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EFI, AI, and O2 sensor Q

3K views 13 replies 8 participants last post by  petronila 
#1 ·
Like most of us I imagine, I cut my teeth on carbureted bikes, and this EFI stuff is new to me. I am planning on new exhaust for my '09 T100 (2-1 and no airbox) but am a little puzzled about the practice of elimination the AI and the O2 sensors. As I understand it the AI is mostly an emissions thing and with a no-airbox setup it translates to some popping of any unburned fuel. Eliminating the AI just reduces this but has no purpose beyond that. Is this correct?

Which brings me to the O2 sensors, which are also typically binned in a hurry (required to be removed for the TTP map). It was my understanding that O2 sensors read the oxygen percentage of the exhaust and enable the computer to adjust the fuel mixture accordingly. Is this not one of the advantages of EFI?? Living in New Brunswick I can be in the "mountains" one weekend and at the ocean the next, seems to me that the ability to have the bike adjust accordingly is in my best interest. Why is it they are readily removed, these seem to have a far more useful function than the AI.

Some enlightenment would be appreciated.

Gary
 
#2 ·
As I understand it the AI is mostly an emissions thing and with a no-airbox setup it translates to some popping of any unburned fuel. Eliminating the AI just reduces this but has no purpose beyond that. Is this correct?
Yes, roughly. The AI burns off unspent fuel in the exhaust by feeding in air directly from the airbox. This causes popping on deceleration as you say, which is an annoyance to some, however there is the other downside of AI too, namely causing the exhaust headers to run hotter than they would otherwise, which leads to rapid blueing of the pipes. Overall, the purpose of the AI is purely to satisfy emission regulations and has no benefit to the rider/owner. On the contrary, it is an inconvenience - the injection tubes even make it more difficult to get at the spark plugs.

Which brings me to the O2 sensors, which are also typically binned in a hurry (required to be removed for the TTP map). It was my understanding that O2 sensors read the oxygen percentage of the exhaust and enable the computer to adjust the fuel mixture accordingly. Is this not one of the advantages of EFI?? Living in New Brunswick I can be in the "mountains" one weekend and at the ocean the next, seems to me that the ability to have the bike adjust accordingly is in my best interest. Why is it they are readily removed, these seem to have a far more useful function than the AI.
On these bikes particularly, the O2 sensors are another item which exist purely to comply with emission regulations. The O2 sensors work over only 6% of throttle opening, which is the minimum requirement for the manufacturer emission tests. Beyond that the ECU ignores them. 6% of throttle opening corresponds to about 1500rpm (roughly), which is, for example, cornering speed. On my own bike, when I had it new in 2010, at this sort of speed and at the point when the ECU closed its ears to the O2 sensors, the mixture would suddenly richen and the bike would lurch forward. The overall effect was like having the throttle on an on/off switch, and cornering at slow speed was very precarious - I had to feather the clutch on most corners. All that was cured on the day that I removed the sensors.

With regard to being in the mountains on one day and at the ocean the next, there are other sensors on the bike which take care of things like that such as the Barometric Pressure sensor and the Air Intake Temperature sensor. The ECU is also adaptive. What you have to keep in mind about EFI is that the fuel mixture is not controlled by air and fuel screws or jet sizes, it is controlled electronically by means of pulse width (open/closed duration) of the injectors.
 
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#4 ·
Thanks for the reply that was exactly the info I needed. I also have the same on/off throttle in slow cornering, so am very pleased to learn that that will disappear with the exhaust upgrade too. Cheers.
 
#5 ·
With regard to the jerky throttle, there are other factors which may cause you the same problem, or make the throttle jerk worse than it really is. The chain should be kept adjusted to its minimum slack, for example the manual says 20-30mm (off the top of my head), so the best adjustment to relieve throttle snatch is the 20mm end of the range. Also the throttle cable slack should be kept at a minimum.

In my case the bike was brand new with only 5 miles on the clock and was properly set up, so my throttle snatch was completely down to the O2 sensors. When I got to remove them it was like riding a different bike, very smooth on the throttle response. Some on here say that they have cured the throttle jerk merely by removing or disabling the AI, others have found that the chain and throttle cable adjustments have cured it, so your milage may vary.
 
#6 ·
Don't know which model years and which country(s) are affected by this, but: removing SAI without also removing O2 sensors. Will mess up the bike's fuelling because the algorithm used to "correct" O2 voltage as recorded with SAI-on, stays in place. (the ECU tries to assess O2 sensor voltage based on "raw" combustion gases, without the additional burnoff via SAI, thus a correction is needed when SAI is operating). With SAI-off / removed, the O2 sensor voltage correction still happens such that closed loop adaption logs are impacted. Resulting in improper adaptive fuel trims.

So, if removing SAI its best to remove O2 sensors as well.
 
#7 ·
But I thought SAI introduces air after the cylinder combustion stroke, so that secondary combustion only happens as the hot gases are leaving the cylinder. How does that have any affect on Air/Fuel mixtures during the primary combustion stage?
 
#14 ·
Depending on what mods have already been done... pipes done ? AI is done. I'd be inclined to now spend the extra quids to do: O2's removal, intake side (minor, enough for TTP#3), buy and load TTP tune. Have TuneECU available as well. Bike should then be good for traffic + sound + xtra fun rural rides on weekends.
 
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