VonK :: Retired

According to Physics 101, ceramic coatings are <b>insulators</b> against heat conduction because they have lousy heat conductivity. But once they get to cherry red temperatures, they <b>radiate</b> 85+% of their heat just like bare steel. What's more, at the same temperature, a flat black body will reject more heat by radiation than a reflective one. Radiation heat transfer is proportional to the fourth power of temperature.

The OEM S4 manifold is double walled for three reasons:

1. Reduce heat transfer to protect neighboring components from being fried,
2. Strength against deformation from high temperature thermal cycling,
3. Keep the heat in the exhaust gas.

The RS4 used the same manifold as the S4.

When its dark, pop the hood immediately after a drag race, and compare what you see with/without a double walled exhaust manifold. I wonder what happens to the engine bay while seriously tracking with a bare manifold.

DAP@EPL

I would think that a thick ceramic coating would do a good job of retaining heat. Also, a double walled design would be tough to fit with larger piping giving the size constraints of the S4 engine bay.


-Dave Pramanik<ul><li><a href="http://www.swaintech.com/header.html">Swaintech</a></li></ul>

Todd/A.W.E.

I would suggest that the OEM manifolds are double walled primarily for a completely different reason than the ones you suggest:

The outer wall is there to keep the exhaust gasses themselves contained.

The original S4 manifold design during development was tubular, yet suffered from repeated cracking problems. It appears that Audi may have solved this problem by creating a floating intersection between the exhaust runners.

If you cut apart an OEM manifold, you will find carbon build up on the inside of the outer wall. This is because the runners intersect as slip joints, allowing them to move more freely during temperature transitions, and minimizing the chance of cracking. However, this slip type intersection also allows exhaust gasses to escape.


Also, even though the tubing may be glowing bright red when under full load, Swain Tech has documented a minimum of a 20% reduction in temps with the particular coating we use vs the OEM header. At lower loads, the reduction is as high as 70%.

VonK :: Retired

Radiation will still be the same, at cherry red temps (85%). I'm sure the RS4 engineers know all about ceramic coatings.

DAP@EPL

The RS4, while highly engineered, is still a production vehicle. Sacrifices were made in terms of cost effectiveness. Its quite likely that ceramic coating on a mass-produced part wouldn't have been as cost effective as using the existing S4 manifolds.

I think it comes down to the fact that if you want larger collector-style manifolds, there isn't room for a double-walled design. So you are left with the option of ceramic coating or bare metal. Which would you choose?

-Dave Pramanik

VonK :: Retired

The slip joints only pass exhaust gas until the inter-space between the walls is pressurized. This has no effect on the steady state total heat transfer. Coated or not, the temperature of a bare pipe at, say, 1,400°F will stabilize when it loses as much heat from radiation as is supplied by the exhaust stream. If the coated tubing still glows bright red under full load, it will still radiate 85% of its total heat loss to the environment.

I guess my point is, your manifold is designed to improve maximum performance, besides some other goodies. That also means max temps and max radiation.

Summer and a few HPDE events at the track will tell whether I'm just paranoid. :-)

Harald

Imolavirus@BuehnEngineering

first, i dont think that we really want to go to much bigger on the piping

also, from what i've seen/experienced with ceramic coated headers, they really do reduce temps in engine bay/keep heat inside during normal driving and even sprinted driving. i personally have measured temps of my ceramic coated manifolds right after hard street driving, and the temps have been lower than non-ceramic coated. That however was the physical temp of the manifold, not actual exhaust temps

Imolavirus@BuehnEngineering

the s4 isn't the only one with double wall mani's. i'm sure you know the 996tt and 993tt use them as well, and many of those guys are getting better results out of modified stockers that are coated. true they are more of a collector style, but my point lies with the double-wall and ceramic coatings

VonK :: Retired

Porsche exhaust are not in the engine bay, but hang in the breeze. [Here we go again. ]

VonK :: Retired

---&gt; 1,400°F up produces a health glow, as seen on race cars brakes.

The S4 manies are not cheap to manufacture. For instance, they require slip joints and much more intricate fabrication and welding than a bare tubular design.

When I got my Stage 3, I was strongly invited to install the AWE manifolds. I deliberately opted out. It was not a cost question. Besides, I don't quite agree with some of Corky Bells arguments and, in the absence of credible A - B test data, I'm not convinced of all the claimed thermodynamic advantages. But that's another story .......

BTW, I have nothing against ceramic coatings. I've used them myself on my race cars. But, I also know that they don't reduce radiation heat transfer. Anyway, on the outside of components, where flaking off is not a hazard, I'd use them myself.

Harald