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« Last post by Jerry Hall on May 02, 2025, 08:14:13 PM »
My 50 plus years of testing and design work on a wide variety of different types of two stoke engines has shown me the stinger size depends primarily upon the power the engine makes not the actual displacement, how many seconds the throttle is held wide open without closing the throttle for a second or two, the length of the stinger, how many degrees of bend there are in the stinger, the type of bender used when the stinger needs a bend, how well the silencer flows, and how well the cylinder's thermal exchange is into the coolant that flow through the cylinder and head are the other variables that have a large influence on stinger dimensions.
One of the primary design criteria of the majority of OEM pistons is to provide an unobstructed path for heat to flow from the center of the piston to the rings and skirts. My testing, observations and conclusions on the pistons that we use and or is supplied with the majority of the big bore kits has been very disappointing for me. It has been obvious to me the the primary design criteria for our big bore pistons was to make the piston as light as possible in an attempt to make it's weight as close as possible to that of a 250 piston. I believe the weight reduction programs have led to piston designs that are thermally inferior to the designs used by Honda and other engine manufacturers.
The OEM pistons with proper stinger restriction, proper ignition timing, with piston clearances adjusted for the various gasoline engine packages and jetted for best torque and power will not usually have any heat related piston problems. When dyno testing I can incrementally continue to lean the jetting down until the power starts to drop off, without heat related piston crown problems on OEM pistons when making long dyno runs.
I cannot usually optimize the big bore gasoline engines for best power that have forged pistons that have compromised thermal designs. 330 big bore Hondas that make around 70 hp on my dyno that are tuned for best power and best torque, the piston crowns will sag in the middle of the crown or half way between the middle of the crown and the exhaust port side of the piston when subjected to 10 to 15 second long dyno runs or 10 to 15 second long runs up some of the long sand hills. The big bore pistons will survive all day long on the dyno if the dyno runs are a couple of seconds or less long runs. The pistons will usually survive 100 yard sand drag races when tuned for best power and torque.
The problem is:
The majority of the big bore engines have to be de-tuned for Cross country, Desert Racing or the 10 to 15 second long hard pulls up Sand Mountain or Oldsmobile Hill at Glamis to make the pistons survive. Short dyno run times do not usually simulate what your engine experiences on most racing and recreational riding applications. Piston designs cannot be accurately tested for heat related design problems when the dyno runs are too short.
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