So to be clear, Dan. Are you implying that the timing advance curve on the Surefly is currently too aggressive for a 200hp angle valve IO-360? I am ready to pull the trigger on a Surefly unit for the non-impulse mag in my Aviat Husky with the IO-360 angle valve engine with counterbalanced crank. I now find myself taking pause.
I previously wrote "Honest discussion, messy as it may be, is how potential users determine if it's the best choice for their particular set of needs and interests." Your Husky application is an excellent case in point. Your needs are not the same as they might be given an RV application.
The Surefly offers both fixed and variable advance, with user choice of base timing.
The base timing for your angle valve IO-360 is generally accepted to be 20 BTDC. Some folks set 25, an option for some certified installations with fixed timing magnetos, as it will slightly improve altitude performance (think Mooney) in return for higher CHT. The best fixed timing compromise appears to be 23, based on personal experience and dyno work by a now deceased friend.
The Surefly's fixed timing option allows the user to select a setting between 18 and 30, with 20, 22, 24, or 25 being the choices suitable for a normally aspirated angle valve.
Begin by asking yourself what you most want from an EI. If the answer is (for example) "easy hot starting", then a Surefly installed with fixed timing is a fine choice. It will definitely start easier, it will run further lean of peak than you can with a mag (assuming you increase the spark plug gaps), it's easy to install and time, and you ditch the required overhaul period. It will otherwise act just like the mag it replaced. So, there is no reason for pause. If you like the product, do it.
The issue is how you set it, not the product itself.
It's a Husky. STOL performance at higher elevations is probably on your list of "must do well" items. That means best power mixture, 100 to 175 F rich of peak. Operating rich of peak, there is no significant compromise with any fixed timing in the low 20's.
As previously noted, you'll be able to pull very lean for those times when fuel conservation is critical. Optimized LOP timing (more advanced than the fixed values noted above) generates a notable speed increase with (for example) a 6-cyl Rocket a higher altitudes. A Husky is at the far other end of the drag scale. Optimized LOP timing would offer very little return, because more power doesn't make a draggy airframe go a heck of a lot faster.
Let's look at variable advance. Here is the Surefly advance schedule. I've added some notes, specifically a pressure schedule for standard atmosphere, a scale for advance, and two RPM lines. The manifold pressure values Surefly has posted on the right margin are kinda fuzzy (23 to 25 inches, for example), so it's hard to know
exactly what timing value results from a particular MP and RPM. A block schedule would be more accurate, but we'll go with what we have.
Bumping around at less than 5000 feet will keep timing at 25 BTDC or less. CHT's will rise some with rising advance. CHT considerations must get greater emphasis in the Husky application, as lower airspeed means lower available dynamic pressure. That said, I don't see a significant problem here, ROP or LOP.
With increased altitude, the schedule moves progressively toward one suitable for a parallel valve engine run high, fast, and lean. Assume 2400 RPM for cruise, and by 8000 feet the system has advanced past the 28~29 range we're finding optimum for an angle valve run LOP. Max advance for 2400 appears to be 31.5, not unreasonable for LOP but
way past optimum for ROP. It's not a horror story, in that it should not break the engine. However, it is pointless given that it will come with a performance decrease, and a higher cooling load.
Let's return to STOL fun, 2700 RPM at best power mixture. Given a strip at higher altitudes, 2700 RPM has the system advanced to as much as 36.5 degrees, well beyond desirable timing for best power mixture. CHT will be high, a combination of lower air density (less cooling mass), lower airspeed (less dynamic pressure to drive the cooling mass), and peak cylinder pressure moved closer to TDC. I think it would be a very poor choice for a Husky, but again, it's the setting, not the product itself. The product gives you a choice.