Long story, so here goes: My plane has a 24V battery in the tailcone, and the Surefly has to be connected to the battery side of the battery contactor for the STC to be valid for the installation. (This is because if the battery contactor fails and you lose the power bus, the Surefly still runs the engine off the battery) As 24V AGM batteries are in the $700 class nowadays and there is a (albeit small) leakage current on the Surefly when the engine isnt running, and leaking current kills batteries, I heeded the advice from Skip Koss from Concorde and put in a fuse holder in the instrument panel that would let me disconnect the Surefly power from within the cabin to shut off the power when I wasnt flying. The trouble is that would leave an unfused length of wire from the battery to the instrument panel. So to ameliorate that, I put a 15 A fuse in the fuse holder in the tail and left the 10A fuse in the panel. The Surefly was then installed at the annual, and I left in the original plugs and harness, even though Surefly recommends replacing both the plugs and harness when installing the Surefly.. Ran great, seemed to climb better, and I definitely got book speeds in no-wind conditions in cruise. But there was an oddity: during climb between 8500 to 10,500 the engine would balk unless I used full rich mixture. Huh. Weird. Well, they DID say to change the plugs and the harness. So I changed the plugs to an New-Old-Stock of Autolite UREM38Es I bought 16 years ago when they were $9 each. Well, that made matters worse. Now the engine was skipping randomly, losing 100 to 150 RPM, then surging. Happened in climb, cruise, and descent, once even when I was on takeoff and only 50 feet off the ground. Very sphincter-puckering moment, there. Egads, was it the plugs? Harness? I thought maybe there was some momentary bridging of the electrode gap, so I widened the plug gap. For one flight that seemed to work fine, but the intermittency came back a week later. Gads. So I changed the harness with a new Champion (the old one was the Aero-Lite that I put on 9 years ago) Nope, same problem where a full-power runup on the Surefly magneto alone would have a 200 to 300 RPM drop in the runup, then seem to work fine, and Id see 100 RPM drops in cruise when both mags were on, then surge again when the ignition kicked back on. An in-flight mag check saw a 200 to 300 RPM drop on the Surefly alone, and running fine (no RPM drop) on the right magneto. On two flights I had 6 misfires between initial climb and turning to the downwind leg, so I just came back to the airport on the right mag. While all this was going on I called the Surefly tech line many times, and Bill and Tom were paragons of patience and good advice. I was all set to pull the mag and send it back to verify it was working, and Bill convinced me to do a pull test on all the electrical connections from the battery to the Surefly, check the engine ground strap, and to check the P-lead grounding on all ignition switch positions with an ohmmeter. (The Surefly installation LED blinks the code for the advance timing settings whenever power is interrupted, so if an intermittent occurs in the power wire, it would show up on the LED). So I removed the 10A fuse from the instrument panel, moved it to the fuse in the tail cone, and re-did all the crimp connections for the power wire, then yanked each lead while watching the LED. Another odd thing happened when I pulled the 15A fuse out of the inline fuse holder in the tail cone, namely, the fuse just dribbled out, Normally theres a spring that holds a small preload force on the fuse to ensure contact, but it seemed like there was little or no force on the fuse when I pulled the 15A fuse out. There WAS spring force on it when I reinstalled the 10A fuse, so its possible the brad inside the fuse holder cocked a bit when I put in the 15A fuse just enough to keep the preload off of the fuse, thus making and intermittent/poor electrical connection. Subsequent pull test on the wires never got the LED to blink, so that confirmed the power was rock-solid to the Surefly. The first runup had a bit of a stumble, but seemed to clear up with a high speed runup (like a typical fouled plug) and next test flight ran perfectly. The hiccup where I needed full rich mixture from 8500 to 10,500 was now gone. and I got up to 12,500 with relative ease. So, upshot after this long story: Yes, it works. According to Bill at Surefly, the spark advance starts when manifold pressure falls below 24 inches of MP, so power wont fall off with altitude as much as it does with the constant-timed magnetos, so rate-of-climb will be improved and, in my case, I get actual published book speeds even though my plane was in a major accident in its youth. BUT: make sure you get it installed right! Sometimes even the little things can bite you on the rump and affect how it all works. Mine seems to be running fine so far. Hopefully your installation will learn form mine and will go a lot smoother. Best of luck to all. Happy flying.

Cutlass G