margin) ignition timing should be more advanced for LOP v ROP operations, less advanced for high power/high manifold pressure operations. Am I correct thinking that optimal ignition should be more advanced with high altitude operations (all else being equal)?
Because my home base is at 7k agl, I went with higher compression pistons/cylinders cutting slightly into the detonation margin ? not a concern at 7k agl, but could be an issue at sea level if the ignition is advanced too much. Should I be thinking about adjusting the ignition timing based on my operating altitude? Or may I simply rely on the MAP curve built into the electronic ignition I have since the high altitude operations is no different than a lower power operations?
 
Based on my search of the archives and my amateurish understanding of how our engines work, the optimal (in terms of power, efficiency and detonation safety margin) ignition timing should be more advanced for LOP v ROP operations, less advanced for high power/high manifold pressure operations. Am I correct thinking that optimal ignition should be more advanced with high altitude operations (all else being equal)?
Because my home base is at 7k agl, I went with higher compression pistons/cylinders cutting slightly into the detonation margin — not a concern at 7k agl, but could be an issue at sea level if the ignition is advanced too much. Should I be thinking about adjusting the ignition timing based on my operating altitude? Or may I simply rely on the MAP curve built into the electronic ignition I have since the high altitude operations is no different than a lower power operations?

Generally speaking, leaner AF ratio wants more advance, more RPM wants more advance and lower MAP wants more advance. The incremental retarded timing at high power (i.e. higher MAP) is to reduce detonation potential, not to improve performance.

Higher altitudes mean less O2 and therefore less fuel per CF of air for the same mixture. I am unsure how altitude affects advance, but speculate that it has no impact, beyond the reduced MAP and therefore advanced timing.

Larry
 
Study this, made available courtesy of the author:

https://www.danhorton.net/Misc/Nigel Speedy - Ignition Advance .pdf



Not necessarily.

Don't disagree. Trying to keep it simple. Point I was trying to make is that many High performance engine tuning protocols will include some incremental ignition retarding, beyond optimized performance, for the purpose of reducing detonation potential in high MAP situations. At least in engines without knock sensors.
 
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Don't disagree. Trying to keep it simple. Point I was trying to make is that many High performance engine tuning protocols will include some incremental ignition retarding, beyond optimized performance, for the purpose of reducing detonation potential in high MAP situations.

Yes. pulling timing back when rich and low costs you very little power in exchange for more detonation margin. It's a good bargain in my book.
 
Thank you very much, Dan and others!

Here are my conclusions so far:

With my EI, it?s not advisable switching ignition advance maps in flight. And it doesn?t distinguish between ROP or LOP. I could switch the ignition advance maps on the ground, but that will require an additional piece of equipment to make it practical.

Considering the rather small increase in speed and efficiency from ?optimizing? the ignition advance, I think a good, hustle free way to approach it is to pick one advance map that is conservative ? no more than the plated advance for the engine (25 degree for a typical parallel valve lyclone) at high MAP and with the max advance of 7-10 degrees above the base level.

Alternatively, for getting a couple of percentage points of extra efficiency and speed at LOP operation, it might make sense to have a ?ROP? advance map and a ?LOP? advance map. The ?ROP? advance map would be similar to the above mentioned ?conservative? map. The LOP map would have the same base level for high MAP/power operations with a greater max advance (perhaps as far as 15 degrees over the base level. Since it would be prudent to choose the advance map on the ground and not in flight, I would only use the LOP advance map when I would be taking off from an altitude of, let?s say, of 6k DA (where my engine would be making less than 75% rated power) AND I expect to go LOP shortly after takeoff and for the duration of the flight. In all other situations (takeoff/flight at lower altitude ROP or LOP or takeoff/flight at higher altitudes but ROP) I would use the ?ROP? advance map.
 
I did some experiments with changing the timing on my new IO390 recently and here is what I found which jibs down with others who have experimented with this.
For ROP operation and lower altitude (below 8000) I get see the following
Max Advance timing to 30 BDC, highest CHT, Lowest EGT, speed X -- using this as my base line
Max Advance timing to 24.4 BDC, lower CHT, higher EGT, speed X +1 knots
Max Advance timing to 23 BDC, lowest CHT, highest EGT, Speed X ? 2 knots
 
Different results

I did some experiments with changing the timing on my new IO390 recently and here is what I found which jibs down with others who have experimented with this.
For ROP operation and lower altitude (below 8000) I get see the following
Max Advance timing to 30 BDC, highest CHT, Lowest EGT, speed X -- using this as my base line
Max Advance timing to 24.4 BDC, lower CHT, higher EGT, speed X +1 knots
Max Advance timing to 23 BDC, lowest CHT, highest EGT, Speed X – 2 knots

I have resisted entering this thread because my information is for an angle valve IO-390 with standard compression and I believe the OP has a parallel valve with higher than standard compression. Because of that I suspect that my data is of little use to the OP.
With that said, Bavafa and I appear to have reached different conclusions for the standard IO-390 (angle valve). I started at 20BTDC and advanced until I found no speed advantage only additional CHT. For my engine the optimal ROP advance was 23BTDC more advance was both slower and higher CHT. I actually retard my advance, in accordance with Toobuilder, for takeoff power because I found very little power loss and I assume it provides detonation margin. I cannot prove or disprove the additional margin.

