I am trying to set the record for the most TLA in a thread title.
I have dual PMags and an EI Commander. Today I did a cheap and cheerful test to look at the effect of ignition timing in the cruise. All testing was done at 10,000' PA, 5 deg C OAT, WOT, Prop 2500 RPM and leaned to 25F LOP. After this the only thing I changed was the maximum allowed advance.
The way the engine has been configured to date is Max RPM 3072, Max Advance 35 deg, 0 deg advance shift.
I made a change to max advance (reduced in 2.8 deg steps) and sent the change to both PMag simultaneously. My engine has been happy with 36 deg advance till now so I was not concerned about sending it less advance.
The actual advance reported is always 1.4 deg more than the Max Advance you program in the EI Commander map. What I show below is the advance as displayed by the EI Commander, which is 1.4 deg more than I asked for.
At the very left of the picture you see the fuel flow reducing as I lean the engine. From this point on throttle prop and mixture are fixed and all changes are a function of advance only.
Effect of ignition timing, reading left to right, the step changes in EGT on the top graph are where the timing changes are made.
36.4 deg, 182 KTAS, time 28 - 31
33.6 deg, 180 KTAS, time 31 - 34:30
30.8 deg, 176 KTAS, time 34:30 - 38
28.0 deg, 173 KTAS, time 38 - 42
25.2 deg, 171 KTAS, time 42 - 44:30
22.4 deg, 170 KTAS, time 44:30 - 48
36.4 deg, 182 KTAS, time 48 -
Timing is then reset back to 36.4 deg and this is shown where the EGT rapidly drop again on the right hand side of the graph.
The CHT's on my aircraft always take about 5 minutes to stabilize after a power/mixture change and I did not wait that long, so the exact effect on CHT could be a little different to what I summarize below.
In general as I reduce advance from 36.4 deg I get the following changes:
KTAS reduces by ~ 1 KTAS / deg advance
EGT increases by ~ 10 F / deg advance
CHT reduces by ~1 F / deg advance
If you get the ignition timing badly wrong it can hurt the engine. If you feel like experimenting I suggest you buy a bore scope before hand and take photos of all your pistons and valves so you have a baseline. This way if anything goes wrong you will be able to make a better determination of potential damage done. Do any timing changes high enough and close enough to a nice long runway so that you can easily conduct a power off landing. Make sure you are proficient in power off landings. Educate yourself as to the likely/expected outcomes of any ignition timing change so you can judge if the outcome of a change is normal. Be proficient in the use of the device that you are using to make changes so that you don't spend too much time eyes inside. Try to preserve a proven map/setting that you can return to quickly if the one you just tried turns out to be inappropriate. Although your engine might be broadly similar to mine there are no doubt subtle differences, so what works for mine might kill yours. The data shown is just meant to be one of many ways to do an ignition timing test, it is not a suggested setting for your engine.