I'm new here, but one way or another I now have 3 of these including one with a full SUGO kit which has been neglected for many years but I am about to restore at great expense. I noticed that some of the regulars here have mentioned that the R hand cylinder has a tendency to detonate and damage the piston. This is a problem I have been thinking about, particularly as the bikes I have are likely to be running on 97 octane pump fuel.
So, my thoughts are these.
1. The reason the R hand cylinder is more likely to detonate than the L might be because the charge in the R hand crankcase is at a higher temperature. That might in turn cause the R hand piston to run hotter which in turn heats the gas in the R hand crankcase creating a vicious cycle. I am wondering if anyone has thought to mount a thermistor near the intake for the transfer ports to measure the gas temperature, or alternatively to mount an infrared temp sensor on a stand off looking up at the under side of the piston. Since the piston is only passively cooled, it would provide an indirect monitor of the combustion chamber temperature.
2. Detonation often occurs because the temperature of the charge in the squish band is too high. This can be reduced by narrowing the squish band to keep it smaller than the boundary layer of gas next to the metal of the cylinder head and the piston edges (less than 1mm). If the metal surrounding the squish band is too hot, detonation will still occur. So anything that will keep either the cylinder head or the edges of the piston cooler will mitigate detonation. So the TZ250s running on lower octane fuel had copper rings inserted on the squish bands of the cylinder heads to act as a heat sink and keep it cooler (as well as re-designed top hat profile combustion chambers with a larger volume to lower the compression ratio).
I read of a racer who tried ceramic coating the piston crowns (thermal barrier insulation). The result was that the ceramic coating on the edges of the piston chipped and fell off. He put it down to differences in thermal expansion, but I wonder if that wasn't actually detonation damage caused by the hot ceramic surface. The next time he tried it, he chipped off the outer edge of the ceramic coating first, and apparently it worked much better. My thought is that perhaps coating the centre of the piston (maybe 32mm dia) with a ceramic coating would lower the piston temperature and the outer portion which makes up the squish band would run cooler.
About half of the thermal energy transferred to the cylinder comes from the hot turbulent gases exiting the exhaust port. If this had a ceramic coating to insulate the metal from the exhaust port gases it would allow the cylinder to run cooler. That would facilitate heat transfer from the piston to the walls of the cylinder, keeping the outer part of the piston and the squish band cooler.
Keeping the piston cooler would also lower the temp of the gas in the crankases, keeping the incoming charge cooler and further cooling the squish band.
Anybody think these ideas could have merit?
Experiments on two strokes where the combustion chamber is entirely coated with a ceramic thermal coating show that this makes detonation much worse at anything over 4000rpm because the ceramic surface gets too hot, and the cooling of the cylinder head is no longer effective in keeping the squish band cool. So coating the entire surface of the piston might well cause more problems. But coating only the central portion of the piston and the exhaust port might work by reducing thermal energy transfer. Some thin (0.003") ceramic coatings are advertised as "reflective". I don't think this would work any better than polishing the pistons, since it would soon be coated with carbon and stop working. But the thicker coatings (0.012") which work by insulation would perhaps be more effective.
Anybody tried anything like this?