[Richard Hudgins]

Machiavelli

As to the statements about stainless as a header material the following:

The Thermal Conductivity - k - (W/mK) of various materials used in headers are as follows:

Carbon Steel 54
Ceramic coated Carbon steel 2.36
Stainless Steel 16
Inconel 8.4

Note: 1 W/(mK) = 1 W/(m degrees C) = 0.85984 kcal/(hr m degrees C) = 0.5779 Btu/(ft hr degrees F)

It is true that stainless makes a longer lasting header then carbon steel under racing conditions. The life increase is minor in my experience and I have had many problems with cracking at welds due to differential material thermal expansion of the base and filler alloys. (This can be a problem with carbon steel as well if you use a really wrong filler material but this is rare)

I have had the best luck with Inconel for race headers. I have never had a crack or failure of any type using this alloy. It is used by all of the F1 guys and most LeMans Prototype cars as well as the turbocharged engines in many series.


Also you still need a thermal barrier with stainless. They still heat up to match internal temperature it just takes a 3.5X longer than plain carbon steel. Also the lower heat conductivity makes them take much longer to cool down which can give you problems with shutdown heat soak. (This is also true for Inconel and why you see the F1/LeMans headers in a gold foil lined and covered containment.)

Whereas ceramic coated carbon steel takes much longer to achieve 100% heat soak and typically never does due to duty cycle and airflow removing the transferred heat away at almost the same rate as conductivity.


I would still use the ceramic coated carbon steel if I were you. The heat transfer normal to the wall is the lowest of all and will keep under-hood temperatures down. (By a factor of 4 relative to stainless from the above table.)

You must remember that you are normally going to be running at part throttle and your exhaust temps are going to be quite low. (Street use)

For the times that you are going to do track work your full throttle time will seldom be over 60% of a given lap and I doubt that you will be putting in more than 10 lap stints. Overall header temperature or corrosion should not be a real factor in the life span of your system.

But you can make stainless units and then ceramic coat them and have, as Dr. Pangloss in Voltaire's Candide so eloquently stated, the best of all possible worlds.

(The real best would be ceramic coated Inconel $$$$$$$$$)

With any luck this will make some overall sense.