Expert: Marc Ebelini - 2/13/2006

Question
    Hello. How are you?
    I am a flight instructor for airplane. I could not find clear answer about piston engine CHT & EGT.
    Why does CHT peak become before EGT peak?
    EGT probe is located inside exhaust manifold. So the temperature indication must be instant. CHT probe is located outside cylinder. The indication has some delay I think.
    Also I would like to know how we can realize engine has problem by the indication of CHT & EGT. I know only both of high temperature show engine over heat and detonation.
    Thank you for spending your time.
                                    Yumi

Answer
Do not mix CHT with EGT as they represent two different problems. CHT has to do with cylinder head temperatures that are more related to cooling and not necessarily fuel problems. You need the CHT to determine best cowl flap positions and angle of climb. If you have only one CHT then it should be on the rear cylinder because that one usually runs the hottest. Climbing at steep angles can cause the CHT to rise to dangerous levels even with cowl flaps open should your aircraft have them. While fuel can cause high CHT, the EGT is used to determine the best fuel ratio. Over leaning can raise the CHT but by that time, you have cause a lot of damage.
I do not advise EGT gages unless all cylinders are monitored. If you use only one probe and in turns out to be on the richest cylinder due to induction leaks you can end up over leaning the other cylinders. Traveling long distances at high altitudes makes EGT monitors a great tool for best economy and performance.
Hope this answers you question. Location of those probes as well as their calibration is very important. Although the temperature calibration is important matching the probes are equally as important.