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  #1  
Old 04-26-2016, 06:35 PM
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nigelspeedy nigelspeedy is offline
 
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Default Effect of Ignition Timing on TAS/EGT/CHT when WOT & LOP

I am trying to set the record for the most TLA in a thread title.

I have dual PMags and an EI Commander. Today I did a cheap and cheerful test to look at the effect of ignition timing in the cruise. All testing was done at 10,000' PA, 5 deg C OAT, WOT, Prop 2500 RPM and leaned to 25F LOP. After this the only thing I changed was the maximum allowed advance.

The way the engine has been configured to date is Max RPM 3072, Max Advance 35 deg, 0 deg advance shift.


I made a change to max advance (reduced in 2.8 deg steps) and sent the change to both PMag simultaneously. My engine has been happy with 36 deg advance till now so I was not concerned about sending it less advance.

The actual advance reported is always 1.4 deg more than the Max Advance you program in the EI Commander map. What I show below is the advance as displayed by the EI Commander, which is 1.4 deg more than I asked for.

At the very left of the picture you see the fuel flow reducing as I lean the engine. From this point on throttle prop and mixture are fixed and all changes are a function of advance only.

Effect of ignition timing, reading left to right, the step changes in EGT on the top graph are where the timing changes are made.

36.4 deg, 182 KTAS, time 28 - 31
33.6 deg, 180 KTAS, time 31 - 34:30
30.8 deg, 176 KTAS, time 34:30 - 38
28.0 deg, 173 KTAS, time 38 - 42
25.2 deg, 171 KTAS, time 42 - 44:30
22.4 deg, 170 KTAS, time 44:30 - 48
36.4 deg, 182 KTAS, time 48 -

Timing is then reset back to 36.4 deg and this is shown where the EGT rapidly drop again on the right hand side of the graph.

The CHT's on my aircraft always take about 5 minutes to stabilize after a power/mixture change and I did not wait that long, so the exact effect on CHT could be a little different to what I summarize below.

In general as I reduce advance from 36.4 deg I get the following changes:

KTAS reduces by ~ 1 KTAS / deg advance
EGT increases by ~ 10 F / deg advance
CHT reduces by ~1 F / deg advance

If you get the ignition timing badly wrong it can hurt the engine. If you feel like experimenting I suggest you buy a bore scope before hand and take photos of all your pistons and valves so you have a baseline. This way if anything goes wrong you will be able to make a better determination of potential damage done. Do any timing changes high enough and close enough to a nice long runway so that you can easily conduct a power off landing. Make sure you are proficient in power off landings. Educate yourself as to the likely/expected outcomes of any ignition timing change so you can judge if the outcome of a change is normal. Be proficient in the use of the device that you are using to make changes so that you don't spend too much time eyes inside. Try to preserve a proven map/setting that you can return to quickly if the one you just tried turns out to be inappropriate. Although your engine might be broadly similar to mine there are no doubt subtle differences, so what works for mine might kill yours. The data shown is just meant to be one of many ways to do an ignition timing test, it is not a suggested setting for your engine.

[IMG][/IMG]

Cheers
Nige
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Last edited by nigelspeedy : 04-26-2016 at 06:39 PM. Reason: add time stamp data to go with ignition settings
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Old 04-26-2016, 06:46 PM
lr172 lr172 is offline
 
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Your results are as expected. As timing advance increases towards optimum, more of the energy (i.e. Heat) remains in The chamber doing work instead of going out the exhaust. This reduces eGT and increases CHT. The increased work increases HP and ultimately TAS.
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Last edited by lr172 : 04-26-2016 at 06:53 PM.
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Old 04-26-2016, 06:53 PM
Bavafa Bavafa is offline
 
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Quote:
Originally Posted by nigelspeedy View Post
The actual advance reported is always 1.4 deg more than the Max Advance you program in the EI Commander map. What I show below is the advance as displayed by the EI Commander, which is 1.4 deg more than I asked for.
Awesome info, many thanks for it. Would you also have the fuel flow for each of those settings to see who the speed reduction is reflected in the fuel consumption?
I have recently got a second PMAG and EIC but still working out some bugs. One thing that I saw after installing the second PMAG and now both on version 40 and timed right at TDC, I noticed higher CHT as well as EGT. I programmed my EIC to reduce the timing it by -1.4 (not max advanced) and that seem to have helped.

