Head Temp Gauge

Accurate temp control is always in important factor in EFI as may functions are controlled by changing engine temp.  We have developed a Chip-based, highly accurate, (1%) that reads up to 600f degrees.  Why is this important?  Fuel and Temp (EFI) using temp to increase and reduce timing and fuel based on engine/manifold temp.

We first attacked the problem like this:

  1.  Non-GM Thermistor sensor installed in the head port (GM must have thought it was a good location)
  2.  Chip is written where we can calibrate to any EFI system or a change in the thermistor.  This chip can also run the factory head temp gauges if modified.
  3.  Uses its own power regulator.  We have used direct power, but the power of the Corvair is so noisy, that it disrupts or destroys the chip.  I could foresee the an early model with a gen being very destructive to a chip without filtered power.

Why don’t we use it?  COST AND SIMPLICITY.  Brown EFI uses temp measurement to the extent to run its systems, but could be used for much more.  We tried some experiments using thermocouples, but had to be closely located to the sensor.  Due to the complexity and higher cost of the Thermocouple, it was abandoned.

We used a regular temp thermistor (water gauge sensor) which is accurate to about 275f  located strategically to mimic water temps which fool the computer.  We were able to do this for less than $20, so there you go.

I can see as the Corvair Thermistor becomes more and more scarce, that this would be a good replacement. I estimate the retail cost of thermistor to gauge, with internal power supply to be approx $99 and Monza head temp using x27 Stepper motor would be around $199 (Self-contained).  I have a hundred hours in programming on this thing, so I’m keeping the code for a future market that may include more than Corvair.

This project started life as a head temp sensor for EFI computer and evolved into the Monza head Temp gauge.

Monza Head Temp Gauge

Monza Head Temp Gauge (Prototype) using an X27 Stepper motor (500f, but would read all the way to 572f)

Update: 6/6/17 –  Clark’s Corvair is selling the Vairtrix thermistor which I think is the same design as above (Arduino) except using a Thermocouple instead of thermistors.  It will be interesting to see how it works in the long term and how the Dirty Corvair power treats the chip.  (watching with interest).

Update:  7/18/17 – We have been using the Prototype in the pic above since Apr with pretty good results. Having the unit and your engine properly grounded is so important as I have already destroyed a chip. I currently have it accurate within 5 degrees, but maybe overkill.  The higher the temp, the thermistor resistance band tightens and small resistance changes makes the pointer move a little at higher temps… this is normal.    I re-wrote the code to monitor from 100f-600f degrees and took a little wiggle out using averages…..We will see.  Changed the title of this page from EFI temp to Monza Late model head temp Gauge.   I have gauge faces ordered and waiting to see how they will come out..getting a little excited.  @ted

Thermistors are designed, 3 prototypes built and are 1% variance. The gauge is accurate within 5 degrees, so its really overkill for this project.

DRAFT GAUGE INSTRUCTIONS:  Do not open back cover.  This will void warranty

Remove/Drop dash from your late model enough to insert the Gauge into the Monza dash.

Red Wire – Switch 12VDC (On when key is on)

Black Wire – Ground (Ensure this one is well grounded)

Green Wire – Sensor at engine (You will need to run an 18-22 AWG wire from the engine compartment to rear of dash).

Sensor – Ground black wire and connect green to green wire that goes to dash.  Screw the 3/8-16 into the Corvair head on the driver’s side front.  Do not over tighten and be careful not to damage threads or wiring during assembly or removal.  The depth of the sensor is the same as the snap-off indicator that the Corvair engine uses to prevent damage to the sensor or the head.  Notice its a 2 wire (w/ground) and was added for the Megasquirt people as a 2 wire is specified and required for those applications.

Set Pointer:  The last thing is to set the pointer to 100f degrees.  Power the gauge and remove the sensor wire, this will hold the needle at 100f.  If it’s not perfectly aligned with the 100f degree mark, slightly move with your finger until aligned.  Remove power and re-apply power and should move back to 100f degrees.  Connect sensor wire and installation is completed.

Insert the gauge into the dash and reinstall the dash, ensuring the dash grounds are connected.


Normal Operation:

Gauge will go to “Zero” position when gauge is powered on to set/calibrate 0 degrees.  After calibration, the pointer will indicate the temp of the engine.  If the engine is less than 100f degrees, it will show 100f degrees.  Temp and Gauge will show head temp in 5 degrees increments.


Temp changes when I turn on fan, headlights or brake lights?  Ground is not proper to dash.  Ensure a good ground exist on the vehicle (from battery ground to engine to body to dash)

Temp reads max:  Sensor wire grounded/Sensor Failed.

Temp reads 100f when the engine is obviously hotter than 100f:  Sensor wire disconnected/sensor failed.  Test by grounding when you should move to the max (600f) position.

Gauge does not move when powered:  Ensure you have 12VDC and ground

Sensor check – Should read 10000 ohms at 75f degrees

You will need to send your LM Monza clock blank for conversion to a temp gauge.