2hgh0zl.jpg


The link below shows DanH had information from a dyno run that correlates well with 23BTDC ROP

http://www.vansairforce.com/community/showpost.php?p=1277327&postcount=39

As for the OP, from most of my reading (but no actual testing) the parallel valve does seem to get more advantage out of more advance.
 
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I have resisted entering this thread because my information is for an angle valve IO-390 with standard compression and I believe the OP has a parallel valve with higher than standard compression. Because of that I suspect that my data is of little use to the OP.
With that said, Bavafa and I appear to have reached different conclusions for the standard IO-390 (angle valve). I started at 20BTDC and advanced until I found no speed advantage only additional CHT. For my engine the optimal ROP advance was 23BTDC more advance was both slower and higher CHT. I actually retard my advance, in accordance with Toobuilder, for takeoff power because I found very little power loss and I assume it provides detonation margin. I cannot prove or disprove the additional margin.

2hgh0zl.jpg


The link below shows DanH had information from a dyno run that correlates well with 23BTDC ROP

http://www.vansairforce.com/community/showpost.php?p=1277327&postcount=39

As for the OP, from most of my reading (but no actual testing) the parallel valve does seem to get more advantage out of more advance.

Hi Marvin,
I read your report with great interest and I acknowledge that my tests were not nearly as exhaustive and methodical as yours. I can also not guarantee that my timing is set dead on as +/-1 degree is very possible when you are reference the timing mark to set the timing. So, these results applies to my setup, using PMAG and EIC to control the timing.
With that said, my results were rather consistent. I did each test about 3 or 4 times to rule out any mistake or other variables. The A/P was driving the plane and the change in CHT/EGT was rather rapid and very consistent in temp changes. The change in speed was consistent but would take a bit to catch up. One other change was in oil temp but that would take longer to show itself. The lower advance (23 BDC) showed the lowest oil temp as well as CHT and the highest EGT.
 
I'm a bit confused. The focus here seems to be on speed gains (HP increase), but I thought that the advantage of advancing timing at lower MAP was supposed to be the ability to go more lean (economy, cleaner burn) rather than power increase. This due to the fact that leaner mixtures burn slower, which requires ignition advance to keep the pressure peak in the right spot in crank rotation for max efficiency.

Perhaps some would see this as two sides to the same coin, but I've been working on the assumption that there are at least subtle differences in the goals.

Did either of you try to 'iterate' leaner mixture & timing advance?
 
I'm a bit confused. The focus here seems to be on speed gains (HP increase), but I thought that the advantage of advancing timing at lower MAP was supposed to be the ability to go more lean (economy, cleaner burn) rather than power increase. This due to the fact that leaner mixtures burn slower, which requires ignition advance to keep the pressure peak in the right spot in crank rotation for max efficiency.

Perhaps some would see this as two sides to the same coin, but I've been working on the assumption that there are at least subtle differences in the goals.

Did either of you try to 'iterate' leaner mixture & timing advance?

Optimum performance (i.e. max power output) in a cylinder requires proper ign timing. The optimal target advance is always changing, based upon the factors mentioned and a few others not mentioned. A sub-optimal advance will reduce the power output of the cylinder from it's peak potential. You are correct that a lean mixture requires more advance than a richer mixture to achieve the peak power available. Advance has a very limited impact on how far Lean you can go. However, EI usually has a hotter and longer spark than a mag and can ignite a very lean mixture better than a mag. Therefore allowing an EI engine to go further LOP than a mag engine. This benefit is unrelated to timing. It's important to note that mags vary in spark power, based upon their wear. As the points or cam wear, the egap changes and this reduces the spark energy delivered by the mag.

Larry
 
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It's the mixture setting that defines the power. That's why peak or LOP is considered "economy" operation. That said, the best timing is that which provides the most work for a given mixture. The easiest way to measure "work" is maximum speed. If you wanted to run at 200 LOP (for example), you would still want to adjust timing until you found your max speed at 200LOP. This would be "optimum", but also would be much slower than optimized timing at 50 LOP.

So no, timing is not advanced to "make" the engine run leaner; it's advanced to optimize the lean mixture
 
That's pretty much as I understood it. But the reports I'm seeing seem to say that they set <some> mixture, and at that mixture, they find that advancing the timing doesn't help. What I haven't seen (and I realize that it's hard to really quantify), is details on where the mixture is for the results seen. I suppose that it's at 50* lean of peak, but don't know.

So, if the results being published are all taken at '75% altitude', full throttle and 50* lean of peak, great. But if there are varying MAPs in the various reports, and varying levels of leaning, won't that have a significant effect on the results?
 
I can't talk to the "other" reports, but I can say that the ability to adjust timing "on the fly" is almost exclusively limited to the SDS products. I have that product and have repeatedly accomplished a timing sweep on the same flight, in the same air and can confirm that there is a very noticable power increase with advanced timing compared to ROP timing.
 
