I recently read in one of diesel magazines that it will depend a lot on how much boost and fuel the engine is getting. If I remember correctly at WOT the difference (manifold being higher) will be in the 300+ degrees range, at normal driving (cruising) it will be around 200 or so different and when at idle (0 boost) the difference is basically zero. I will try to dig up the article tomorrow and post the issue and data. BTW, my pyrometer is located post-turbo as well.
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