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V3 MicroSquirt® - QuickStart Guide

Air Cooled Engines and Other High Temperature Applications

To use your MicroSquirt® controller with an air cooled engine, you will have to decide where the best place is for the coolant sensor: in the oil, or on the cylinder head (CHT). There are various arguments for and against using either CHT or oil temperature as the 'coolant' temperature input on air cooled motors. A lot depends on whether the motor is substantially oil cooled or not. Since the CTS input is used for warmup enrichment, you want something that responds fairly rapidly, so this is highly engine-dependent.

For high temperature applications (i.e. air cooled engines with a CHT), with your MicroSquirt® controller you can set the #unset EXPANDED_CLT_TEMP by changing:




Then the upper temperature limit should be 600 degrees F. This is from the INI notes:

; FAHRENHEIT (Expanded/Normal):
;     Low Limit: -40F/-40F
;     High limit: 600F/300F
;     Low danger: 150F/50F
;     Low warning: 200F/150F
;     High warning: 325F/200F
;     High danger: 350F/220F
However, you have to calibrate the thermistor table(s) appropriately.

However, note that the tuning software limits the temperature range. The thinking is that if you are at an extreme it is probably a bad or missing sensor, so it goes to a default value. This isn't a big deal in TunerStudio though, you can change these limits in the ms2ReferenceTables.ini file. You can adjust these limits if EXPANDED_CLT_TEMP is set. Currently only the CLT sensor respects EXPANDED_CLT_TEMP, the IAT does not.

IAT min = -40, max=350 if outside that range it goes to 70 CLT min = -40, max=350 if outside that range it goes to 180 CLT with EXTENDED_CLT_TEMP min = -40, max=400 if outside that range it goes to 350 default

Here is the section of the ms2ReferenceTables.ini file that controls that:

; tableLimits (optional) = intentifier, min, max, defaultVal
; will set the default value if value is outside the min and max limits.
tableLimits = 001, -40, 350, 70
tableLimits = 000, -40, 400, 350
tableLimits = 000, -40, 350, 180
So the user can just edit the one line in ms2ReferenceTables.ini using notepad.exe or something similar to change it from:

tableLimits = 000, -40, 400, 350

to something like:

tableLimits = 000, -40, 600, 350

or similar, and it should work once the user re-burns the table.

The additional problem with very high temperatures is that the difference in voltage gets very small (and your MicroSquirt® controller can only read discrete voltage steps, about 5 milliVolts for each of the 1023 steps from 0.00 to 5.00). Also, if the volts at the processor pin gets very near 5.00 (~4.98) the values defaults to a safe value because it assumes there is an open circuit.

The ADC count result from the voltage divider circuit (above) is:

ADC Count = V/Vs = ADC max count * (Rs/(Rb+Rs))
      = 1023 * (Rs/(Rs+2490)) for a MicroSquirt® controller (10-bit ADC)

Here is a simple ADC count calculator that shows the number ADC steps for a given bias resistor and sensor resistance. If the ADC count values are very high or very low at your sensor's resistance (at a given temperature), the temperature range may be restricted:

Bias resistor:
Voltage supply:
Sensor resistance: Ωhms ADC resolution:
ADC Count:

For air-cooled use, side of the argument says to use the CHT over the oil, as the oil takes over twice as long to get to operating temperature than water in a water-cooled car does. The engine does not need to run rich for long periods, only enough to keep the car driveable while it is warming up. Once the cylinder head is up to temperature, the car is usually quite driveable. For an air cooled engine you can drill and tap into a fin in the head for the CHT sensor.

The other side says that it does not matter if the oil warms more slowly, you can just set the warm-up enrichment to come off at a lower temperature. In that case, having the coolant sensor fitted in the oil (sump) will work nicely. Search the archives for extensive discussions on these points. It is your decision.

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
your MicroSquirt® controller and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a your MegaSquirt® or MicroSquirt® controller is legal for your application.
©2004, 2011 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of your MicroSquirt® controller boards from Bowling and Grippo.