It’s been a long time since I started this thread and I have learned a bunch. That, coupled with the many others who have become avid SDS users means that it’s time to update this thread with more refined information. I hope the other users will join in and add their experience.
Since starting this thread I am now a full SDS EFI user and some of that journey crosses over to the “ignition only” product. What follows is “ignition related” unless otherwise identified. Also note that it has become apparent that effective tuning of this system is dependent on many factors: 6 vs. 4 cylinders, PV vs AV, or C/S vs. FP play a role in the numbers you see. Bottom line, don’t use my methodology as a recipe unless you have the similar engine configuration to me. The methodology should provide some guidance however. That said, The following has been effective on my stock compression PV 540 with a “heavy” Hartzel metal C/S prop as installed on my Rocket.
Also noteworthy is the fact that the SDS product uses RPM and MAP as tuning variables. I depart from Ross’ normal guidance in the sense that I use the MAP signal as the primary logic for engine demand. The RPM tuning in my case is a fairly simple linear slope that provides a conservative baseline that can be further tweaked with MAP settings. This is because with a C/S prop and the resulting narrow RPM range I fly, the MAP is the most dynamic and responsive to my needs. Recent experience helping a new SDS customer set up his fixed pitch Cozy illustrated some of the differences in my technique compared to what ultimately worked for him. It was also a clear reinforcement that a one size fits all ignition product generally leaves a lot of performance on the table for most.
All that said, let me cover a few of the “optimized” points on the curve that I did not address early in the thread.
IDLE TIMING: Anybody remember twisting the distributor on an old car to set the base timing? The vacuum advance hose was disconnected and the engine was running without benefit of the MAP advance component. Remember how labored the engine sounded and how much it perked up as soon as you hooked up the vacuum hose? Well, with SDS you can find your engines “happy place” right in the chocks with a little thought and a few keystrokes. Using the LOP (or “flight test switch” as I like to call it), with the engine warmed up, note the MAP and RPM of your engine. Zero out the LOP value, select the LOP to active, and press the up arrow to add timing. In most cases, the engine will respond favorably to advance and continue to add RPM and smooth out until it peaks and begins to decline. The peak of RPM and/or smoothness is your engines “happy place”. Note that advance value and edit your curve to apply that timing to that RPM and MAP. While editing, Smoothly ramp down the MAP advance to accommodate advancing the throttle and make sure it is all removed with any MAP above that required to taxi. Congratulations, You have now optimized your curve to the idle region. Took all of 5 minutes. Note that it will likely run so well that you will have to go back and adjust the throttle blade stop screw to pull the RPM back to your normal level. Expect the timing adjustment to add 100+ RPM to your normal value.
TAKE OFF POWER: High MAP at takeoff power is one of the more challenging areas of engine performance testing. Even with 100LL and well performing subsystems, some of us struggle with detonation and high CHT’s. This is in no small part to the fine balance that the timing of legacy magneto ignition systems had to contend with. With a fixed system, one had to strike the balance between too much advance at takeoff power and not enough when high and lean. That’s a tall order when the timing is fixed. But is the “data plate“ value optimum for takeoff power? Not always, as it turns out. Not a lot of experimentation in this area so when I had my 540 on the dyno I decided to try get some data. I published a thread on this site describing the process but to save you reading it, it turns out the PV Lycoming is largely insensitive to timing when this rich. My aim in the dyno test was not to see where peak timing occurred, but rather, how much timing could be removed before power fell off. Turns out, that value is A WHOLE BUNCH! So in my case, since I am primarily interested in auto gas use and therefore detonation prevention as the main consideration, I knowingly edited my ignition to remove a bunch of ignition advance in those MAP values associated with TO power. As a reference, I pulled 7 degrees OUT of the data plate baseline of 25 degrees. Yes, at a sea level airport, I’m only showing 18 degrees advance on my takeoff roll and initial climb. Dyno testing shows this value is only costing me a couple of HP (out of 336) but if you think that hurts performance in my airplane, I’m happy to take you for a ride. And obviously, my CHT’s are no problem - ever.
Those of you testing at home, there’s a couple of options. Start pulling 1 degree out and measure TO distance until it becomes obvious that the performance is declining faster than the CHT/detonation margin benefit, or just remove a few degrees and forget about the raw numbers.
MAP SENSOR FAILURE PROTECTION: Thanks to a report from another member concerning the failure of another brand ignition system, we all are aware of a previously unknown failure mode. Thanks to the flexibility of SDS, we can mitigate that failure mode through discrete programming. SDS, like several other EI vendors uses a GM style MAP sensor. This is a commodity part and has billions of hours of history - but they can fail. And the failure mode of this device is that it “tells” the EI brain that the MAP is zero at failure. In response to this low MAP signal, the brain box applies full advance (let’s call it 15 degrees) to the top of whatever RPM timing exists (Let’s say 25 degrees). And if you are on TO roll and the RPM timing is at 25 and the failed MAP sensor calls for 15 more, then you are lifting off at 40 degrees of timing. That’s a bad day.
So the mitigation for this failure is simply to navigate to the lowest 4 or 5 “slots” on the MAP page and zero out the advance. Typically, these settings are well below the MAP you will see at any point with the engine running, so they are effectively hidden from the useful part of the map. However, IF the MAP sensor fails it will ask for those very positions and the brain will deliver ZERO ADDITIONAL advance to the existing RPM timing. In the takeoff scenario above, the failed MAP would result in a timing of 25 at takeoff. No problem, thanks to a few keystrokes.