A pyrometer is used to measure Exhaust Gas Temps. Most mount them pre turbo and some post turbo. When pulling a load, it is wise to keep temps below 1300 or so (pre turbo) - someone will give the specifics - there is debate on this also. This gauge is also used to monitor shutdown temps. If the engine is shut down before the turbo has a chance to cool, "coking" will occur. I'd explain, but I'd probably screw it up and get flamed by someone who really knows. You'll get more details from other members.