In drag racing we all coat our headers with a ceramic coating like Jet-Hot or Airborne Coatings Co. It does wonders to lower underhood temps and keep the exhaust gas hot and flowing. Seems like the exhaust manifold and the turbine housing on our CTD's would be an excellent application of this. Keep all that heat in the exhaust gasses, able to do more work in the turbo and lower underhood temps to boot. Also would make that ugly rusted manifold look alot better too.

Anybody do this????


Zino

 

There was a HUGE discussion on if gas flows better hot or cold. One guy had done lab type test too. So I would say that holding in the heat on a motor that you want to do everything to eliminate exhaust heat would seem not the way to go. Jet Hot looks cool ;D but I was under the impression it was only for looks, no proven performance gains addvertised.

 

You could also do the Black High Temp ceramic coating, I did all my hot stuff in this on the Twins then did the cold pipe in Aluminum ceramic(Jet Hot..HPC).

Jim

 

Make no mistake about it, keeping the heat in the exhaust gas is a good thing. Its density is much lower which lowers its momentum, thus making is easier to negotiate exhaust system bends and restrictions. This has been proven over and over again. On a typical NHRA superstock 350 SBC which is about 580 HP, Jet Hot Coating is worth 10 to 15 HP, dyno proven many times. With these coatings, were not talking about holding the heat 'in the motor', we are holding the heat in the exhaust gasses. Underhood temps are also typically 40°F cooler as well. Since the turbine of our turbos is a heat machine, operating on the energy delta between inlet and outlet, keeping the inlet temp as high as possible ( within the limits of the turbo material) would maximize the turbine efficiency.
I am really thinking its a win win situation coating the manifold and turbo.

Zino

 

I agree, and have done other cars in the past....figured I ll do the dodge when I get the new turbo.....and swap to the other manifold.......The down side is the high temp stuff is only available in flat black.....and trust me.....you'll need the high temp stuff....

On a side note I run to a 1600 degree ceiling for short periods.....I just wouldn t want 1600 for 15 minutes pulling a hill with a 48 ft trailer....

 

Zino,

The engine config you speak of is normally aspirated.

Our engines have a serious restriction in the way (the turbo)

Turbos work from expansion of hot gasses. The heat, in nearly all cases..... is there in sufficient quantities to turn the turbine.

It does look cool though and in certain, but not all cases it could be beneficial.

Part of the reason ( besides strength) the manifolds and turbine housings are sooo thick is that air is a good insulator. A thin header will hold in more heat than a thick ol manifold anyday. The thicker material will discharge the internal heat faster.

Don~

 

Don, I don t agree with your wording. Hold more heat in a thinner manifold? The thicker the manifold the more heat it absorbs and does not transfer. The heat is traped for lack of a better term in the manifold. More energy is in the thicker manifold. The thinner manifold will cool quicker because it has less energy / heat stored. One reason most oem turbo setups are cast is to keep heat in....produce better low speed drivabilty via quicker spool up characteristics.

Also , to the best of my knowledge all indy car teams have virtually every part coated.....not just for longevity (only a few hours tops) but because the dyno has shown that if you keep the heat in the exhuast, it flows faster thus giving better turbo performance. As it cools it gets denser and thus needs less area to flow as well.....realistically we should make our exhuast smaller as it gets further away.....like a 5 to 4 to 3 inch at the tail pipe......but the only way to tell would be to measure velocity at each point....and ofcourse market it to a public that always wants the biggest....haha

I m not sure if what I read is what you wanted to convey...

 

1320,

I was afraid when I posted that a thinner manifold will run warmer or will hold more heat than a thicker one internally I might get a repsonse or two.
Thermodynamics will tell you a different story though.

The wall thickness of a particular material will strongly influence the heat transfer, in that the thicker the material, the faster the heat will travel through it. This seems contrary to logic at first, but consider how fast heat would be drawn out of a high conductivity, infinitely thick aluminum manifold as opposed to a very thin piece of stainless steel surrounded with a nice insulator like air. Stainless is used as the exhaust material in Indy and many other applications because it is durable. Low rates of oxidation with high heat. Indy cars have very long tubes when compared to a diesel engine. Indy rpm ranges are very high as well. Long tubes help @ 10,000 plus RPM. Short tubes like ours ( less than 5 inches on some runners) are better for the low rpm ranges the diesel operates at.
Second rule: The total surface area of the manifold has some more to do with heat rejection. More surface area.... more heat loss. Cummins manifolds are short and have low surface area compared to the Indy manifolds that are measured in feet, not inches. Thermal coatings help to retain the heat from the chamber to the scroll in the long distance. With the EGT issues many of us face, we have ample , even excessive retained manifold heat to the turbine. Cylinders 3 and 4 are really high. Pre turbo temps that read in the 1500 degree range ( the boat many of us are in ) is considered excessive to all turbo manufacturers. Coating will just raise this average temp the turbine sees.

