Exhaust Temperatures

by Richard Atwell
(c) Copyright 2004-2011

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Keeping your exhaust from rusting takes some care. When it's done right you'll make without the need for maintenance it last even in harsh climates.


Temperature:

I took these exhaust readings from a 76 Westfalia after it had been driving on the freeway for an hour.

exhaust temps

The exhaust headers are pretty damn hot and so are the afterburners (u-pipes). This is one location you do not want to have an exhaust leak: besides the noise it will heat up that area of the underside unnecessarily at traffic stops.

The exhaust continues to cool on its way down the sustem until it exits the exhaust pipe at 300F.

The heat exchangers show an increase in heat from front to back indicating that the exhaust tubing and internal heat-sink are heating the fresh air that the impeller fan is blowing through.

egr temps head temps

Paint Longevity:

Why is it so hard to paint the exhaust?

Regular paints and power coatings are good to 400F; Engine enamels 500F; Brake caliper paint 900F; Exhaust paints 1200-1500F and up.

So, given these temperatures why is high-temp paint recommended by everyone for painting mufflers?

After the engine is turned off and the hot-soak period begins, the parts of the engine that previously had ambient air flowing through them (intake manifold, fan shroud, heat exchangers) will increase in temperature while the already hot exhaust parts will start to cool down in temperature immediately. The exhaust tends to be made of thin metal and cools down quickly compared to a solid block of aluminum. Despite this increase after shutoff, it's not enough to warrant the use of hi-temp paints on the engine pieces such as the tinware.

It would seem that hi-temp paint is only required for the headers and normal paint will do for the rest of the exhaust. My feeling why paints (hi-temp or regular) fail is because the exhaust components aren't prepped properly: thorough sandblasting, no finger or hand oils, application of tack cloth, light coats, flash times, etc.

Even with proper prep, I can tell you if you paint it too thin it will burn off, and if it's too thick it will flake off. There is an "art" to painting the exhaust and making it last. It took me three attempts to get it right and the only paint that worked for me was VHT.

Keep in mind that the average EMPI or aftermarket VW muffler is only painted to keep it from rusting on the shelf. It will rust instantly once it is fitted to a running engine so you have to strip and paint it right after you buy it.

Another issue is that some hi-temps paints require high temperatures to cure properly. Take VHT and it's cousins (sometimes referred to as BBQ paint): it wants to be cured for 30 minutes at 250F, then 400F and then 600F. This is beyond the scope of the average home oven and although the exhaust will get that hot it's impossible to keep the headers at those temps for the required amount of time while the engine is warming up to try to cure them in-situ: the headers get to 700F very quickly. This means it's not entirely suitable for exhaust headers either. As for the muffler, it doesn't get hot enough to cure properly either but I have found it the longest lasting anyway.

So, in general is 1200F paint better suited than 400F? I really don't know. I'd really like to know what kind of paint VW used on the heat exchangers even though it didn't turn out to be that durable either.


How to Paint and Cure:

The only low cost paint I know of that works is VHT Flameproof Spray paint. It's has a ceramic based silicone paint with a matte finish that has lasted fo me. My muffler looks as good as it did the day I painted it, many miles ago.

As with any painting process, prep is key. You must clean the muffler of all the old paint and rust and let no oils/grease from your fingers touch it.

Once you are ready to paint, do it at 70F temps and apply two light coats (just enough to achieve coverage) and let each coat dry 72 hours. Because you cannot cure the paint without a large (not for cooking) oven that gets hot enough to follow the VHT instructions you should install the muffler on the bus and let it idle in the driveway for 20 minutes and then let it cool back down to air temp. Do not go driving or the paint may peel off as the cool air passes under the muffler and the hot air rises from it.

Gene Berg gave this curing advice but I wouldn't follow it because I don't know what kind of paint he used:

After painting, you start the engine and idle it for 1 to 2 minutes. Let cool and then run for 1 to 2 minutes at about 2500 RPM. Let cool and repeat for 4 to 5 minutes. This cures the paint slowly from the head out.

Follow my advice instead for VHT. As for painting the heat exchangers, you'll find that it will take 2-3x as much work with paint stripper to remove the paint from a Genuine VW heat exchanger than it will to remove the paint from a Dansk heat exchanger. Be sure to test fit the Dansk HE before you attempt to paint it.


References:

History:

06/04/04 - Created
09/07/11 - Fixed broken photos, added translate button, updated footer