Quote:
Originally Posted by Discoveror
I came across a Good dyno-tuning outfit which meticulously showed (using a wide-band oxygen sensor and meter ... on the dyno) that a Honda Grom ECU made for a near perfect (stoichiometric) 14.7:1 air:fuel ratio ... UNTIL the last 10% of throttle opening where the air:fuel ratio dropped to ~13.2:1 - to make maximum power at wide-open throttle.
So, I ran my 2023 Grom at ~5,000 rpm (NOT wide-open throttle) for awhile, killed the ignition (withOUT closing the throttle), pulled the spark-plug and, sure enough, the plug center insulator was white as snow. Is that what a perfect 14.7:1 air-fuel ratio looks like on the spark plug center insulator?
Today we're Forecast to reach ~50*; I'll try running wide-open throttle - to see if the same approach shows the sought after light TAN plug center insulator, which used to be depicted as 'perfect' on Champion spark plug charts in the back of gas station bays?
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A stoichiometric mixture will leave next to nothing on a plug insulator during a short term plug reading on a new plug. Of course, the plug will color a bit over time even with a stoichiometric mixture as engines do burn some oil that gets past the piston rings under conditions of high engine vacuum (typically deceleration) and also what oil vapor comes through the crankcase breather into the airbox.
If the mixture is in the 13-ish : 1 ratio at WOT, I still wouldn't expect much additional coloration of the plug insulator on a short plug reading run. 13:1 is still fairly lean for a maximum power scenario unless the engine in question is in a relatively mild state of tune mechanically (compression, cam, etc.). Air fuel ratios of 10.5:1 - 12.5:1 are generally considered safer for maximum load conditions on an engine that may be approaching conditions of pre-ignition under high load demand. The additional fueling provides significant cooling that staves off the ill effects of high combustion chamber pressures and temperatures. Of course this comes with a trade off of much higher exhaust emissions of CO and HC. These will ideally mostly be cleaned up after the fact by a catalytic converter but the CAT typically won't tolerate those conditions for an extended period of time without suffering heat-induced degradation. That said, modern metallic substrate CATs do tolerate those conditions much better than ceramic substrates.