If constructing a long tube header and moving the rpm range higher the coatings may be needed, but most of us are not in that catagory.

Don~

edit- your statement on exhaust pipe reduction as it cools is exactly right. Yes, the velocity is lower as it cools afer the turbine.

The idea scenerio: Converting the flow from axial to turbulent as fast as possible, slowly reduce the pipe diameter to retain velocity, but like you said...whos gonna believe that one

 

[b]The idea scenerio: Converting the flow from axial to turbulent as fast as possible, slowly reduce the pipe diameter to retain velocity, but like you said...whos gonna believe that one

I would, it's Jet Engine theory! Convergent/Divergent ducts! Used and all non-high by-pass jet engines (fighters) for thrust.

Jim

 

Don, I have to take exception to a few things you said: The one about long tubes good for high RPM and short tubes good for Low RPM. I'm quite confident you got that one backwards. Probably just a typo.

Slowly reduce the flow from axial to turbulent.... Lost me on that one. Axial is as the name implies in the line of the axle. So axial flow leaving the turbo is straight away from it. In the turbine housing though, there is all kinds of radial flow. Did you mean convert the flow from radial to axial?? As for turbulence, you only want it one and ONLY one place...in the combustion chamber to promote mixing and complete burnout. Anywhere else, it just inceases flow resistance...

As for convergent then divergent, that only works once the flow is supersonic. This is a common type of nozzle arrangement on a steam turbine as well. But at low steam throttle pressures and flows, the flow is not sonic yet and the convergent/divergent nozzle is actually a detrement However once past critical pressure for the nozzle, and the flow becomes supersonic.... look out!!! Then the velocity just keeps goin up up up.... Subsonic flow, and a convergent/divergent nozzle will only speed up the flow through the convergent part, then slow it down in the divergent part. Rest assured our exhaust flows are SUBsonic...

Rate of heat transfer through a wall is a function of the materials' coefficent of conduction, not wall thickness. Additional wall thickness just adds mass, which as in dynamics increases inertia, in heat transfer it increases the time it takes to either heat up or cool down. Once steady state, the rate of heat transfer through the wall is the same....

No doubt 1,500° is BAD JUJU for the turbo, I agree on that one.

Again, hate to keep goin back to drag racing gas engines, but :we all use step headers which are constructed exactly opposite from what you are describing:: The first length of header tube leaving the head is the smallest, then 12" or 14" later it steps up 1/8" in diameter,, another 12" or 14" it steps another 1/8". Again proven time and time again on the dyno to help the powerband accross its ENTIRE range. The reason being, just past the exhaust port you are interested in velocity, this helps scavenge the cylinder, after that you just need to move the mass out of there, and dont care about velocity, just mass flow. So as the gasses are cooling, they are becoming more dense and need more cross-sectional area to flow through... Scavenging is not an issue on a turbocharged diesel engine, so velocity is not an issue either. Thats why you cant really over pipe a diesel. Put a 5" exhaust on a gas 350 though... lol// you'd have a 250 RPM powerband if you were lucky..... Lol

Boy that was long... Sorrryy!! :

 

Axial Flow = The spinning gas exiting the turbine housing, after the wheel.

Turbulent flow = In a straight line. Not rotating.

When speaking about gas flows....turbulent is straight flow.
Turbulent flow travels a shorter distance through the pipe than axial flow. This is why it is important to reduce the distance traveled by the gas. If it is spinning past the turbine wheel the faster you convert to turbulent the better. The shortest distance between two points is always a straight line. Not a circular or axial path.


Turbulence "in-cylinder" is different. Yes its a gas flow, but not the same meaning.

Step headers and gasser engines are a totally different thing when speaking of exhaust gas pulses. Step headers, long tube, short tube, etc, all depend on pressure waves from the exiting gas to bounce back to scavenge the cylinders. TurboDiesels are not that way at all. You can not bounce anything back to the cylinders past the turbine wheel after the exhaust has exited it.

Many turbodiesels actually have exhaust flow that exceeds Mach speeds. The speed of the exhaust wheel and exiting gas actually determines if the rotation of the exhaust is clockwise or counterclockwise. There are some tuning tricks in that statement, BTW.

