Finally figured out what to do about the enrichement sender. The megasquirt requires a "coolant" sensor. This input is used by megasquirt to enrich the fuel mixture when the engine is cold. There is a temperature versus percent enrichement table in the megasquirt configuration window.

I though about what to do here for a while. One idea was to use the oil temperature, however discussing this with other folks we concluded that the oil temperature takes too long to react to the engine temperature. Since the 911 engine is aircooled and there is no water coolant what is usually done is a cylinder head temperature sensor is used to control cold enrichement.

While sitting in my workshop today, my eyes fell upon an old power supply from a PC. Looking closely I saw it had a sensor that is used to monitor the air temperature in the PC. This is probably used to increase the speed of the fan when it gets too hot. This sensor is a thermistor. The resistance varies with its temperature. This is exactly what the coolant temperature sensors use.

 In the true spirit of DIY I decided I would build myself a sensor for this function. I cut the sensor off the power supply and measured the resistance at room temperature and at 100 degrees Celsius. It looked like a close match to the sensor that the Megasquirt expects.

My idea was to mount this to a standard ring terminal. Here are the two parts:

The sensor was inserted into the ring terminal. I needed to make good thermal contact between the thermister and the ring. I could not squeeze the connector on the sensor as is normally done with a wire, because it would crack the thermister.

One of the two wires from the thermister was also placed inside the ring terminal with the thermister and would be soldered to the ring. This way one terminal is grounded and the other goes to megasquirt. To get good thermal contact between the thermister and the ring, I poured solder on top of the thermister. This provided a good thermal path between the thermister and the ring. You can see below that the thermister is covered with solder.

The wire on the other end was glued to prevent  strain on the thermister. The place I chose to mount my sensor was on one of the studs that comes up from the intake. This bolts down the intake runner to the intake side of the cylinder head. I figured this is a good enough place to monitor engine head temperature.

After hooking it up, I started up the laptop and hooked it up to the megasquirt ECU. Under the real time display in the "megatune" software I could see that the "coolant" temperature was reading 53 degrees F (Actual temperature was 40F, so it wasn't too far off). I started up the engine and within five minutes of idling it reached 100 degrees F. After I shut the engine off it went all the way upto 120 degrees F. So it looks like this works.

The absolute temperature reading isn't really that important here. All that is needed is to tell the megasquirt how much percent enrichement to add based on what the sensor reads. This will be fine tuned once the fuel injectors are installed. I am still running on CIS upto now.