I tried this for analyzing fuel mixture. Don't be surprised by considerable variation in temp. from these guns. I found the average idle temp on header pipes not far from the heads was around 980F on a Costco temp gun. Shooting the exact same spot on multiple round tubes is difficult, and minute changes in vertical distance away, angle or position can change the indicated temp by 100F or more. You can't tell if its exhaust gas temp variation, or whether its because the spot you checked is not identical on all 8 tubes. Or if its just a cheap gun.
The lower the temp, the less all this seems to affect the gun reading. So its much more accurate on radiator header tanks and useless for checking exhaust mixtures. My 2ยข....