Originally Posted by
cathurga
Sr Telemaster, as Elmshoot says, go back and read a bit, there is a lot of information at hand. I think the MOST important fact, which JieM has shown in his pictures, is that the timing is way too advanced for a gasoline engine, by quite a way. The other thing is that measuring TDC on all cylinders, reveals that the TDC occurs at intervals of 126 between 1 & 2 cyl, 108 between 2 & 3 & 126 between 3 & 1, so there is a variance in the crank spacing between them, as a result of the main rod design.Its not misaligned, its designed like that.
That being the case, getting the timing to 30Deg BTDC on each jug, means having to space the magnets 126-108-126 on the circumference of the prop hub. If you have an FG engine, go ahead and measure it. If you have a reason to question our logic, we would accept your theory as we are all scratching our heads, here, and you are welcome to scratch along with us....or lead us in the path of a solution.
If you can find a way to measure the piston distance from TDC at your 30 degree BTDC ignition timing, I think you might be surprised. At 30 crankshaft degrees the position of the piston in the bores will vary between cylinders.
A crude yet effective "gauge" could be fashioned form a round toothpick with the point trimmed back on one end. Doing your best to keep the toothpick angle consistent, measure the distance to the top of the #1 piston at 30* BTDC. Make a reference mark at the edge of the spark plug hole on the toothpick. Now rotate the crank 120*. Again insert the "gauge" until it contacts the piston crown. Make another reference mark like you did for #1 cylinder. Repeat for #3 cylinder.
Is the linear distance of the piston crown from TDC the same for all 3 cylinders? As a dynamic value, the position of the piston may be more important than the crankshaft degrees as the cylinder pressure will not be equal in all if the cylinders when the spark occurs of it is timed by crankshaft degrees.
Cylinder pressure is the main factor in detonation so perhaps some variance in ignition timing would be optimal for this engine given the variance in piston position relative to TDC in the bores of the 3 cylinders?
On glow ignition radials, it is cylinder pressure that determine ignition timing so it would be self correcting in a radial glow ignition engine. Perhaps the variance in ignition timing is designed in and merely pulling 15* out of a 120* crankshaft degree magnet spaced ring to reflect 30* BTDC timing for #1 cylinder would be correct.
Sitting here mulling over the variance from #2 & #3 cylinder timing has me wondering of a leading and trailing geometric effect of the linked rods compared to the master rod is a factor here.
BTW; I checked my FA-450R3 master rod and as close as I can tell, the link pin bores are at 120* spacing relative to the master rod bores.