Hi Brian. Its been a while since I looked at this master rod / link rod compensation angle stuff. I know a lot has been posted on the forum previously, we could probably search for links. I have some of my files before 'the great PC crash' but not all. I still have my step-by-step (graphical) layout methodology, which I pilfered by someone on the old Yahoo R&R forum. I think I can make a PDF of that & post.
I might be remembering this all wrong so we may have to dig a bit deeper. But I seem to recall: assuming equal link rods/pistons/cylinder dimensions & radial layout, you can satisfy one parameter but not both. In other words if you have equal 72-deg layout on the master rod (365 deg /5 cyl) then you will have TDC occurring at 72-deg rotation. However the throw will be different resulting in different compression ratios across the cylinder. If you adjust the angle of where the link rod bottom end pin lays out on the master rod, you can achieve equal CR. But I think TDC will no longer occur at 72-deg, it must occur at some other crankshaft angle. On a glow engine, balanced CR is important if it varies too widely. The gas will fire when it fires, we have no direct ignition control anyways. On a timed ignition engine, I would assume the equal crankshaft angle is more important.
For example the Edwards 5-cyl has equal 72-deg layout, which by itself, results in significantly different CR across cylinders (spiky looking CR plot). But I think the design calls for altering the head shims or shaving the top of the liner lips to compensate. Maybe an Edwards builder can chime in. By contrast another radial I looked at, possibly the Kinner, shows an angular compensated master rod layout & more equal CR compared to master. And the parts are all machined to same dimensions.
'Carl Sorensen' does that sound familiar to anyone? He wrote an article & Excel spreadsheet? That's what I used to adapt into these summary plots. I really should go back & clean up the tools.
