So with WOT in the climb(base timing 25 deg) ignition will start to advance from 7000’ up to a max of 38deg at 10,000… depending on rpm.
Any time the power is reduced below 75% the ignition will start to advance to a max of 38 deg at 65% or less.. depending on rpm….assuming I have interpreted the graph correctly :-)
PeteD wrote:
If the manifold pressure drops at 65% power then it won’t be 65%?
You maintain 65% power as altitude increases (and WOT manifold pressure drops) by increasing RPM?
Fixed pitch
Thanks for the data @PeteD, it looks roughly to me like you start to see significantly more advanced timing at 5000 ft and above in cruise.
Yes, with a CS prop you increase RPM to maintain 65% power as MP drops off with increased altitude. This decreases engine efficiency a bit but you have no choice if you want to maintain power and fly high where airframe efficiency increases more. With a FP prop you run in this mode at lower altitude too, limiting power with the throttle and not running with low RPM and high MP, as you can with a CS prop. As a result the benefit of vacuum advance will have a bit more impact for planes with fixed pitch because they tend to run with more vacuum (lower MP) in more situations.
My low compression (7.5:1) 150 HP rated Lycoming O-320 runs fixed 25 degree BTDC timing if I remember correctly and is certified on 80/87 fuel. Nowadays it’s run on 100LL so there is lots of detonation margin. One could likely advance the fixed timing a couple of degrees more and have no issues at full power, but it wouldn’t be correct per the MM. You can also lean it a bit in climb, especially over 3000 ft as long as CHT is OK.
A dumb Q: is this system with one old mag still in place?
If you cruise at 65% at 1000 you also get advance, the mag doesn’t know how high it is…only what the manifold pressure is.
I use the constant egt method in the climb, which works from 1000 up.
If you cruise at 65% at 1000 you also get advance, the mag doesn’t know how high it is…only what the manifold pressure is.
That’s true but would be a bit less true with a CS prop where you can lug the engine at low RPM with higher MP at low altitude versus throttling it to the same degree. That adds engine efficiency in a different way, with lower friction and pumping losses, but can’t utilize as much ignition advance.
The STC is to change one mag. With only one mag changed and with vacuum advance you are lighting off the charge on the electronic ignition and following up slightly later with the mag. Dual electronic ignition might work better but one is good and works too.
Thanks much for posting your results.
One point I forgot to mention, with 65% power and therefore some advance, doing a mag check in flight there is minimal mag drop on the surefly and the engine is smooth, the other mag behaves like it’s on one mag!
Before someone talks about faulty ign switches, the mag drop is normal on the ground, as the rpm is too low for any advance.
The mixture control behaves slightly differently too… it’s as though the egt curve is flatter on top, pulling the mixture just reduces the fuel flow with out changing the egt as much or running rough
The idea of 2 sureflys brings with it battery backup systems etc, CAW potentially requiring expensive battery replacements at timed intervals, negating some of the benefits.
I’m more than happy with the present arrangement :-)
I had a pirep of a friend who installed SureFly mags, it seems to push him to fly LOP instead of ROP
The good news: his engine now runs smoother when leaned but these mags seems to be aggressive on CHT for rich operations
Have you noticed that much increase in CHT when ROP?
Did a straight climb today to Fl100, CHT remained at 360 so more or less as usual, maybe 10 Deg more , ISA +5 @fl100
Would agree with your friend LOP certainly doable now, whereas previously no chance.
