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Question on LS+Slick to Dual PMAGS

Airzen

Well Known Member
Hi All,

On my new to me RV-10, the ignition system consists of a Slick mag together with a Lightspeed Plasma III.

I have been fairly happy with dual PMAGS on the 8 that I built a few years ago and now considering PMAGS on the 10 as well. I am considering the following options:

1) Replace the slick mag with one PMAG
2) replace both with dual PMAGs
3) Do nothing
4) Consider other electronic ignition choices

I am inclined to rule out #4 as it'll require far more work and downtime when compared to other three.

#3 is not really an option as the Slick will approach 500 hrs soon - so something has to be done.

I'd love to hear from folks who have faced similar dilemma. LS is working fine on the airplane - but there is a desire to keep things similar/uniform across the two ships.

Thoughts/suggestions?

Ashish
 
Option #1 for now...

When my Slick needed removal to comply with their SB I decided to remove and replace it with a Pmag. At the time the Slick still had about 150 hours left before the 500 hour overhaul. So now I run a Pmag on the left and a Plasma III on the right. So far I have about 50 hours on that combination and it works great on my IO360. I figure when the Lightspeed starts having issues I’ll replace it with a second Pmag. IMHO you can’t beat the ease of installation with a Pmag and the weight savings over a Slick.
 
If your LS system has the equipment on the nose of the engine today, then adding a SDS CPI2 system won't take any more time to install. You will be in the area anyway (to remove the LS stuff). Installation time aside, the CPI2 will give you far more flexibility in operation than the Pmag.

You asked for opinions - mine is to go with CPI2.
 
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+1 for Toolbuilder comment

I have two Pmags and in my opinion, the only reason to use them is ease of installation to replace a mag. That said, you probably need new plug wires. I cannot imagine anything more reliable and trouble free than crank triggered ignition that you define the advance curve.
 
I know of one plane running one Lightspeed and one pMag. They play well together. The owner started off with two Lightspeeds but when he had a series of issues with one box he replaced it with a pMag. His plan is to replace the remaining Lightspeed when it dies with a second pMag.

Considering you have a flying plane, here is a similar approach:
- Fly as is, when it comes time for mag work, replace with a pMag.
- Fly like that until the Lightspeed becomes a problem - then replace with a pMag.

For the six cylinder pMag you set the bottom and top of the timing curve during installation. Assuming you have a stock IO-540 engine I recommend 25 BTDC for the bottom, and 34 BTDC for the top. This is what is flying on my old RV-10 and it has demonstrated significant efficiency gains over the original mag install.

Carl
 
25 degrees for the bottom is much higher than required for TO power. I run 18 degrees on mine (8.5 CR). I also run 31 degrees at idle, 29 in cruise ROP, and 33 LOP.

It's a complex curve that's easy to program if the system is flexible enough to support it. Not sure the 6banger Pmag can deliver a curve like that - the 4 banger version sure can't.
 
25 degrees for the bottom is much higher than required for TO power. I run 18 degrees on mine (8.5 CR). I also run 31 degrees at idle, 29 in cruise ROP, and 33 LOP.

It's a complex curve that's easy to program if the system is flexible enough to support it. Not sure the 6banger Pmag can deliver a curve like that - the 4 banger version sure can't.

I would not recommend running an engine at take off power at anything other than data plate timing.

Your timing curve is essentially the same as mine, and after 1000+ hours on dual pMags I can report that it matches my flying just fine.

Carl
 
I'm installing EFI/electronic ignition on my O-320. I'd be VERY interested to learn more about that complex curve.

Indeed, it will be easy to implement, but I've got no idea as to what to aim for with this curve. Michael, would you care to elaborate on this a bit more?

Thank you!!
 
I'm installing EFI/electronic ignition on my O-320. I'd be VERY interested to learn more about that complex curve.

Indeed, it will be easy to implement, but I've got no idea as to what to aim for with this curve. Michael, would you care to elaborate on this a bit more?

