the marks on the piston on the SAME side as the exhaust port (never seen them elsewhere) are yes, from the exhaust gasses impinging upon it. the engine say, covers from 1000 rpm, til 6000rpm.
somewhere in this range, its out of tune. at some point, the exhaust is going to get smashed against the piston while the ports closed. even when it IS in tune, it still smashes against a fair portion of the piston as the piston rises to cover the port. the piston doesnt just suddenly close the entire port instantaneously, nor does the pipe just suddenly decide to stop pushing. in fact, the piston on a ht never completely exposes the port, and then, it only opens it fully for an instant as it goes through BDC! the rest of the time, that pistons in the way, impeding flow, creating turbulence.
i rarely see these "burn marks" on a NON piped 2 stroke. burning looks completely different. its a lot...drier looking. blistered. and looking at the underside of the piston crown is a dead giveaway. tan is good. blistered black is not so good.
they ARE NOT burn marks. it is simply oil from the exhaust getting smacked against a a stationary surface.
ignition timing has nothing to do with pipe resonance. port timing and length is all. (yeah yeah, and temperature and atmospheric pressure)
it will affect total pressure placed upon the piston crown, and exact timing of where that pressures "peak" is. when the peak is too close too tdc, or even on the wrong side of it(! way advanced !) its like hitting a brick wall. the force line from con pin to crank pin to crank shaft is too "straight" to develop rotational power.
now, if the peak is too far behind tdc, the piston is descending faster than the gas is expanding, heat is lost, power is lost, la de da de da.
find the perfect spot, which does change as rpm changes, and you will get maximum torque at a given rpm. which is why theres advance curves and companies like honda can invest millions of dollars into just ignition timing... timing is critical, but does nothing to pipe resonance. all other variables the same, the pipe stays as a fixed system. a harmonica tuned to C will always play C. while that exhaust port is closed, the pipe may as well not be there. (impractical, maybe)
when everything is running as planned, combustion has completed and finished before 10 degrees ATDC, if not earlier. so there is, depending on engine, another 50 to 100 degrees of rotation before the ex port opens.
all that is happening is hot gasses expanding and forcing the piston down by doing so. the "flames" you see occasionally escaping exhaust ports, are merely incandescent particles of carbon. (fourstrokers with straight pipes!) the closer to blue, the more complete the combustion, the less free carbon present.
a flame is a byproduct of fast, heat liberating combustion. exothermic.
rust is a slow, cold combustion. you dont see any flames on rust. endothermic. an oxy torch is fast heat liberating combustion. exothermic.
hydrogen burns with an almost clear flame as it contains no carbon at all.
and then, i dont know about sbp pipes, ive never tried one. seen them, never tried one. why should i?
a real pipe, made to the exact, calculated dimensions required, will knock your socks off. without having TOUCHED anything else. (except main jet)
meh. my only major port job was ruined by a ring locating pin letting go. i stuck to pipes and cleanup only after that. too depressing. if it snapped a conrod or something it would be different, but a locater pin letting go? that makes you wanna sit in a cupboard and cry...
though....im getting over sulking a bit...this to do list just gets longer.... longer... neverendingly longer....
arrrrrgh! i hate commitments! why cant i just be a billionaire and do things instead of having to live day by day and never get ahead?