Twinny wrote:Adam_Winstone wrote:But he does give you % of bore details for exhaust port width in the porting chapters, which then needs to be applied to any piston/bore diameter that you port.
You'll find that most people today will suggest that you can go wider with decent modern rings, however, you then get into a debate about port shape and how many miles you should expect out of a kit before it drops a ring and knackers itself! Dyno figures always show a power increase from wider ports... then again, many people waiting on the hard shoulder might think differently.
Good question... no simple answer
Adam
PS - I might be thinking of a different 2-stroke tuning bible (certainly does) but I think Dave does make comment in the tuning chapter.
hi adam has it got something to do with position of ring pegs in the piston
cheers john
No, the TS185 piston should fit without any need to repeg the piston. The advice that you've been given re: 65% of bore is exactly what I would work to also. Yes, plenty of people will say that you can go larger... yes you can.... but I agree that ring and barrel wear becomes too rapid, often resulting in only getting a few thousand miles out of it before it goes pop! Take 1000 miles out for running in and this doesn't leave you much before you're doing it all over again
Even with the top quality rings available these days, and allowing for a chamfered port of the right shape, go too wide and it will fail. People become convinced that it will be ok because it seems to cope well for a couple of thousand miles, then it goes belly up and there will commonly be evidence of nasty wear where the ring has been popping in and out of the port, wearing the radius of the port until the ring can cope no more.... oops! If the exhaust had been left that few % narrower then it is likely the ring would not have started to drop, therefore, not cause the wear that seals the top end's fate.
Each to their own. Want it to look good on a dyno and a few Sunday runs then go as wide as you like. Want it to make reasonable power and last for thousands of miles then 65% is what I would aim for.
Another problem that is coming to light with these decent flexible chrome rings is that the hard chrome edge to the ring is very good at avoiding wear... but... unfortunately, many of the aftermarket kits (iron and iron lined alloy) are very soft on the bore, meaning that the bore wear is rapid especially around wide exhaust ports.
Best of luck.
Adam
