I can't help much but the scangauge will tell you what the incoming air temp is. the symbol is fIA and there is a degree symbol before the lower case f (might be a "c" if you have it set up for celsius).
as far as issues, still contemplating it myself. several people on here say that it helps a lot but I haven't gotten around to it yet so I have no personal experience.
if you have a grill block, I would first just block off the opening in the air box and make a new opening that takes the air from the engine compartment. that is my plan when I get around to it. I also plan to make the opening in such a manner that I can add a tube to it later if I need to get the air right off of the exhauste manifold. maybe even put some kind of connector on it so that a pipe can easily be added.
these are just thoughts that I have had for my own car. the scangauge will get you started. I plan to proceed with caution and to use baby steps.
Be the change you wish to see in the world
On my 99 SL1 it does not make a lot of difference until I get to about 140 IAT. I like to run 160-200. I have never heard pinging or anything like that. While cruising the 99 held 37 degrees ignition advance at 220 WTf and 200 IAf at an altitude near 3000 feet. Yes at 200 F there is a significant power reduction, maybe as much as 25% torque reduction. Also at the higher of these temps the engine misses a bit around 2500-3000 RPM when accelerating. Different plugs and different gap did not make a difference to this miss. The ceramic on the plugs turns white when running like this even on an engine that used a quart of oil in 1500-2000 miles.
The best bet for monitoring the temp is a scangauge but it has been done with remote outdoor thermometers for cars with high enough range.
I have made a 240 mile round trip with the '99 that got me just a bit under 45 mpg. I fought a stiff head wind outbound and got 38 mpg. When coming back the wind had died down a bit and I stayed about 50-100 feet behind a truck running right at 65 mph. The trip back had me at 52 mpg but the round trip was 44.5 with the odometer a percent or so under reporting. Before IA heat I struggled to make 40 mpg with this car so I would say this gave me about a 10% gain. When I filled up the Scangauge calibration was within 1%. This is not a lot better than a stock Saturn might do in the Summer. My tequnique was just mild hypermiling and the air temperature was around 50 F.
I also have my tires at 60 psi and the grill almost all blocked with pipe insulation with the air dam under the radiator removed. I have about a 10 inch opening in the center lower grill slot and no pan between the bumper and radiator on this car. I think that is all I have done to the 99. It does not coast near as well as the '01 SL1. I have not looked into this. I plan on getting rid of the 99 and finding an '01 or '02 SC1 three door.
My '01 runs similar but with more torque. It has Somender Singh grooves in the head. Maybe they help or it may be the different programming. I think the '99 has a Delco computer and the '01 has a Motorola. The '01 ignition advance maxes out at about 33 degrees and does not seem to retard with heat while cruising. Other wise it is about the same as the 99 except it gets a bit better mileage.
Others have reported ignition retard but it might be at conditions I am not monitoring. I guess I should point out that my knock sensor is not unplugged.
I hope I have added a bit without confusing the issues too much.
i do have and use a scangauge, but i see that the factory location of the iat is in the air inlet. i would imagine that it needs to be relocated even if the stock air inlet was just capped. have you relocated yours? know if readings are affected by the exact location, or would any location with air flowing past it show similar readings?
also, is 200 as hot as you would want to go (no gains over 200)? or is it just difficult to create/maintain temps any higher?
Yes I relocated the IAT sensor. I used a 1/8-27 NPT tap. Technically I suppose that is the wrong thread but it worked fine for plastic. Mine is still in the front of the air cleaner box but low enough to clear the air filter. I have considered moving further downstream but have not done so.
Yes 200 is the practical limit for the crude systems I use. The power loss is quite substantial above 200. I have not tried to run above that. Others think the mileage suffers above 180. They may be right. I don't measure stuff as carefully as a lot of folks.
I have thought of incorporating a heater core or other water to air heat exchanger into the system for quicker warm up but I have not come up with a really easy way yet.
air density at 200 is about 75% of the density at 32 (both readings in Farenheit).
On my 94 Civic VX I just reversed the air snorkel and shortened it slightly, so it draws air from the area of the top radiator tank and radiator hose. Its also closer to the exhaust manifold. It seems to reduce power somewhat, but my last 3 tanks were 66, 55, and 62 MPG (rounded off to higher number IE 65.744rounded to 66 others were at mileage or above).