My point is still, that exhaust manifolds that are thicker will transfer more heat, at a faster rate, from its inner diameter than a thin pipe will. Remembering that the pipe temperture is not constant depending on load. As you said.... the rate of heat conduction. Ask yourself if the thick material pipe will transfer more heat from its inner core to its outer layer since it has a higher conduction rate than the thinner pipe that is surrounded by a layer of insulating air. Yes they are both surrounded by air, but they are not both the same thickness.
The average heat retention whether speaking of a drag car or a turbodiesel will always be higher with a thinner pipe. If both pipes are of the same material.

Don~

 

Sorry, with all due respect Don, you have your terminology wrong big time. Turbulent means that the Reynolds number is greater than about 4,000. The Reynolds number is a unit-less number which is a function of the gas density, velocity and characteristic length. Turbulent flow is flow which is non-homogenous, mixing rapidly, churning etc. Laminar flow is flow which is in a straight line, with all flow paths parallel. Transition flow is the flow between laminar and turbulent. Turbulent flow in pipes results in flow shearing which results in exponential increases in flow friction ( bad thing). Gas Flow in a straight line is Laminar flow. An excellent example of flow going from laminar to transition to turbulent is the rise of cigarette smoke from a standing cigarette in a room with no wind. The smoke leaving the cig will be a thin smooth straight line for a foot or so, then as its gaining speed you will see it start to get wavy and wider, then finally its got lots of eddies and swirls its real wide and dispersed. Another good example of Laminar and Turbulent flow is that of a calm smooth glass like river (Laminar) and a churning crazy whitewater creek (Turbulent).

Axial flow is flow along a line parallel to the shaft of a fan, compressor or turbine. Not rotating flow.

Rotating flow is often referred to as swirling flow, or radial flow.

Yes you want to stop the swirling flow as soon as possible after the turbo to minimize the distance traveled.
Yes, the pulses are in fact the key with the naturally aspirated gasser engines.
Yes, the thicker exhaust manifolds will transfer more heat from the inner diameter during a transient condition of increasing temperature of the gasses relative to the manifold. (such as going from idle at 300° F, to a romp down the quarter mile). But steady state, manifold and exhaust gasses at the same temperature (Like a long 5 minute pull up a grade) there is no difference at all.

The majority of what you have said is correct, but you got your words wrong for sure.

No flaming or disrespect intended, just trying to stay factual

Zino

 

Zino,

Turbulent flow is all your gonna get from a exhaust pipe.

An easy way to explain turbulent flow is....like a river or stream. Like water following the natural path of the river bed. Yes, mixing is somewhat taking place in the pipe, but you cant get any better than turbulent flow in this case ( exhaust systems ) Laminar is impossible in exhaust sytems. Laminar requires a steady state velocity and few turns/twists.

Axial can be described as the path the gases take after the turbine. The turbine wheel is horizontal ( for the most part) and spins the gases in an axial manner. At least the terminology of turbo manufacturers is "axial flow". read: MacInnes ( the patent holder of divided turbine housings)

So I still say the fastest way you can design an exhaust pipe is to convert axial flow to turbulent flow as fast as you can without breaking the boundary layer. Generally a 15 degree gradient. Turbulent flow is the closest thing to a straight line flow you can get. Since "laminar is outta the question". Looking down the pipe and thinking of the velocity of the gas, the tempertaure of the gas, and the twists and turns it takes is just like looking down a river of water. Twists, turns, some mixing, homogenization, etc.=Turbulent flow.

Could you imagine how long it would take the flood waters to get to the ocean from the Mid-West if the flow was axial or spinning? Same volume of water? It would pile up fast. Or swell up fast

A laminar flowing river would be impossible on earth, so would a laminar exhaust pipe after a turbine.

I also still say the best pipe you can get for the heat conditions in our trucks will have short tubes for low end torque (with a turbo in the way), be constructed of thick material for long term durabilty and weight capacity, not be coated...unless you really want to run higher rpm and increase the surface area and length of the pipe and could use it. The coating does look nice though! NO rust.

Radial flow would be radiating outward from the turbine wheel. Like "exploding away from". Turbines discharge the gas in a flame type shape in the same horizontal plane as the turbine wheel/shaft. Axially.

Turbines like the ones on the Cummins take the air in tangentially and it exits axially. ALWAYS! The other side of the turbo...(the compressor side) takes the air in axially and it exits tangentially.

Finally, I must disagree on the RE number figures you gave. In all cases of hydrocarbon exhaust gases the RE number would need to be above 2000, not 4000, to be turbulent

Don~

 

Does anyone have a picture of this?

 

Here is a cheesy graphic from yesteryear that shows the flow into and out of the turbo.

Don~