I have been looking into the option of biasing the air intake temperature sensor to induce a perceived drop in temperature, and a corresponding decrease in air-fuel ratio.
This is basically the "booster plug" concept, but DIY and potentially adjustable.
The concept has been commercialised and can be bought here: Booster Plug but it is a one-size-fits-all solution, and fairly pricey for what it is (IMHO)
Initial work on a DIY version was inspired by Davide Cavaliere, with notes here: Monocilindro Blog and more insight from here: F800 Riders
Essentially the idea is to take advantage of the fact that the ECU tries to maintain some target air-fuel-ratio (AFR) and uses a measure of the air temperature to estimate/predict changes in air density. Assuming the ECU is computing or using a look-up-table of Boyle's Law or the Ideal Gas Law, or equivalent:
This might look something like:
p V = n R T i.e. (Pressure) x (Volume) = (amount of gas) x (some constant) x (Temperature in Kelvin)
For the ECU, the term 'V' is the stroke volume of the engine, so it's fixed, and 'R' is a physical constant, also fixed..
Rearranging to express the amount of gas in terms of the other factors:
n = ( p V ) / ( R T )
So we see that the amount of gas in the cylinder increases with increasing pressure, and increase with decreasing temperature.
The AFR is a weight-by-weight ratio, so when the ECU perceives adrop in temperature, it will assume there is more oxygen per stroke-volume, and apply a corresponding increase in the fuel quantity.
If this temperature drop is biased a little, with some added components, then the ECU will add more fuel than it was designed to, making the ratio a little more rich than it 'wants'.
The entire effort is based on the idea that these machines run a little lean "out of the factory" in order to meet emissions regulations - and that a small increase in fuel provides a better driving experience.
The commercial version, and the version described by Davide, both prescribe a fixed adjustment, of about 6%. In the post here. Unfortunately the temperature sensor on the Monster 796 is tucked away under the tank and a bit difficult to get to - so experimenting with it becomes a bit of work. So I plan to use a little wiring harness to bring the interface out to the frame so I can more easily swap out components and experiment a little bit.
![226383]( "226383")
I plan to attempt a few different circuits:
The estimated performance of three of these (short-circuit, 2.5 kOhm, and 5.0 kOhm) are shown in the plots below. The top plot shows how it would estimate temperature, versus the true temperature - where the nominal is correctly estimating the temperature and the two modifications (NTCT: negative-temperature-coefficient thermister) show an estimate that is lower than the true temp. The middle plot shows precisely how big the bias is, and how it varies over temperature. Because I cannot perfectly match the beta factor of the thermister, it is not perfectly content over temp. The lower plot shows the expected change in fuel, assuming that the ECU attempts to keep a constant air-fuel-ratio across temperature.
![226384]( "226384")
I plan to use a standard OBD2 diagnostic tool (and probably JPDiags M3C) to check that the intake air temperature estimate is being biased as expected.
But, of course, measuring that the injectors are actually responding to this change would be far preferable.
As I move along with the project I can update here with the status/progress, in case anyone is interested.
Has anyone any thoughts/advice on this - does it look like a sensible approach?
Comments/warnings/advice are welcome.
This is basically the "booster plug" concept, but DIY and potentially adjustable.
The concept has been commercialised and can be bought here: Booster Plug but it is a one-size-fits-all solution, and fairly pricey for what it is (IMHO)
Initial work on a DIY version was inspired by Davide Cavaliere, with notes here: Monocilindro Blog and more insight from here: F800 Riders
Essentially the idea is to take advantage of the fact that the ECU tries to maintain some target air-fuel-ratio (AFR) and uses a measure of the air temperature to estimate/predict changes in air density. Assuming the ECU is computing or using a look-up-table of Boyle's Law or the Ideal Gas Law, or equivalent:
This might look something like:
p V = n R T i.e. (Pressure) x (Volume) = (amount of gas) x (some constant) x (Temperature in Kelvin)
For the ECU, the term 'V' is the stroke volume of the engine, so it's fixed, and 'R' is a physical constant, also fixed..
Rearranging to express the amount of gas in terms of the other factors:
n = ( p V ) / ( R T )
So we see that the amount of gas in the cylinder increases with increasing pressure, and increase with decreasing temperature.
The AFR is a weight-by-weight ratio, so when the ECU perceives a
If this temperature drop is biased a little, with some added components, then the ECU will add more fuel than it was designed to, making the ratio a little more rich than it 'wants'.
The entire effort is based on the idea that these machines run a little lean "out of the factory" in order to meet emissions regulations - and that a small increase in fuel provides a better driving experience.
The commercial version, and the version described by Davide, both prescribe a fixed adjustment, of about 6%. In the post here. Unfortunately the temperature sensor on the Monster 796 is tucked away under the tank and a bit difficult to get to - so experimenting with it becomes a bit of work. So I plan to use a little wiring harness to bring the interface out to the frame so I can more easily swap out components and experiment a little bit.
I plan to attempt a few different circuits:
- a simple short-circuit, keeping the bike in 'stock' operation, no bias to the temperature
- a modification with a 2.5 kOhm thermistor, with a 3950 beta factor, which should provide somewhere around a 5.0% increase in fuel richness;
- a modification with a 3.3 kOhm thermistor, again 3950 Beta factor, providing around a 5.7% boost in fuel
- a modification with a 5 kOhm thermistor, again 3950 Beta factor, providing around a 7.5% boost in fuel
- and possibly final modification, with a 10 kOhm thermistor, again with a 3950 Beta factor, which should provide somewhere around a 12% increase in fuel
The estimated performance of three of these (short-circuit, 2.5 kOhm, and 5.0 kOhm) are shown in the plots below. The top plot shows how it would estimate temperature, versus the true temperature - where the nominal is correctly estimating the temperature and the two modifications (NTCT: negative-temperature-coefficient thermister) show an estimate that is lower than the true temp. The middle plot shows precisely how big the bias is, and how it varies over temperature. Because I cannot perfectly match the beta factor of the thermister, it is not perfectly content over temp. The lower plot shows the expected change in fuel, assuming that the ECU attempts to keep a constant air-fuel-ratio across temperature.
I plan to use a standard OBD2 diagnostic tool (and probably JPDiags M3C) to check that the intake air temperature estimate is being biased as expected.
But, of course, measuring that the injectors are actually responding to this change would be far preferable.
As I move along with the project I can update here with the status/progress, in case anyone is interested.
Has anyone any thoughts/advice on this - does it look like a sensible approach?
Comments/warnings/advice are welcome.