Can you please elaborate on the above statement as why that is?
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Last edited by Bavafa : 04-26-2016 at 06:56 PM.
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Old 04-26-2016, 07:16 PM
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Nigel, this is a 180 horse parallel valve?
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Old 04-26-2016, 08:54 PM
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nigelspeedy nigelspeedy is offline
 
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Default Fuel Flow

The Fuel Flow was constant at 8.4 gph, shown as the slightly noisy but essentially flat blue line on the top graph, scale on the right hand side. All changes are due to timing only.

Putting in a - offset reduces the advance across the whole map. Think of offset as sliding the sloping line left/right. The max advance sets how high the slope can go. Unfortunately you cant change the slope though.

The engine is a parallel valve IO-360 stroked to 380, 10:1 compression, dual PMags, and the heads were ported/polished. Really feels as though it likes a good bit of advance. Down around 25 deg it just feels sluggish and subjectively feels ever so slightly less smooth but I don't have any data to support that.

I also did a test on the advance on idle quality but I haven't crunched that yet.

Cheers

Nige
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Old 04-26-2016, 08:55 PM
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Default Timing Offset Effect

By changing the offset in a negative way you have less advance. I.e. the spark happens latter, so peak pressure happens further after TDC. Less work inside the cylinders so lower CHT, more wasted out the exhaust so higher EGT.

Was your idle smoother?

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Nige
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Old 04-26-2016, 09:51 PM
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Interesting data. With this and what TooBuilder published when testing timing on his Rocket a couple months back, I hope people can now see the real benefits of EIs with adjustable timing curves compared to running 24-25 degrees fixed timing across the board.

Thanks for posting!

Our customers are seeing a similar sweet spot at altitude with similar engine specs at around 35 degrees true timing.
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Old 04-26-2016, 11:20 PM
Bavafa Bavafa is offline
 
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Quote:
Originally Posted by nigelspeedy View Post
By changing the offset in a negative way you have less advance. I.e. the spark happens latter, so peak pressure happens further after TDC. Less work inside the cylinders so lower CHT, more wasted out the exhaust so higher EGT.

Was your idle smoother?

Cheers

Nige
I could not detect any effect on my idle and/or smoothness. Prior to V40, I had set my timing a couple of degrees aft TDC and this was just to duplicate that. I was really surprised that both my CHT and EGT were higher, compared to the past, without the -1.4 degree (A Curve) and set at TDC. Also, the way I understood the EIC, which I still need to read more on it, setting this way does not effect the max advance. Feel free to set me right here
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  #9  
Old 04-27-2016, 01:37 AM
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nigelspeedy nigelspeedy is offline
 
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Default V 40 changes

My understanding is that one of the changes in the V40 software update to the PMags was a retard to timing when the engine was at very low RPM when being turned over on the starter motor. So with V40 the plugs should fire a few (unsure how many, but I think about 4 deg) deg ATDC to avoid any issues of kickback with a light weight flywheel and/or prop. Once the engine is running faster than a couple of hundred RPM (again unsure exactly how fast) the timing then transitions to BTD and at idle speed gives about 19.6 deg BTDC.

Putting in an advance shift indirectly affects the maximum advance under some circumstances. For example if you wanted to allow 40 degrees of maximum advance but you also put in -10 deg of advance shift you may never be able to get to a MAP & RPM condition that would provide 40 deg total. This is because you can't adjust the sensitivity of the PMag advance curve to MAP & RPM, or put another way you can't adjust the steepness of the slope of the advance curve. If you have a maximum advance set, changing the timing shift will not result in an exceedance of the maximum you have set, but a negative offset may result in never being able to reach the maximum.

My engine with 0 deg timing shift idles at 700 RPM and 23.8 deg advance. If I put in a -1.4 deg shift the idle quality improves with 21.0 deg advance. The down side is that inflight at altitude I can't get as much as 36 deg maximum advance which the engine likes for best LOP cruise performance. So given that it spends much more time cruising than idling I may have to sacrifice a little smoothness at idle to allow the PMags to give the large advance necessary for best cruise performance. The ability to fully control the timing curve would be an advantage over the current PMag with EI Commander configuration.

Cheers

Nige
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  #10  
Old 04-27-2016, 06:40 AM
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I am impressed, thanks for posting.
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