Plenums, Do They Run Lean On Rear Cylinders?

[Headsex]

Basically they are Temperature sensors that sit in each exhaust port right up near the head.

http://www.racepak.com/Sensors/Temperature.php

Wide band sensors will tell you AFR, and AFR only.. Spark has some, but little effect on AFR when you change them.

With EGT (thermocouple).. AFR and Spark effect the tempreture.

Obviously there is a maximum temp you dont want to pass. because well, things start melting!

Look up EGT's and how they can be effected.. With Diesels, its really the only way to tune them!