I know understand

Hi Marvin,
I read your report with great interest and I acknowledge that my tests were not nearly as exhaustive and methodical as yours. I can also not guarantee that my timing is set dead on as +/-1 degree is very possible when you are reference the timing mark to set the timing. So, these results applies to my setup, using PMAG and EIC to control the timing.
With that said, my results were rather consistent. I did each test about 3 or 4 times to rule out any mistake or other variables. The A/P was driving the plane and the change in CHT/EGT was rather rapid and very consistent in temp changes. The change in speed was consistent but would take a bit to catch up. One other change was in oil temp but that would take longer to show itself. The lower advance (23 BDC) showed the lowest oil temp as well as CHT and the highest EGT.

Hello,
With this additional post I now understand what you were saying. I looked at the data from low to high advance. You posted data from high to low advance, but the data was not linear (i.e. Base, +1, then -2 for the speed). If I take into account that we could have “zeroed” slightly differently we are both saying the best ROP advance is near 23BTDC plus or minus our “zero” point. The SDS system allows for finding TDC with a strobe light and a software advance/retard. That makes finding TDC very accurate. So I did not consider that we had different starting points. Thx
 
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?Optimal ? most desirable or satisfactory?. This is a definition from a dictionary. This terms seems to be quite subjective ? ?optimal? is in the eyes of he beholder.
For me, it is not the max/best power. It is more of the best compromise between safety margin/best power/efficiency/economy within the constraints of my operating environment (high elevation home base with an occasional foray to lower altitudes) and given my equipment setup (higher compression piston/cylinders and EI with a recommendation of not changing timing in flight).
The article Mr. Horton posted was indeed very relevant and helpful to my analysis.
I purposefully didn?t mention any specific product fearing that the thread will quickly gets derailed towards a discussion of advantages/disadvantages of one EI brand versus another ? there are plenty of those threads here already.
 
Yours is a tough question to answer because "optimal" IS subjective. Two nearly universal truths I've found:
1. Best power at LOP is very "peaky", but is unlikely to hurt the engine if even significantly wrong timing is used. Don't worry about playing with the razors edge here.

2. Best power when low and fat is very flat and too advanced timing can induce detonation easily. Be very concerned about playing with the edge.

Because of the radical differences between ROP timing and LOP timing, a single map is going to be a compromise. I'd err on the side of conservatism and make sure the timing can't hurt you when down low and or at rich, detonation prone mixtures. It's safer to give up a few knots when high/lean.

...Or get an ignition that can do both.
 
With this additional post I now understand what you were saying.

I think Mehrdad's data and yours are a match within the scatter.

Yours is a tough question to answer because "optimal" IS subjective.

...and varies with equipment.

For example, you've gathered timing data with a 540 in a Rocket. More advance means higher CHT. Subjectively you balance higher CHT against a speed increase, and find the benefit to outweigh the penalty, as the speed increase is significant. That's cool. It's a Rocket. Speed as a first priority is a reasonable choice.

Assume the same cylinder type and size. Four cylinders in a less slippery airframe will net a smaller speed benefit. At the same time, it will be harder to offset the CHT rise; available dynamic pressure for cooling rises with velocity squared. Just within the RV community the pressure difference is significant; at 8000 feet, moving from 195 to 170 KTAS means 25% less pressure. The speed benefit is smaller and the cooling penalty is larger.
 
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True, but the byproduct of LOP ops is significantly cooler CHT. Even in the RV-8 I struggle to keep the temps warm enough. In this case the added heat from more power (optimum advance) should be easily absorbed by a properly built cooling system.

More heat added to an already taxed system (as found with a climb at VY) is generally a bad thing, but more heat added to an abnormally cool condition like LOP ops is a welcome addition - especially if it comes with a speed increase.
 
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True, but the byproduct of LOP ops is significantly cooler CHT.

A straw man. More advance means higher CHT....an absolute fact. With or without advance, too cool when LOP is an invitation to reduce mass flow and lose some drag.

With the RV-8, same conditions as the Rocket, the speed gain with the best speed advance setting is less, yes?
 
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A straw man. More advance means higher CHT....an absolute fact. With or without advance, too cool when LOP is an invitation to reduce mass flow and lose some drag.

With the RV-8, same conditions as the Rocket, the speed gain with the best speed advance setting is less, yes?

Sure more advance means more CHT, just like more throttle means more CHT. That's not to say more CHT is "bad" so long as the engine is running at best power for conditions. Said another way, yes, it's normal for CHT to plummet when LOP (especially with fixed timing), but that's a result of the limited timing. The too cool CHT is not a design choice, it's an unfortunate result. Even with timing optimized for best power LOP, the resulting CHT is still well under that seen on takeoff. When I activate the LOP advance on my CPI I see a 3 knot increase and a corresponding CHT bump of less than 10 degrees. Even with this bump in CHT, I'm still too cool by 30 degrees in the summer.

As to the debate concerning the merit of reduced cooling for drag reduction or increased power production with proper timing - why not do both? Get the engine performing correctly LOP with optimum timing, and then since it is still going to be too cool anyway, tighten up the cooling airflow.

I am not a proponent of artificially hamstringing the engine with late ignition timing as a cooling strategy.
 
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