Not sure what your saying here...Maybe my ESR 350 cylinder does not qualify for Big Bore Status but I have run it and run it hard for over 4 years and never had any piston problems running the TRX11 straight from ESR....It might have something to do with my complete package as the cylinder and cases were both modified...Either that or I'm just lucky?
What does the under side of your piston look like after it has 30 minutest of hard running on it? Is the whole under side of the piston shinny and jet black? Is the whole under side of the piston jet black with crust in the center or on the exhaust side of the piston? Does the crown of the piston show some minor sagging in the center or half way between the center and exhaust side of the piston? If your answer is yes to any of these you MAY have too much restriction for the current combination of stinger length, stinger, muffler inside diameter and the power your engine is developing. The flow rate of this area of the pipe depends upon how much power your engine is making.
I have had a few customers that have purchased complete ESR big bore engine kits (cylinder, head, pipe reeds, carbs, CR ignitions) and could not get a weekend of riding before the pistons failed. Some could never get 5 minutes of hard riding. Some pistons had the top of the piston caved in, some had the exhaust side burned off and others had 4 corner seizures. I bored and fitted a new piston and told them to find the problem before they ride it again because it was going to make the new piston and bore look just like the old one. Most of the guys assembled their top ends, played with the jets, installed a lower compression dome, but would continue to have the same type of piston failure. I suggested that they send the cylinder and head back to ESR so that they could look for any defects like the interference fit between the liner and aluminum casting or problems with coolant flow through the water jacked. ESR gave their stamp of approval that nothing was wrong with the cylinder kit and returned it to them.
Eventually I convinced one of the guys to they bring me his bike, let me inspect and assemble it, put it on the dyno and tune it. I made some jetting changes, timing changes, tried different types of fuel but the engine did not respond to the changes I made like they should have. I finally spent the time to hook up some sensors to the TRX 5 pipe and found that the pipe was severely over restricted. I started at the silencer and worked my way toward the pipe. I found the diameter through the bend in the stinger was collapsed approximately .100" from what looked familiar to me when my pipe bending dies need replacing. I found weld boogers inside the stinger at the junction of the stinger and the tail cone had a severe misalignment with the stinger. I made a new 1 1/8 OD 18 gage stinger with a good mandrel bend and then ran it again. I had to richen the carb up a few sizes with the new stinger to get the power optimized. The jet that now made the most power had a rich misfire when tested with the stinger/bad welds that came on the pipe. Pipe pressures were borderline high for the power the 310 cylinder was producing but the stinger replacement eliminated his continuous piston problems.
Another guy was having piston failures just like some of the other guys with the big bore ProX and ESR cylinder kits. This guy had a Pro X 330 with the TRX 11 CM. I put it on the dyno and ran it. It had tuning symptoms just like the first one that was over restricted. I instrumented the engine and pipe and check it and it was over also restricted. I found it had the same craftsmanship applied to the critical junction of the tail cone and stinger as the TRX5 pipe. After optimizing the flow through this portion of the pipe with a new 1 1/8 stinger it was still over restricted. I had to install a 1 ¼” OD stinger and new silencer. The jetting had to be richened to optimize the power like the first engine and the engine turned into an easy to tune and not one that required constant jetting attention.
The word spread fast among the group of guys that had big bore cylinder kits that were having repeated piston failures. The majority of the guys with big bore kits were using what I refer to as 250 pipes with 1 1/8 OD 18 ga stingers. I have repaired numerous pipes that had the flaw at the junction of the stinger and tail cone with good mandrel bends made of 1 1/8 18 gage tubing. The 1 1/8 OD stinger produces pipe pressures that I consider marginally high on the low HP big bores and the high HP 250s. This modification eliminated the majority of the piston failures if the guys were using good fuel and had the carbs tuned well.
On the higher HP engines I have to use 1 1/4 OD 18 ga stinger and silencer cores to get the restriction right. I use a 1 3/8 stinger and silencer core with adjustable restrictors for the big motors with a lot of power.
I have been developing tuned exhaust for two strokes for over 40 years and have seen and tested just about every possible combinations of stinger size, muffler core size and pipe restriction inserts on a wide variety of different two-stroke displacements. It takes a lot of development time (money) to get every section of the pipe working together with the right over all pipe restriction to make the widest possible power band and an easy to tune reliable engine PACKAGE. I am seeing this problem becoming wide-spread among shops that are developing products on their dyno and not using what I feel are testing procedures that will find these design errors before they are mass produced. I think that a large number of shops and manufactures are being forced to use the wrong testing procedures in their quest to be able to record and advertise dyno results that are not representative of what the engine is experiencing on the race track. It is much easier to get good over-rev by OVER RESTRICTING the pipe than spend the necessary developmental time to get the rest of the pipe dimensions where the engine wants them to be without adding the unnecessary heat to the piston crown.
I am not trying to bash any one brand of pipes. I am trying to drop some hints to those that could benefit by taking a look at their test equipment and testing procedures so that these problems will be discovered during the development of their products. By doing this I think that they will be able to better recommend specific pipes for certain cylinder kits making power within a certain power range, rather than recommend one pipe for low end, one pipe for top end and one pipe for top end on everything from a 250 to a 370 cylinder kit regardless of the power it makes.
