89 per the owners manual. Below that is an early combustion event because the fuel ignites easily (compression) and the pistons are not where they are supposed to be. 91 is hard to ignite with cylinder heat and ignition resulting in an even more incomplete burn. Meaning less energy to push the piston. Spark plugs are also sold in heat ranges. Hotter spark plugs and higher compression engines for high octane. Vehicle never burn all the fuel. Does it work? Absolutely! They all do. We are talking atoms here but there is a difference. Unused fuel also creates carbon on the valves, pistons, etc... overtime. Fuel is also a lubricant and coolant among many other things. Low octane burns sooner and all of it is gone meaning no coolant for the cylinder and valves. The combustion chamber heats up past effective threshold, the chamber can create NOX at 2500F which kill everything. 91 does not burn so easily meaning excessive coolant in the cylinder resulting in lower combustion chamber temperatures. Low temperatures do not like to burn which is why we used to choke our engines, to create operating temperature. Unburned fuel is wasted energy. An old school champion spark plug re14mcc4 for example.
R= resistor 6-15 k ohms E=14mm x 1.25 taper 14=Heat range this one is medium hot (1-23engines 53-63 is performance indycars 75-95 is industrial engines) MCC=electrode type 4=gap 1mm or 1.14mm or .045"
If the spark plug can not dissipate heat it may crack or prematurely fail. Excessive heat is transferred to the cylinder head and to the cooling system if it does not burn a valve. Now there is an effect on the cooling system...etc... If the cylinder is cold the plug will foul out. Again does it work, yes, yes it does but not to the best of its ability. Are there going to be variations, yes. Someone on here can school me on their application. What I show are industry standards and what billion dollar companies have engineers research. Although if they did it right there would not be mechanics...
