hey jerry whats the most likely cause of the piston underside going black ? carb to lean, stinger diam to small, ign timing to much retard. what about plug to hot or exh timing to high
The underside of the piston “going black” is ONLY an indication of the surface temperature of the underside of the piston and the time the surface has been at a temperature high enough for the fuel and oil vapors that comes in contact with this surface to coke. The underside surface of the piston crown does not care where the heat comes from or what caused the temperature to rise to a level for coking to occur.
The underside of the piston going black does not tell us anything about the air fuel mixture. The presence or absence or the length of what many tuners call the mixture ring at the base of the porcelain insulator in a spark plug, does not tell us anything about the air fuel ratio. These two areas where soot or carbon form are just indicators of the temperature of these two specific areas and cannot measure air fuel ratio. At low temperatures the porcelain insulator will remain white until it’s temperature us raised to the temperature where fuel and oil coke to form deposits. As the temperature of the insulator increases the deposits start to burn away at the tip of the insulator and the dark ring of soot or deposits move downward. If the temperature of the insulator is raised sufficiently high the dark ring completely disappears. The insulator can withstand high enough temperatures to burn the insulator to a clean white appearance. The aluminum above the underside of the piston cannot withstand high enough temperatures for the underside to burn off the dark layer of soot and carbon. The piston crown (surface of the piston that is exposed to direct combustion chamber heat and pressure) will often burn off the majority of the carbon when combustion temperatures are excessively high or the piston crown is not designed for the long periods of wide open throttle the engine may be experiencing
If you have a brand new piston and lean the jetting enough to drive the engine into run-away detonation or……. reduce the stinger diameter enough to drive the engine into run-away detonation or……. advance the ignition timing enough to drive the engine into run-away detonation or ………..run fuel with low enough octane to drive the engine into run-away detonation or……… install a heat range that will cause instant pre-ignition that will drive the engine into run-away detonation, there will not be enough time for enough fuel and oil vapors to contact the underside of the piston to make it black before a hole is melted in the top of the piston.
If we mistune the carb, ignition, or some other component enough to drive the piston crown temperature high enough for these crankcase vapors to coke on the underside of the piston, but not high enough to drive the engine into occasional detonation counts, the discoloration process begins. The degree of blackness depends upon the actual temperature of the underside of the piston and the amount of time this surface is exposed to the mixture in the crankcase at a high enough temperature for the crankcase mixture to coke on the underside of the piston. Hard crusty carbon deposits will form on top of the black areas where the temperature is raised even higher. I do not know what temperature is required for the crusty carbon deposits to form on the underside, but when I see the dry carbon the piston crown temperature is very close to the temperature where the crown is about to melt.
I believe the majority of the current Wiseco and Wossner piston crowns I have looked at are too thin and not designed to dissipate the heat from the center of the piston into the rings and skirts at the rate at which many of the engines are capable of driving heat into the top of the piston. I had a few phone conversations with one of the engineers at Wiseco about 10 years ago just after they started marketing the Pro-lite line of pistons. I was having piston crown overheating on a lot of the new pistons on engines that were run at sustained high RPM and at lot of wide open throttle. These engines were used for road racing, shifter karts that were run on big tracks, micro sprints, flat trackers, and some pro moto-crossers. I did not have this problem on the older pistons on the same engines prior to the new Pro-lite line of pistons.
According to this engineer the new owners of Wiseco (Dover Diversified) needed to drastically raise the price of their pistons to pay for all of the new overhead (Stock holder dividends, and all of the CEOs, CFOs and unneeded management teams that drain the money from companies that are publicly owned. Prior to this buyout Wiseco was a family owned company. Marketing had to come up with an excuse to justify the higher prices so they came out with the Pro-lite line of pistons. Engineer were told to make the pistons a lot lighter because the average racer and shade tree engine builders believe light pistons produce more power than heavier pistons. A light piston helps reduce the stress on con rods and bearings but often does so at the expense of reducing the thermal loads the piston can take before it fails. As we increase the power of the engine the thermal load of the piston needs to increase not decrease.
The majority of the weight was taken away from the outer edges of the piston crown because that was an easy place and about the only place left where weight could be removed. The Wiseco engineers did not want to remove the material in the outer edges because they were decreasing the size of the pathway where heat flowed from the center areas of the crown into the rings and skirts. As a result of the new Pro-lite line of pistons I feel Wiseco instantly became a company that primarily produces pistons for drag racing and or cannot be tuned for max power if used for applications that require longer periods of full throttle. Testing and observation of customer pistons has shown me that the new thin parallel piston crown thickness will not tolerate sustained full load without the temperatures in the center building until crown sags or blows through. A piston designed for sustained and or ultra high power levels will have a piston crown that is thinner in the center and the thickness exponentially increasing toward the outer edges of the piston. Cut any of the OEM pistons in half through the piston crown and tell me what you see. You will generally find a piston crown designed to dissipate heat and not a piston that was designed so that marketing can brag about how much lighter there pistons are than an OEM piston.
I feel all of the pistons being made for the big bore after market cylinders are also designed for drag racing and to have lesser effect in upsetting the balance of a crankshaft and a connecting rod that was designed for the weight of a 250 piston. I believe the aftermarket cylinders and the piston manufactures the cylinder manufactures are using were attempting to minimize the stress on the OEM connecting rod and balance of an OEM crank, led to a line of pistons that will not take the thermal load that the new cylinders are capable of producing especially when the ports and pipes are taken to the next level of power or higher.