Thank you!!

The following will not be a “tuning guide” but does illustrate some truths about the Lycoming engine behavior and just how much conflict the ignition system has to deal with to be effective:

First off, Lycomings respond very well to an ignition advance well above the data plate value FOR THE IDLE CONDITION. Anyone with an SDS system can verify this on their very next engine run. Just start the engine, let it warm up, and start punching up the advance. Every keystroke will see the engine smooth out, probably speed up, and sound “happier”. There is a limit of course, and where the “right” advance number lands is based upon prop inertia, compression ratio, number of cylinders, fuel mixture, combustion chamber configuration, and probably the phase of the moon...

Next up is the fact that at 100% power, the timing is fairly insignificant from a power production standpoint – “close enough” actually works fairly well as it turns out. I have performed this experiment in the air as well as on a dyno. Yes, the data plate value of 25 delivered peak power, but retarding the timing for takeoff by 6 degrees resulted in only a 1.2% reduction in power in a recent dyno run.

https://vansairforce.com/community/showthread.php?t=182190&highlight=dyno

Why take even the slight reduction in power? Because it buys me a significant detonation margin and reduced CHT in climb. I’m interested in detonation margin because I intend to run auto gas in my airplane as a primary fuel. I’ve recently demonstrated that capability. I also have a neighbor who has a high compression engine in his Rocket and struggled with an occasional “rattle” on TO even with 100LL. After my dyno experience, he’s pulled the timing way back on his SDS CPI for TO power and it transformed the airplane. No more rattle, and climb CHT is way down – all with no perceptible reduction in performance.

Finally, you have the substantial difference in cruise power “optimized” timing for ROP vs LOP ops. Even at the same MP and RPM, the ignition advance requirements are different. As soon as you decide to go LOP, you MUST make a timing shift to stay in the sweet spot.

So the challenge for an ignition system is to satisfy the conflict between delivering a huge advance at idle, an equally huge retard at TO, then the “optimum” curve at altitude for best power, then again “optimum” for LOP. With the legacy ignition products discussed in this thread (LS and Pmag), you can “shift” the existing curve to cover some of the desired end points, but you can’t get them all in the box like you can with SDS. To some people, that level of optimization is irrelevant – and that’s fine. But the fact remains that this capability DOES exist, the benefits are real, and it is a product discriminator for SDS.
 
Thanks Michael,

So for a ROP setup, a good starting point would be your settings of

Idle at 31 degrees. That is: Low MAP and low RPM

Cruise (ROP) 29 degrees. Meaning MAP between 16 and 24 inches and RPM anywhere up to 2400 or so

Climb: 20 degrees. However with my fixed pitch Catto not a whole lot of difference between lower altitude cruise and higher altitude climb, and yet a significantly different timing requirement. Difference will probably be most notable in the RPM - MAP of 24" with 2600+ rpm means climb at higher altitude, while at 24"/2400 rpm it appears I'm low down and in cruise.

This is going to be an interesting curve to put together! Looking forward to this one...
 
Please don't take my earlier post as any form of checklist or path to build your curve. I posted that info to illustrate that an optimized ignition is much more than a simple curve, and it is rarely interchangeable between engines. Your 320 has a shorter stroke, less cylinders, and a much lighter prop than the engine in my Rocket. You will need to test to see what works best for your engine. But yes, you appear to have the jist of it.

Most conservative approach is to define the operating environment you are trying to optimize and then test those in isolation (idle, for starters). Once you have all the corners of the envelope figured out, stitch them all together to build your curve.
 
Obviously, yes.

I'll start with the data plate timing and work my way up from there. It's good to know a general direction to go in though.

One thing: With just a fixed pitch prop to work with, it will be pretty much impossible to emulate cruise power settings. Or I must find myself an autumn gale force storm to point the nose of the aircraft into :rolleyes:

I guess I'll be making a good number of short hops between timing trials...
 
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