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  #1  
Old 02-27-2020, 10:18 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,051
Default CPI timing optimization ?

I have two CPI units factory programmed for conservative settings. When it is time to optimize them per Toolbuilder's CHT/MP observation protocol, is it OK to make changes on one unit in flight, and then modify the second unit to match it?
My reason to do this is space limitations suggest a temporary mounting to access the CPU for experimentation, and then permanently mount it with the second unit in the avionics bay . I don't anticipate fiddling with the units once they are reasonably and conservatively set for my engine and fuel preferences.
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Larry DeCamp
RV-3B flying w/7:1 0320 / carb / Pmags / Catto 3b / digital steam
RV-4 fastback w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #2  
Old 02-27-2020, 10:32 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by Larry DeCamp View Post
I have two CPI units factory programmed for conservative settings. When it is time to optimize them per Toolbuilder's CHT/MP observation protocol, is it OK to make changes on one unit in flight, and then modify the second unit to match it?
My reason to do this is space limitations suggest a temporary mounting to access the CPU for experimentation, and then permanently mount it with the second unit in the avionics bay . I don't anticipate fiddling with the units once they are reasonably and conservatively set for my engine and fuel preferences.
Not really. The closer you get the peak pressure to occur at the optimum angle , the more optimized the power output is. Peak pressure angle is modified via the combination of both sparkplugs firing position, relative to TDC.

You can leave one ignition fixed a some timing and only modify/optimize the other, but you can't modify one and then apply those settings to the other. The results would change from the experimentation you conducted.

Generally on a twin plug set up, you want both plugs to be advanced the same. Two equal flame fronts meeting in the center is optimal for many unformly shaped cylinder combustion chamber, like the lyc. That said, it's doubtful you would notice any significant difference in performance if they are different by a few degrees.

Larry
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Last edited by lr172 : 02-27-2020 at 10:43 AM.
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  #3  
Old 02-27-2020, 01:22 PM
rv6ejguy's Avatar
rv6ejguy rv6ejguy is offline
 
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Location: Calgary, Canada
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Larry,

If you have a parallel valve engine, we did a bunch of flight testing recently on Les Kearney's RV-10 with an EM-5 system looking at timing vs. mixture vs. TAS which might give you some ideas: https://www.youtube.com/watch?v=7YvXmq7vwzY

The easiest way to find optimal timing with a CPI is to just reset the LOP advance amount in both units, flick the switch and watch the TAS.

We had perfectly smooth air and the autopilot holding altitude for us.

Optimal timing changes noticeably with mixture, especially LOP.

ROP at high MAP, somewhere around 24-25 degrees seems good. At lower MAP (15-20 inches) maybe 1-2 degrees more than that. LOP, 30-32 depending on MAP and how far LOP you are.

We were up at 16,000 for this test so MAP was fairly low.

There is another video where we went to FL200 and did some testing too: https://www.youtube.com/watch?v=FgXH-1VJl44
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 445.2 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
https://www.youtube.com/channel/UCiy...g2GvQfelECCGoQ


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  #4  
Old 02-27-2020, 02:18 PM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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I installed the CPI unit in Bill Beatons HRII with the cabling & vac line long enough so he could hold the control head in his lap while programming. After which, the unit was just velcro'd out of the way when programming was complete.

I installed another CPI in a RV-7A that hinged from the bottom edge of the instrument panel. When programming was complete, the unit rotated forward and latched out of sight, out of mind.

I would suggest you set both CPIs to engine factory set (25 adv. or what ever) for initial runs and flights to confirm proper operation before introducing custom curves.

If you make a modification to one unit, you have to make the same to the other. Remember the engine will be running on the unit that sparks first (most advance), the second unit would just be along for the ride at that point. Ross's suggestion of using the LOP switch setting to progressively test advance settings is a good way to manage your progressive customization.
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  #5  
Old 02-27-2020, 04:01 PM
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Toobuilder Toobuilder is offline
 
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Its been said before but bears repeating. You are looking for the "optimal" PCP/crank angle. This will show up as the best speed (most mechanical work available). The PCP/crank angle is a result of the initiation of the combustion event. The timing of the combustion event in a dual plug ignition is a composite of the timing of the two plugs. If one plug is later than optimal, it drags the composite to retarded; if one is early, it drags the composite to advanced.

