Pete....No expert but this is mine and my friends experience on this...........Post turbo temps will be roughly 250 to 300 degrees less than manifold temps....my thermocoupler is on the down pipe. From what I was told from autometer and several diesel specialists, the gauges "red"area is designed to read post turbo on the down pipe. My egt's are at 1100-1150 pulling Potisi at 48-52 mph. Still have plenty of pedal left but I don't want to melt anything. If you get the time I would put the coupler on the downpipe. Every gauge kit I've seen(ispro, auto meter and banks) instruct you to mount on the down pipe. This being said, I also took my infrared heat gun and checked the down pipe temp and compared it to what the gauge read and they were very close....just my .02