Seems there is a whole group of 250r guys running 350 cylinders with 250 pipes. They start blaming the pipe. A larger stinger diameter will let the pipe cool your motor down.
I see this every week. Guys need to wait until they have the money to do it "right". You will usually have problems if you are trying to upgrade to a big bore cylinder and running mismatched components until you have the money to purchase the rest of the necessary components
My experience has been that putting a larger stinger and muffler core on a 250 pipe that was developed for the OEM cylinders is often a band aid . Less restriction on the outlet of the pipe will help keep the piston temperature reasonable on hard pulls but will often not make the power that a pipe that was specifically designed and tested for the big bore cylinders and porting being used.
Even though the same basic molds appear to be used on the Pro X 310s, 330s, 350s, and ESR family of big bore castings, the scavenging ports effective discharge angles and areas change when a given casting is machined for a different bore size. Cylinders that scavenge differently and have different displacements need different pipes to match the unique scavenging patters of the different groups of displacements.
Old pipe formulas and pipe software used one size of stinger for 125cc engines, another size stinger for 250s and so on without regard to how much power the engines made. The amount of exhaust being pushed through a given diameter stinger is a function of the power being made. If your big bore cylinder makes significantly more power than a good running 250, the 250 pipe is more than likely going to be two restrictive and piston overheating WILL occur.
I think that it is the engine builders/parts supplier DUTY to inform the customer as to what pipe, porting, head, carb, reeds etc. and starting point on jetting they should be using for a given engine package. If the engine builder cannot furnish this information you need to go to builder or supplier that can. This knowledge only comes from expensive testing on the engine builders part. If you are following all of your builders recommendations for engine package components and tuning specifications and are still having problems....you need a builders that has done the necessary testing and development. A customer should not have to go through a handful of $150.00 pistons to work out the bugs that was in a engine package that was supposed to be already developed.
Customers often shoot themselves in the foot trying to select their engine package components themselves rather than finding a builder and following his recommendations to a "T". I have some customers that do not listen to my recommendations and use the wrong pipe, porting, head etc.because they do not want to spend the money for the tried and tested components and do not understand the importance of using components that compliment one another.
