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Custom OS For Dual Fuel or Dry NOS Example


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Our daily driver is a Turbo Territory running a dual fuel system. I have been using this to test the Custom OS for many months but have finally gotten around to wiring in the LPG "on" signal into the rear O2 sensor and will show what you need to do to configure this. This would clearly be very useful for people running NOS as well with the NOS "on" switch supplying the rear O2 sensor. One of our customers (Independent Motorsports) has been doing this for quite a while: 


Getting Started/Prerequisites

Before we start if you configure your you will need to license your Custom OS and leave the setup as default. You may only need to modify the stoichometric table (auF100014) to reflect your fuel. Because my base fuel was 98 I left the table alone:



I will be modifying this table very soon :) 

Now using the guidelines in this post:


I built the Custom OS with my 98 tune in both the in the base tables and the Flex tables in order to ensure the car ran properly as both 98 and LPG injectors were "flow matched". Installed this tune in the car and it ran well for many months testing the Custom OS. 

Rear O2 wire up, or OMG look at the Fuel Trims :) 

After the rear O2 sensor was wired up the car behaved exactly as it used to on 98 but as soon as the LPG system activated the PCM started pulling masses of fuel out. Without the rear O2 sensor being connected the voltage on the rear O2 sensor was showing 0.02V and similar when the LPG system was not activated. When the LPG system activated the rear O2 was being supplied with 13.8V (battery voltage) and the reading on the rear O2 sensor was 1.667V  (5.0/3.0 V for the curious). The car was pulling over 10% fuel. Remember the Stoich table earlier? 

Why was there too much fuel when LPG was on?

Lets look at why this is happening. So starting with the Voltage to Alcohol conversions table:


What Ethanol contend would the conversion table be supplying at 1.6666V? 

Moving along you can see that the ethanol content would be 0.3333. Now going back to our stoich table the calculated stoich would be ~12.72. Now given these injectors were flow matched you can see why the car was pulling over 10% fuel out as 14.64/12.72 ~ 1.15 so the car is getting an extra 15% fuel. 

Your could easily fix this by making the stoich table all 14.64 but lets have some fun and take advantage of this.

Out with those poxy 64Lb injectors, in with the nice shiny 95lb injectors

Just for fun and to demonstrate what can be done with these tables I am going to turf my GT500 petrol injectors (Bosch 0 280 158 117, running about 4.5 bar pressure to get them up to the flow of the LPG injector) and replace them with (Bosch 0 280 158 040  approximately 95lb injectors at 4 bar). Note: there is more to this than swapping the injectors you have to get the offsets to be mighty close to the LPG injectors as well as the high and low slopes but in this example I knew the offsets were close enough to get it running well).

LPG injectors flow about 64 lb/hour and petrol about 95lb/hr. When the LPG injectors are on we will need to ensure the the pulse width is the same as it was before. 64/95 = 0.673. In order to achieve this we want the Stoich value at 0.3333 ratio to be 14.64 * 0.672 = 9.852. How cool is that, one of the values used for E85 Stoich!  Anyway I am happy to modify the standard stoich table. Next I am just going to change my Voltage Conversion table to use 0.0 as the amount alcohol when the switch is off (LPG off) and 85% when the switch is on:

image.thumb.png.74bc9484cdf1888b94a15a943083f32b.png0.1 is > 0.02 so it contains the 0 point and 0.5V is just a point that ensures the switch is on (as the voltage is 1.6667V). As soon as the switch is on the Stoich value will use the 0.85 point (9.765) in the Stocihometric table. This is what I did to the standard conversion table to get it to align up:


Now the value for 0.85 will be 9.852.

Next the Blend Tables need work so I have these configured:


 So now when the LPG system switches on, the Voltage conversion table gives our alcohol content as 0.85, Stoich is 9.852 and the high MAPs will be used.

How did it go?

In a nutshell it worked pretty close to perfect and I have the fuel trims on both petrol and LPG well under 2% again.

And now I can compensate for the lack of injector flow using the Base Fuel table in the LPG tune:


and have a standard table for running on 98.

Dry NOS.

Dry NOS system you would modify the Voltage conversion table to give you the increased fuel for when the NOS system kicks in. So, for example if you wanted to have an extra 20% fuel you would modify the table so that Stoic was 14.64/1.2 = 12.2. i.e. you would want a Alcohol content of approximately 0.43.

You can do a similar thing with the base fuel that I did with the LPG injectors to change the amount of fuel in different points in the rev range to vary the required fuel above/below the 20% extra we have already done.


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  • 2 months later...

hi ebbarra, 

If you need a sensor to trigger the change of MAP then we are looking at the power steering. We are also investigating the Cruise Control switches to allow customer change over without a sensor input. 

Is the car an automatic or an manual?  Clutch pedal switch may also be an option as I believe it is in the loom but not wired up for automatics (not confirmed).

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