A friend emailed me asking for clarification in the “big step” update:
Did you get the stepper motor working correctly? In the vid it went to full open then I wasn’t sure if that was the secondary’s @ 60% or the mains.
I replied:
Yes, working well. At least working as well as I can test with it removed from a running engine.
The video depicts the subthrottle’s opening all the way under MegaSquirt control. This, via the cam, opens the main throttle a bit. MegaSquirt then works it back towards about 60%, by which time the main throttle is closed, as I manually adjust the CLT knob on the stimulator. CLT is megaspeak for coolant temperature, intended to represent the running temperature of the engine.
The engine would (likely) not warm from freezing to 160F in 15 seconds, so the throttle would close in many smaller steps instead of the 3 or 4 big jumps shown here as I manually turn the CLT knob on the stimulator. Also, the actual throttle opening needed for warmup will probably be less than the full amount provided by the subthrottle cam. I wont know that for sure until it’s on the engine and the engine is running
Perhaps obviously, neither Buzz nor the VW have “coolant”, at least not a self-contained recirculating coolant. Sometimes people use oil temp for this, but the engine is usually well into operating temperature before the oil gets very warm. Cylinder head temperature is a better representation of CLT on aircooled engines. On Buzz, I JB Weld’d the CLT sensor between some cooling fins cylinder #2.
I was lucky that the fins on Buzz’s head were spaced similarly to the size of the default MegaSquirt CLT sensor. What’s more appropriate, especially for a VW, is something more like this cylinder head temperature sender which goes under the spark plug like a washer. Trouble is that this is a thermocouple, which produces a small voltage based on temperature. MegaSquirt wants a thermistor, which varies resistance according to temperature.
I may hack an IAT (intake air temp) sensor, which is electrically the same at the CLT, and see if I can get it crammed between some fins.
Another complication with the VW is that you not only have to deal with the cooling tins, but what they represent: cooling air forced over the cylinder head, cooling the fins while you’re trying to read their temperature.