Author Topic: List of Big Bore Pipes - 1-1/8" Stinger Diameter - TRX250R  (Read 5547 times)

Offline Jerry Hall

Re: List of Big Bore Pipes - 1-1/8" Stinger Diameter - TRX250R
« Reply #15 on: March 04, 2020, 07:35:48 PM »
Inside stinger diameter can get complicated real fast and involves a lot of variables and I can usually answer this question on stinger sizing quicker using the dyno and monitoring the piston crown temperature rather than trying to model the system using partial differential equations.  Listed below are the variables that must be considered and some of the basic science on how these variables affect stinger sizing.

1. The horsepower of the individual cylinder not the displacement is the most important variable when sizing stinger diameter and length.
    a.  A 1 HP cylinder creates a given quantity of exhaust gas and wasted heat.  A 100 HP cylinder creates 100 times more exhaust gas and wasted heat than the 1 HP cylinder

2.  The time between power pulses must be long enough for the exhaust to exit the pipe on each engine revolution

3.  The flow rate of the stinger and muffler has to provide the proper amount of back pressure at the exhaust port to work with the diffuser and tail cone shape profiles.
     a.  Anytime length is added to pipe or tubing, the flow rate is reduced.
     b.  Anytime a bend is added to pipe or tubing the flow rate is reduced.  The total number of degrees of of bend in the stinger/muffler is cumulative.  One bend of 50 degrees will offer approximately the same amount of restriction as five bends of 10 degrees each in the same length of tubing.
     c.  Turndowns on the end of mufflers reduce the flow rate but help fool the sound meters that the officials use when conducting sound level testing.
     d.  Anytime the inside diameter is reduced due to using cheap tube benders or worn out tube bending mandrels, the flow is reduced.
     e.  Anytime the inside diameter is reduced due to a weld booger inside the pipe flow is reduced.
     f.  The transition between the tail cone and stinger entrance should be straight and not bend at the junction.
     g.   The muffler's perforated tube reduces the flow rate.  The hole pattern and hole diameters affect the flow rate and the sound attenuation of the muffler.

A combination of "a" through "g" can be used to obtain the desired flow rate (back pressure) of the stinger/muffler.  On some engines the length of the stinger/muffler will show a slight benefit due to the tuning of some of the secondary wave activity in stinger/muffler tract but the proper restriction is most important. 

Small displacement cylinders are usually more thermally sound and their pistons will usually allow more back pressure to be run at the exhaust port. Some of us engine builders can get a lot of power from the old two stroke engines that have been updated and are not using 25 year old pipe and cylinder port technology.

It has been my experience that the two stroke piston crown profiles (minimum thickness and thickness profiles) are poorly designed on most of the forged pistons being manufactured for our sport, especially the ones we have to use on the big bore cylinders.  I believe the piston manufacturers have gone overboard in trying to make the lightest piston possible at the expense of not being able to dissipate the heat that is proportional to the power that can be found in some of the newer cylinders. Most of the time thin piston crowns will not sag or burn through even after 500 runs on the dyno, when the dyno runs are in the 2 to 7 second range, but will sag or burn through when being run for WOT 10 to 15 seconds when tuned for max power up a long steep sand mountain or across a dry lake bed in a Baja race.

Back to the stinger topic:
The stinger inside diameters (exhaust flow rate) is adequate on many of the big bore pipes until the power level is pushed to a much higher level.  If this high level is reached it may be necessary to increase the inside diameter of the stinger/muffler combination or the length of the stinger/muffler combination may have to be substantially reduced or bends removed the the stinger/muffler combination to increase the flow rate so that the back pressure does not cause a piston crown failure.

A decision has to be made if the power level drops, the over-rev portion of the power curve is substantially reduced after the stinger inside diameter is increased or other measures were taken to improve the flow rated through the stinger.......  Am I willing to sacrifice power for reduced piston crown operating temperature?.....or...... use a stinger/muffler combination that makes the most power&over-rev and shorten the number of seconds that the engine is run at WOT with 30 seconds or so of rest between each WOT period?


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