I run both (pre-turbo and post-turbo) pyrometers on my truck. The difference between the two is proportional to how fast the pre-turbo temp is changing. If I am really screaming up a hill with a load (and the pre-turbo temps are rising fast), I might see 1200 pre-turbo but only 800 post. If I had been going downhill before climbing that hill (and hence had a very cool post-turbo temp initially), I'll bet I would see a higher temperature differential. Since the differential is dependent on how hard the truck is running, there is no magic number which will equal 1250° pre-turbo.
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