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HOWTO: Nitrous/Water Meth controller with fail safe multiple tunes

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Ever wanted to have nitrous automatically controlled with no external controller? What about staging a 2nd fuel pump? How about water meth?

Using the following instructions you can automatically stage nitrous to turn on at a specific rpm/map pressure and only in a specific tune. You can also wire a feedback signal back to the PCM to automatically change the tune to one with different fueling/spark depending on the application.

For other applications please read this guide.

Eg for water meth you may want to use a low level switch to automatically switch to a tune with more fuel and less spark when the tank runs dry. 

Step 1. Open the custom operating system wizard and select Multi Tune


Next pick "Flex Fuel Sensor" and select "Custom Voltage Input Table"


Next step is to add the IMCC enable/disable rpm setpoints to the multi tune.


Also add these scalars to tune 2,3,4 on the next page.


This will allow you to configure which tunes the nitrous/water meth is switched on. The default is 3800rpm.

Finally we need to enable the IMCC output auF1703 by setting it to 1


Nitrous Setup

To configure nitrous to turn on in tune 4 and during launch control to assist with spool set the following values. Make sure to set these values for all 10 tunes, both the 98 (nitrous) set and the e85 (no nitrous) one.


To confirm the above is working you can datalog the "IMCC" value in the datalogger, a value of 1 means 0v, a value of 0 means 5v is available at pin B03/1106. Note that when the IMCC is referred to being closed the PCM supplies an open circuit, when the IMCC is open it supplies 5v.

Now that the IMCC output has been configured to setup the nitrous output we need to setup the no nitrous tune (98) with normal spark values. This can be done by adjusting the "Tune 1 Base Tune for 98" spark map.

Next we need to tune the nitrous spark/fuel maps. This can be done by tuning the "Tune 1 Base Tune for E85" spark and fuel maps.

First we need to set the stoichmetric ratio correctly. In our case we are using 98 octane fuel so we need to set the entire stoich table toe 14.64, this is as we are repurposing the flex fuel tables to use with nitrous and not alcohol content.


Next step is to configure the analog input used for feedback to toggle the nitrous maps on/off. To do this we set the high/low map usage to toggle at 1.1v to 100% of the e85 (nitrous on) tables. Below 1v we get 100% of the 98 (nitrous off) maps.


The final step is to wire up the nitrous to the IMCC output and then wire up the feedback to the Rear O2 sensor.

The recommended way to do so is to cut the nitrous system when the clutch is let out, and also have a master override switch in the cabin. This means even if you have a nitrous map selected (tune 4) from the steering wheel, you can still disable the nitrous system if needed.

Using the schematic below we can see the following parts are needed.

  • Cabin override on/off switch with normally open contacts.
  • Toggle push button switch with normally open contacts to be installed on the clutch bracket.
  • A 5v relay with normally open contacts that is rated to enough current to power the nitrous solenoid.



Note. You do not have to wire nitrous enable feedback to the PCM, and can instead assume when the IMCC output is on, nitrous is also on. This means skipping the steps to enable the flex fuel logic and instead modifying the spark/fuel values in tune 4 98 octane directly, and not in the e85 (nitrous on) tune. This is a simpler way to set the system up.

If the nitrous is not actually on and tune 4 is selected, then the tune will simply run retarded spark and rich fueling which will not risk the engine so this may be satisfactory.

Water Meth:

If you are planning to install a water meth system with a low tank level cut out switch follow the instructions above for nitrous and instead use the following schematic which uses a second relay for the low level switch


Note: You do not have to wire a water meth low level switch and can instead rely on the fact that water meth drops the charge temperature, knowing this you can datalog the expected temperature drop, if its not within the expected range you can add significant timing retard to the IAT spark adder table auF0220, this will enable the engine to still safely function in the event of a water meth failure. The recommended way however is to use a flow sensor or low level switch.

Staged Fuel Pump:

The above logic can also be used to trigger a staged fuel pump or any other accessory which requires a window switch.




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  • 4 months later...

I've got a water meth kit (Aquamist HSF3) that I'm planning to install soon. It can output 5V when the tank is low, or it can invert the signal and switch from 5V constant to 0V when the tank is low.

So given that info,  it seems I could just run a dual tune with a switch style input, run the fail safe wire to the rear O2 through a 10M ohm resistor and after tuning for the timing, extra fuel and extra boost, let er rip tater chip.

Anyhow that's my thoughts, anyone care to share their 2c?

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  • 6 months later...

Ok just for reference the on condition is rpm > rpm setpoint AND map > map_sp it then latches on until rpm drops below the setpoint. So this means it will stay on during decel which may be undesirable. 

From what we can tell only one customer has ever used this feature in the 4 years it has existed. So we want to minimise support if it doesn't do what people want. 

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That's exactly how it should be. In fact I'd be the opposite and have rpm always armed and trigger straight off map. 

I'm waking up to the idea that e85 is being priced stupidly lately. When using it in a daily application where 95% is cruising, the water meth is looking like it would be a better proposition economically.  

Please keep it. 

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