So the short answer is no, you cant optimize the advance on one system and apply that setting to the other system without over advancing the resulting composite.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #6  
Old 02-27-2020, 04:31 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
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Default Thank much to all for input.

I was just hoping to avoid longer wires and hoses. I have the hinged panel under mountings fabricated and thought these responses would be informative to others as well. The switch is a great idea to safely sneak up on optimum.
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Larry DeCamp
RV-3B flying w/7:1 0320 / carb / Pmags / Catto 3b / digital steam
RV-4 fastback w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #7  
Old 02-27-2020, 04:59 PM
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Toobuilder Toobuilder is offline
 
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Given your circumstances Larry, I'd consider locking one ignition out at 25 degrees (or whatever your data plate value is), then play with one to arrive at your optimal value. Once found (let's say it's 34 degrees), remove 4, then apply that to both ignitions. If your engine likes 34 on a single, then it should be pretty close on 30 with both. It's not ideal, but it will quickly get you in the ballpark and will save you many steps of iteration.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #8  
Old 02-27-2020, 08:14 PM
rwtalbot rwtalbot is offline
 
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Quote:
Originally Posted by rv6ejguy View Post
Larry,

If you have a parallel valve engine, we did a bunch of flight testing recently on Les Kearney's RV-10 with an EM-5 system looking at timing vs. mixture vs. TAS which might give you some ideas: https://www.youtube.com/watch?v=7YvXmq7vwzY

ROP at high MAP, somewhere around 24-25 degrees seems good. At lower MAP (15-20 inches) maybe 1-2 degrees more than that. LOP, 30-32 depending on MAP and how far LOP you are.
G'day Ross, I enjoyed watching the video, but one observation I made is that just about every ignition system on the market would probably been running maximum advance (~14-18*) at 17 In Hg. Your testing seems to indicate that about 5 degrees advance is optimal for that particular aircraft.
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  #9  
Old 02-27-2020, 09:28 PM
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rv6ejguy rv6ejguy is offline
 
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Quote:
Originally Posted by rwtalbot View Post
G'day Ross, I enjoyed watching the video, but one observation I made is that just about every ignition system on the market would probably been running maximum advance (~14-18*) at 17 In Hg. Your testing seems to indicate that about 5 degrees advance is optimal for that particular aircraft.
If you're talking total advance, no.

No Lycoming will be making any power at that figure. If you're talking MAP advance plus base RPM (25 + 17= 42 total), this would be WAY too much and lead to a loss of power and high CHTs not to mention likely detonation.

Several of the popular systems running canned curves run too much advance to be optimal and we often see reports of high CHTs..

You saw in the video, there was no speed gain advancing to 32 total at 16,000, even LOP. Running ROP, flame speed is higher, requiring less advance as the video explained.

The optimal ignition timing doesn't vary with airframe type, only the engine type. AV engines need even less total timing than this 9 to 1 PV engine.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 445.2 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
https://www.youtube.com/channel/UCiy...g2GvQfelECCGoQ


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  #10  
Old 03-08-2020, 04:34 AM
rwtalbot rwtalbot is offline
 
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Quote:
Originally Posted by rv6ejguy View Post
If you're talking total advance, no.

Several of the popular systems running canned curves run too much advance to be optimal and we often see reports of high CHTs.
Yes its interesting. The "standard" approach, common across most of the vendors I have looked at seems to be to start at about 25-24 inches MP and advance 2 degrees over base engine timing for every inch of MP reduction until they reach 14-18 degrees.

The same basic approach was suggested by Nigel Speedy in his May 17 KITPLANES article.

Each manufacturer provides just enough of an "artist's impression" of their curve that you can almost figure out how they do it.

The maximum advance would have to be a function of RPM as well, so one would assume you would need to be turning 2700 RPM and very low MP to ever see maximum advance in practice.

I don't know, but I suspect no one has really run an engine on a dyno to optimise too much. To me that's part of the appeal of the SDS system. To be able to start with a known good setup and optimise the 2 or 3 common power settings and cruise heights I fly at. Even better if it comes preoptimized for each engine.
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