FYI: Exhaust radiant heat

[RV7A Flyer]

Originally Posted by dougmattson

I used to work in a diesel dyno lab also. I believe you can see thru the turbo housing and see the turbine wheel.

I don't think the metal becomes transparent...I believe you were seeing the "shadow" of the cooler turbine wheel on the blackbody-radiating housing.

[nucleus]

Pros: All your rubber bits under your cowl will last longer without all that heat, slight increase in power

Cons: You can never weld that exhaust again

[RV8RIVETER]

Originally Posted by Kyle Boatright

Coated internally or externally? I makes sense to me that internally coated pipes would help cool the cylinders, pipes, and the engine compartment, but externally coated pipes might result in higher temps at the exhaust port flange and in the pipes.
.

Coated only on the outside. I wanted both sides coated, but the coating expert was adamant that they only be coated on the outside. He had dyno runs that showed higher hp with only the outer coated versus the inner as well.

Mine were Zirconium coated, which was claimed to not be problem welding any future repair, but since I have not tried or known anyone who has, I do not know if that is true. If they need to be replaced I will go with a home made mild steel copy and coat that, cheaper than stainless.

Also, I edited my previous post to make sure my last sentence wasn't taken the wrong way.

[Toobuilder]

When coated, by collector was a slip fit for all four pipes. Shortly thereafter I welded one of the primary tubes to the collector to lock it in position. No issue with cleaning off the ceramic and welding.

[Bavafa]

Originally Posted by nucleus

Pros: All your rubber bits under your cowl will last longer without all that heat, slight increase in power

Cons: You can never weld that exhaust again

Ironically, my exhaust was welded by Wetterman as part of a recall they had for this specific model. It has held up very well so far. They did not make any comment about welding it, if it was harder/easier or different.

[flyeyes]

Originally Posted by AlexPeterson

What is the theory that says that coated pipes would result in cooler cylinders?

Speaking of theories, what is the physics reasoning behind coating pipes in the racing world?

I have been told by an exhaust fabricator that the exhausts "wick heat away from the heads" and that this and welding are impaired by the ceramic coating.

It seems to me that the exhaust runs several hundred degrees hotter than the head, so any heat transfer would be into the head rather than out of it. A ceramic coating on the outside would make the metal of the exhaust hotter by reducing heat transfer to the airflow in the engine compartment, so there may be some validity to that argument. An internal coating would seem likely to make both the heads and the engine compartment run cooler though.

There are three stated benefits to the ceramic coating:

1) cooler heads and engine compartment. An internal coating should do both of these, an external coating (or wrap) still cools the engine compartment and might make the heads hotter, although I'd guess that's a pretty small effect given the difference in mass of the heads and exhaust, and the fact that there is already a gasket between them.

2) More horsepower, presumably by reducing the temperature/pressure difference as exhaust gases enter the pipe. I don't quite understand this one, it seems to me that rapid cooling would drop the pressure in the pipe and reduce backpressure, but there is lots of empiric evidence in cars and airplanes that it works. I do understand how it would increase the "jet" effect of the exhaust, as the exhaust gases ought be be hotter, and faster leaving. Any ceramic coating (or wrap) should do this.

3) It looks better, presumably causing attractive women to want to hang out around your RV. Given the already very powerful effect of the RV airframe, I'm guessing any incremental effect from the exhaust is going to be pretty small. Obviously only an external coating would have any effect on this parameter.

[tclaire]

Very interesting. I emailed jet hot to find out a little more on how they do the exhaust they replied back that they would coat the inside and outside of the exhaust. Which seems like the correct way to do I am not sure how much weight it's adds.
Tim

[Toobuilder]

Back when I did mine, Jet hot offered the satin silver finish typically seen on car headers. Jet Hot's literature indicates this stuff is good up to about 1350, IIRC. I called the company and told them that I'd be subjecting my exhaust to approximately this temp, but for hours on end. Jet Hot suggested this product would not survive. They steered me to their high temp coating which did last fine. I only bring this up because it has a very rough finish, about like #40 grit sandpaper. I think this roughness would be bad inside the pipes.

That said, are people having good luck with the shiny, smooth silver product?

[SvingenB]

Originally Posted by flyeyes

2) More horsepower, presumably by reducing the temperature/pressure difference as exhaust gases enter the pipe. I don't quite understand this one, it seems to me that rapid cooling would drop the pressure in the pipe and reduce backpressure, but there is lots of empiric evidence in cars and airplanes that it works. I do understand how it would increase the "jet" effect of the exhaust, as the exhaust gases ought be be hotter, and faster leaving. Any ceramic coating (or wrap) should do this.

On large yachts, and smaller for that matter but in particular on the large ones with large diesel engines and/or turbines, water is injected into the exhaust to reduce the velocity and reduce friction. The friction is proportional to the mass flow times the velocity. The mass flow is constant regardless of velocity, while the velocity will be greatly reduced when the density increases. Water injection also nearly removes all noise. There is a huge industry revolving around this, which a friend of mine is working with.

I don't know what exactly is meant by the "jet effect". If the exhaust flows inside the pipe at constant density, there will be a reflection only at the end where the pressure is constant, a pressure node. At that point the flow will have an antinode, or a max amplitude point when the pulses from the engine resonates with the frequency of the hot gases inside the pipe, like a flute. It sure will make lots of noise, but if the power will increase? maybe, I don't know. Maybe it somehow will help evacuate gases from the combustion chamber?

[skylor]

Originally Posted by flyeyes

2) More horsepower, presumably by reducing the temperature/pressure difference as exhaust gases enter the pipe. I don't quite understand this one, it seems to me that rapid cooling would drop the pressure in the pipe and reduce backpressure, but there is lots of empiric evidence in cars and airplanes that it works. I do understand how it would increase the "jet" effect of the exhaust, as the exhaust gases ought be be hotter, and faster leaving. Any ceramic coating (or wrap) should do this.

Much of the flow in exhaust pipes is supersonic. Supersonic gas flow and the related shock waves have some characteristics that are counterintuitive to what we know about subsonic flow. One example is that nozzles behave oppositely for subsonic and supersonic flows. Ever notice that rocket nozzles are divergent, i.e. there area increases in the direction of flow? For supersonic flow, the gas velocity actually increases through the nozzle, while divergent nozzles are used to decrease the velocity of subsonic gas flow.

Anyway, some of these same effects occure inside exhaust pipes, and keeping the temperature of the exhaust gas elevated is all about maintaining supersonic gas velocity and controlling the related shock wave.

Skylor