Originally Posted by
Lead Balloon
Or maybe the laws of chemistry and physics mean the fuel/air being sucked into a cylinder fitted to a radial engine combusts differently than if it’s being sucked into a cylinder fitted to a horizontally opposed engine.
Of course there are variations in how the fuel/air combusts in different engines. Differences in bore, stroke, compression ratio, piston speed, head design, piston design, manifold pressure, spark timing, spark plug location all affect combustion and in particular influence the likelihood of detonation.
The fact that different Lycoming engines use different timing is evidence that there are differences even between engines that appear very similar.
The assertion that all engines are the same is one of the those statements that just fall apart if you look at the evidence.
Originally Posted by
Lead Balloon
Maybe the compression is squashier
The flow of gas inside the cylinder during compression is influenced by cylinder and head design, and is a very important factor in engine design. It directly influences the likelihood of detonation. So yes, it certainly could be different.
Originally Posted by
Lead Balloon
or the spark sparkier?
I read some notes on operating the big radial engines. Part of the leaning procedures was to advance the timing to improve valve life. Maybe they were the earliest victims of the OWT about LOP causing valve problems? Or maybe there is something there?
In any case yes the spark was certainly earlier running LOP.
I reread one of John Deakin's columns where he was describing a Lycoming engine where Lycoming had approved operation LOP. One interesting tidbit was that the timing was advanced compared to engines where LOP operation was not approved.
Many engines run LOP, but I don't know of any engines with variable spark timing that would not advance the spark when running LOP.