Redundant IAT and CLT sensors: suggested mounting locations?
 

I am finally preparing to MegaSquirt my 1997 Miata but could use some insight on a few issues to get me started. These may seem like stupid questions to many of you, but I figured it would be better to ask the stupid questions now rather than to do stupid things and then ask stupid questions. I've searched around a bit and found a lot of useful information, but haven't quite satisfied these specific concerns yet.

Currently the engine configuration is 100% stock with the exception of the M-Tuned coolant reroute. I plan to get the MS unit installed and get familiar with it before adding forced induction. The MegaSquirt is an MS1v3 built by a local guy who ran it in parallel on his '97 turbo Miata. I also plan to install the MegaSquirt in parallel (to satisfy local OBD2 BS requirements and potentially simplify idle control, AC functionality, etcetera). The unit is configured to use its own CLT and IAT sensors rather than trying to leech signal from the stock sensors, and this is how I too would prefer to install it.

I already have the IAT sensor and plan to install it either in the factory airbox housing or in the intake snorkel. I assume that either of these should be suitable locations, correct?

My primary concern however is where to mount the additional CLT sensor. I could easily mount a sensor in the block-off plate at the front of the engine where the thermostat originally mounted, but that area should be seeing cooler temperatures as it is closer to the water pump and supplied by cooled water from the radiator. That particular area is also somewhat isolated from the main flow path of the coolant, so it may have a slightly delayed reaction to changes in temperature as well. I took some readings with my non-contact thermometer and that area appears to run about 15-20 degrees cooler than the inline thermostat housing on the back of the engine. Would this still be an adequate location, or am I right to assume that this would be a poor location for the MegaSquirt's CLT sensor? If I don't have room to mount a second sensor in the M-Tuned water outlet, would it be better to mount the MS sensor on the outlet and relocate the OBD2 sensor to the cooler location at the front? Is there a better/simpler place where I could tap another sensor in? Perhaps on the heater core supply hose?

Also, since I would be running these sensors exclusively for the MegaSquirt, I would use 2490 for the bias (the same as if I were running a standalone) when setting up my tune, correct?

Does the M-tuned have a 2nd metal tube/housing under the intake manifold that connects two hose? Or was that a homebrew reroute that I saw. If not why not tap and mount it there?

You're likely going to want to put the IAT sensor somewhere it can actually measure the temperature of the compressed air before it gets into the cyls... I.e. after intercooler, but not in the intake manifold (sensor will likely heatsoak if its physically connected to the intake)

Truthfully, it's been a while since I installed it, but I seem to recall the only outlets on the neck being the radiator hose nipple, the heater hose nipple, and the OEM sensor. I might be able to mount a tee where the heater hose nipple currently attaches, but it's probably going to be an extremely tight fit if so.
EDIT: Actually, upon closer inspection, there appears to be a 1/8" NPT plug next to the OEM sensor, so I could possibly bore it out and tap it for the 3/8" NPT GM CLT sensor.

At present I am installing it to a stock (naturally aspirated) engine, so I would think that there should be virtually no significant temperature change from the airbox to the throttle, right? I had intended to mount it on the intake tubing right before the throttle once I go turbo though. From the sounds of it, that would still be likely to heat soak though? Where are people generally mounting it?

right before the intake manifold after the i/c

Directly in the charge piping though, correct? I was planning to weld a bung directly onto the pipe that would run between the IC and the throttle, probably very close to the throttle.

that's what i wish i did (put it in charge pipe). i put it right before the throttle body and it gets heatsoaked... not so bad while driving but if you shut the car off for 5 or 10 minutes it'll have a hard time idling b/c the ECU thinks it's 30*F hotter than it really is.

A better place for the IAT sensor would be in the coldside endtank of the IC. At least put it as close to the IC as you can. This way, you'll avoid heat soak from the radiator fans. Until you go turbo, I'd put it close to the air filter, hopefully inside a cold air box.

Gotcha. That makes sense and seems very do-able. Thanks!