Since the LC-1 approach to richening the engine fuel to air mixture does so many good things but is a bit of work to install, I wanted to see if there was an easy way to use the stock Narrowband sensor and "pull" it a little richer than it is designed to run normally. It turns out that after some measurements, and tests, it is possible to do just that--get up to 6% richer fueling using the stock O2 sensor. Here's the story ...
A couple of months ago I installed a second bung in the exhaust of my R1150RT. I wanted to have both a Wideband O2 and a Narrowband O2 installed and running at the same time. The idea was to let the Motronic use the Narrowband in the usual way while I recorded the results of Motronic/Narrowband by monitoring the Wideband. In other words using the Wideband to spy on the Narrowband. Second Bung Install
The Wideband showed that the Narrowband/Motronic pair kept the Closed Loop fuel right at lambda=1 (AFR 14.7:1).
Then I wanted to see if there was some kind of circuit that could be inserted in series with the Narrowband to richen the mixture without having to add an LC-1. The Narrowband sensor is well designed and has a big change of voltage right at Lambda=1. Just a bit leaner and its output drops to 100mV. Just a bit richer and its output jumps to 800mV.
The exact rich output voltage increases from about 700mV at 14.6:1 to about 900mV at 13.8:1. That is a small voltage change for a large mixture change, which means it is too small a change for the Motronic to work with.
Compounding matters, in the rich zone, from 700 to 900mV, the voltage changes as the exhaust gets hotter from higher engine load. Another way to say the same thing is that the voltage that corresponds, for example, to 14.1:1 (slightly rich) changes with engine loading. Compounding the problem further, the Motronic has a clever circuit that figures out if the voltages have been shifted. It uses that circuit to ignore simple shifts of the O2 signal.
I also built a test harness which allowed me to add circuits in series with the the stock O2. I tried a dozen different ideas, including some patented circuits from nightrider.com that work on Harley Davidsons. Nothing worked.
Over the last year, I've gotten to know the owner of nightrider, Steve Mullen. One of his Harley O2 richening products has a microprocessor inside. As designed for the Harleys, it didn't work either. But a couple weeks ago, we discussed and agreed on a different algorithm. Steve coded it up and sent me a new "chip" just for BMWs.
To make a long story short, the new "chip", with some other circuit changes, will pull the Narrowband sensor several percent into the rich zone. The way it works is that the microprocessor module measures the stock O2 sensor voltage, filters it to reduce noise, and then alters the voltage transfer function so that the signal it sends to the Motronic looks like a normal Narrowband sensor that is switching at a richer lambda (range is 0-6% richer).
As it is designed (proto with many extra wires, below) you unplug the stock O2 and plug this device between the two stock connectors. There is a ground wire to connect also. There is already power in the O2 sensor cable so no power connection or new fuse is required. The final product will be about 1/2" x 1" x 3" with two OEM connectors, plus one ground wire.
I have test ridden this circuit at 13.8, 14.1, 14.3 and 14.45 and will post some charts and other data tomorrow.
For those interested, PM me. We will build a couple of modules for pre-production trials for R1150XX with Motronic MA 2.4. Assuming everything works it will then go into production.
Note: If the connectors are made compatible, this would work on any R1100, R1150, R1200 (2 needed).
Prototype test cable harness and small O2 processor device. Final product would be just the processor, two thin cables with OEM connectors and a ground wire—a simple plug 'n play solution.