SDS CPI Tricks and Tips
As some of you know I am a very happy user of the SDS CPI product. I have several threads on this and other boards documenting the installation, tuning, and general observations of CPI, and I will continue to help out when I can in the future. I also get PM requests for help and am happy to do so. In the spirit of useful information I thought I would throw together a quick list of common CPI errors and installation tips in this thread.
Keep in mind that this thread is intended to be CPI specific, and covers the problems/tricks I have seen. In many cases, these are covered clearly in the manual but consider this a “foot stomp” requiring extra attention.
Magnet position: The CPI needs to see the synchronization magnet before it derives the firing angle. The magnet position IS UNIQUE TO EVERY INDIVIDUAL ENGINE! Despite the fact that there is a beautiful machined billet hall sensor mount and a precision magnet drill position fixture, the relationship between the engine’s TDC, hall sensor and magnet location on the ring carrier is subject to significant variation. There are a lot of machined holes between the engine and CPI components that define this relationship, so that means a potential for lot of “tolerance stackup”. Also keep in mind that even in a perfect world, there will still be a different magnet position between the first and second pickup of the dual hall sensor (because one is further away from TDC than the other), and the single hall sensor, which will roughly split that difference. Yes, the default magnet position is close, and you can quickly adjust the “timing” with the up/down keys to achieve a beautiful idle, but the timing “value” displayed is likely to be off by 5-10 degrees. It will be worthless as a reference. So to foot stomp this one point – The magnet position MUST be set before flight. Fortunately, it’s a very easy process. Set the engine to a convenient mark on the ring carrier (TDC, 10, 20, etc) and place a pointer in any easy to access point on the engine, make a mark on the gear with a piece of tape and a Sharpie corresponding to the pointer, and grab your trusty automotive timing light (remember those?). With the timing light set up on #1, set the idle RPM advance on the CPI window to match your selected timing mark. Assuming you went with 10 degrees advance, set 10 degrees in all RPM ranges up to about 1200, and zero out the MP advance. Select the magnet position window on the CPI and plug in the default shown in the manual (should be 80 – 90), start the engine and aim the timing light at your temporary pointer. If the mark and pointer align, you got lucky and you are done. If not, simply key the up/down arrow a few times until you drive the mark into alignment. You are now done. Look at the value displayed in the window and record it in your logbook, because it’s permanent to your engine combination. Note that if you have a dual hall mount sensor, you will also have to set the other set of pickups too. DO NOT simply duplicate the mag position setting from one CPI into another. Setting the magnet position takes only seconds to do and forms the basis for all other timing, so don’t skip it.
Drilling the ring carrier: Drilling the holes for the magnets is easy with the provided drill guide. So easy, it can be done without removal from the engine in many cases. That said, the old adage to “measure twice and cut once” applies here. It’s a bit hard for some of us to get our head around the relationship between the sync and trigger magnets and their location around the ring carrier, and I’m not the only one who has a few “extra” holes… My advice to all of you out there is to slow down, study the installation manual, and when you think you are ready to drill, STOP, and use a Sharpie instead. “Drill” and label your holes with a Sharpie and then go to bed. The next day, compare your artwork with the illustration in the manual with a fresh set of eyes. If it matches, fire up the drill and go for it.
Ignition tuning: Set with the defaults, the CPI provides all the performance benefits of any electronic ignition and works beautifully. However, unlike the other offerings, CPI offers essentially infinite adjustment of timing so that you can tailor it to YOUR airplane and YOUR mission. Despite the myriad of adjustments, you do not have to be a test pilot to find the optimum setting. I have found that one of the clever features of the CPI has an unintended benefit – a way to safely “backdoor” test settings. The LOP feature instantly adds a user definable advance value with the flip of a switch, which also means that you can instantly REMOVE this value if something doesn’t go the way you like. This provides a very safe way to sneak up on an advance setting in flight because it gives you an instant return to a known, safe condition.
To illustrate a very conservative approach to finding a “ROP, cross country cruise” (for example) setting: On the ground, set up the RPM advance to max out at your data plate setting (25 degrees, for example), and zero out (disable) any MP advance. Now you are flying with your engine manufacturers known “safe” condition and can play from that position of certainty. Climb to altitude (7500, for example) and set power, mixture, and let it stabilize at speed. Once satisfied with your “normal” speed, record that value, and all other pertinent engine values, as well as your timing as shown on the CPI window (should be 25, in this example). With data recorded, select the LOP window on each CPI and set it to “1” (degree). Now flip the LOP switch and see what happens. If everything looks good, let that setting stabilize (at least 5 minutes) and record the timing (now 26), and the speed. If you want, grab the temps too. With the switch still active, simply select one more degree (now “2”), let it stabilize, record, and repeat in 1 degree increments until you see the speed peak, then decline. The total advance setting that corresponds to peak speed is now your target for your yet to be built “final” advance curve. What this is doing is finding optimum engine output for that mixture, tailored to YOUR airframe, engine and prop combination. Once your final advance curve is built and you once again are cross country, high and ROP, you can use the same technique to find your “LOP” setting. The only difference is that you will now be at a RPM and MP derived “optimum” advance (let’s say it’s 31 degrees) with the LOP switch INACTIVE. You will pull the mixture to your typical LOP setting, the speed will sag a bit (as usual) and the FF will plummet. Once again, select the LOP window on each CPI and input 1 degree advance. Record your (stabilized) speed and then activate the LOP switch. You will see a bit of the speed come back shortly, so as before, add advance and find the peak speed. The optimum advance setting you find is now your “permanent” LOP switch setting. In my case, my Rocket regains 3 knots with the flip of that switch when LOP.
That’s all I have for now. If I (or anyone else) thinks of any more they should drop it here. CPI is one of the newest ignition products on the block so the pool of knowledge is not as deep as others. Hopefully, this thread will serve as a resource for current and future CPI cusomers.
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.
Harmon Rocket II -SDS EFI
1940 Taylorcraft BL-65
1984 L39C - SOLD
RV-8 - SDS CPI - SOLD
Last edited by Toobuilder : 03-23-2017 at 12:34 PM.