Remember: Closed Loop is when the bike is in part throttle cruise (which is most of the time/regular riding) and needs to be the most efficient. CL "targets" the most efficient 14.7 parts of air to 1 part of gas (stoic). Clean emissions and clean burning. Mods don't affect this because it is still reading what comes out of the engine (regardless of mods) and adjusting for its target. Reads the O2, adds or subtracts a little fuel, reads the O2, adds or subtracts... etc. In theory,
CL operation doesn't need any fuel map at all, it's just reading output and adjusting input to get whatever target output you set.
Remember the O2 is in the tail pipe,
after combustion, so when the motorcycle needs power (more fuel) for big fast throttle changes like WOT. Things are happening too fast to wait to reference the O2 for whats already happened in combustion and adjust for the richer mixture you need for that power
right now. You have to know how much fuel to put in
before its burned and read by the O2. So if it can't use an O2 sensor, how does the ECM know how much fuel to give the engine?
The ECM ignores it, and directly references a pre-set fuel map. Open Loop operation.
The fuel map is a Load/Rpm map that is preset for the ECM to
immediately reference at whatever Rpm/Load its at, when you whack open the throttle. That preset map is built by dyno testing the engine through the
Rpm range at different
loads and is adjusted
manually for the bike to make the most power, 10:1-ish (note: NOT 14.7:1 part throttle cruising). THIS is the fuel map everyone talks about after changing their muffler. CL operation at part-throttle cruise is unaffected because, re-read that part.
OK Aaron!
With Open Loop Learn on, the ECM looks at the O2 sensor during OL to see what the engine is putting out. OL operation is still too fast to adjust like CL operation does, but can see repeated results and can modify the OL fuel map to be more accurate that point, so the next time you are at that point of the map (still in OL) it can add that little bit immediately from knowing its history without having to wait for a reading. Lets say you whack the throttle open at 3500RPM/ 90% throttle and it sees a lean mixture. It's too late to adjust input, so the ECM will add a little fuel to that spot of
the fuel map. So the next time you are at 3500rpm/90% throttle, it will just refer to the now modified map and give the bike what it needs, from what it told it last time, still without waiting on the O2. The O2 is still reading an modifying for the
next time its at that spot of the map.
Beautiful right? Why'd you want to handcuff such a simple system?
*Still greatly oversimplified. Some things are "wrong" but for off-point reasons. K.I.S.S.
