Temperature of the Exhaust System Components

[Kinky John]

Hello everyone,

Done a search on here and a bit of googling but I don't seem to be able to get any figures.

For the purposes of applying paint to the exhaust system in order to increase its life expectancy and prevent corrosion for as long as possible, does anyone have a reasonable idea as to the temperature of the exhaust system along its various points?

I have cans of high temperature paint rated at different temperatures and I got to wondering can I apply the lower rated stuff at the rear silencer while saving the more expensive stuff for the front sections.

Thank you very much
Mark

[Mazza]

I’m no exhaust expert but I think it helps the gases to flow through more efficiently. Also keeps system cooler and longer life of exhaust system - protection from rust.

Some guys I knew (with mk2 golf gti 16v) had the exhaust ceramic coated. Better gas flow and improved bhp... so they said.

[Robsey]

If I read your question properly, you are looking at the temperature of the exhaust at various points.

The obvious answer is that the exhaust is hottest at the exhaust manifold, as that is closest to the heat of combustion. Probably about 600°C.

The exhaust will be cooled by ambient air flow under the car.

The catalytic converter is tucked in close to the body, and wrapped in heat shields away from the cooling draught, as the ceramic interior needs to be approx 273°C to allow the maximum efficient conversion of the exhaust gasses.

After this point, the exhaust is allowed to dangle a little further away from the underside of the car, as it is no longer important to keep the gasses hot.
On a good brisk run, I would suspect that a back box would still get up to approx 100°C to 150°C, so too hot to hold or handle without waiting 10 or 15 minutes for it to cool.

I would expect that most high temperature paints would have instructions and specifications of how and where to use them.

Ultra-high temperatures around the manifolds (or headers) and the front down pipe to the catalytic converter, and then medium to lower rated paints, the further back you go.

To be honest, I think high temperature paint is only for aesthetics.
Most exhausts don't just rust into an ugly mess, unless you buy really cheap and tacky.
And usually they rust from the inside out.

So what would I use it for?
To make the manifold a pleasing colour...

And as I have an SRi back box with oval exit pipe, I believe the original spec was to paint the tail pipe black or something.

The most expensive component on my car is the catalytic converter, and mine lasted fine, without going particularly rusty for 23 yrs, and then it was only changed due to failure of the ceramic honeycomb core to convert gasses sufficiently to pass the MOT.

[James McGrath]

I actually took some infrared pictures of my exhaust not too long ago.
This should help answer your question:

[Robsey]

Much cooler than I was expecting.
I have seen a lot of these paints rated up to 1200°C.

Maybe that is for brake parts or turbos.

[Mk3alan]

Love a good photo!
I had a tour round Vauxhall Luton many years ago and was most impressed by test engines running 24/7 flat out on dynamometers and the exhaust manifolds were a real bright cherry red - the valves must have been white hot!
Love another tour.

Alan

[James McGrath]

Yeah, maximum of 150 degrees, I was quite surprised myself.
That was taken after a short 5 mile journey and the engine up to normal temperature and was idling at the time.

[Robsey]

You got me thinking...
I bought a stainless exhaust for the campervan, but not fitted it yet, as we are a long way from getting the engine fitted.

I got to remembering seeing stainless exhausts in the past changing colour due to heat.
Then I remembered that there was a vague colour change pattern observed for quenching stages of stainless steel.

There are many factors involved that affect the temperature at which a specific colour is produced / reached.
(Material specification / grade,
surface texture / smoothness,
heat absobtion time,
heating method,
localised environment / in air etc...).

But as a VERY rough guide...
And based on the grade used when I was a bench fitter back in the 80s and 90s - 301...

Pale Yellow = 290°C
Straw Yellow = 340°C
Deep / Dark Yellow = 370°C
Brown = 390°C
Browny Purple = 420°C
Deep Purple = 450°C
Blue= 540°C
Dark Blue = 600°C.

Now this is for stainless, not for chrome plated pipes, which will probably colour change at different temperatures.

[Robsey]

Love a good photo!
I had a tour round Vauxhall Luton many years ago and was most impressed by test engines running 24/7 flat out on dynamometers and the exhaust manifolds were a real bright cherry red - the valves must have been white hot!
Love another tour.

Alan

I presume that would be many years ago, because I am sure modern alloy heads would warp badly at just a couple of hundred degrees.

Most temperature gauges are in the red by 120 degrees. The point at which the owners manual says “turn off immediately “.

Can’t beat those old cast iron heads.... unless you failed to put a proper coolant mix in, in the freezing winter months.