Header Wrap

not only does the wrap help to keep the radiator from sucking up extra heat from the pipe, it does a couple extra things as well.

1: it keeps you from getting instant 3rd degree burns when you touch the header. It's amazing how little heat is passed thru the wrap.

2: It helps to keep the shock cooler from not radiating heat into it. Ever look at how close the pipe is to the resevoir?

3: it helps to keep the carb cooler and that equates to recovered hp that would normally be lost from the heat.

4: It keeps the exhaust velocity high since it doesnt lose all that heat out of the pipe any more and that leads to better scavenging of the cylinder and that again equates to recovered hp.

D0 not wash it with a pressure washer or you will be re-wrapping. Been using it for years and it works well.

The first point is undeniable, and is the primary benefit of this "mod".

The second two are marginally true, although the things "cured" are not really a significant problem on a bike that goes more than 10 mph most of the time.

The fourth point is just wrong. It's a myth based on both a lack of understanding of exhaust tuning concepts and of the actual dynamics of gases within the system.

The first point is undeniable, and is the primary benefit of this "mod".

The second two are marginally true, although the things "cured" are not really a significant problem on a bike that goes more than 10 mph most of the time.

The fourth point is just wrong. It's a myth based on both a lack of understanding of exhaust tuning concepts and of the actual dynamics of gases within the system.

Before this turns into one of our regular wrestling contests please explain what experience you have had with header wrap.

Before this turns into one of our regular wrestling contests please explain what experience you have had with header wrap.

my experience with BB Chevys, TT 300ZX's and my YZ450F are "Hands Down" Positive... call it mummy all you want but for the performance gain I'll look like a mummy rather than be a dummy :thumbsup: anytime...

Rod

Before this turns into one of our regular wrestling contests please explain what experience you have had with header wrap.
Somewhat extensive, Philip.

But first, let me offer some observations:

Very few, if any, of the major factory MX teams wrap their exhausts (they do, however shield the bottom of their fuel tanks, and duct cold air over the engine)

Virtually none of the F1 cars (arguably the most powerful and advanced racing engines extant of the planet) insulate their exhaust.

Must be a reason

If keeping exhaust gas velocity high is important, why are the best modern systems designed with successive increases in cross section, each of which decreases gas velocity?

How much heat do you imagine that a single exhaust pulse looses to an uninsulated pipe during the 50 milliseconds or so that it occupies the pipe?

For some reading, start here:

http://www.thumpertalk.com/forum/showthread.php?p=4948537#post4948537

ohhh I get so pimply when you scold me like my mom did. :thumbsup:

The reason you dont see big money using it is because it can cause corrosion of the headers. they all use ceramic coating now, for the same benefits.

The incremental increase in cross section presents an effective way to prevent the sonic pulse from reflecting back into the cylinder.

Got anything to show the decrease in velocity? The reason I ask is because the gasses are still building pressure along the pipe run and can look as a constant pressure for the entire run since the pipe diameter increases linearly along with the pressure. Thus with a constant pressure along the run of the pipe there is no decrease in velocity.

If a 50 millisecond pulse of exhaust can not impart very much heat then either physics is like a bowl of poopy soup or you are way underestimating the heat transfer. That pipe dont get hot from just conduction, it gets 99% of it's heat from convection, you know those little 50 milliseconds of exahust pulse.

The reason you dont see big money using it is because it can cause corrosion of the headers. they all use ceramic coating now, for the same benefits.

The incremental increase in cross section presents an effective way to prevent the sonic pulse from reflecting back into the cylinder.

Got anything to show the decrease in velocity? The reason I ask is because the gasses are still building pressure along the pipe run and can look as a constant pressure for the entire run since the pipe diameter increases linearly along with the pressure. Thus with a constant pressure along the run of the pipe there is no decrease in velocity.

If a 50 millisecond pulse of exhaust can not impart very much heat then either physics is like a bowl of poopy soup or you are way underestimating the heat transfer. That pipe dont get hot from just conduction, it gets 99% of it's heat from convection, you know those little 50 milliseconds of exahust pulse.

When the nationals come to SoCal, I see no ceramic coated exhausts on the factory bikes. As for F1 cars, those that use ceramics do so for the purpose of controlling the temperatures in the immediate area around the pipes, not for any direct performance benefit within the exhaust system. Rust prevention is very rarely an issue in F1, as steel is hardly ever used in building an exhaust system.

As the exhaust valve opens, the power stroke is ending (not over with), and the piston is still on its way down. The fuel from the previous power stroke is 100% involved in the combustion, but the expansion of combustion gases is still going on. The residual pressure of combustion begins the exhaust event for that particular engine cycle. As the valve opens, and the gases burst into the port under pressure, two waves are initiated. The first is a shock wave, which proceeds down the pipe at more or less sonic speed. The second wave is a pressure wave which also travels at sonic speeds, followed by the actual bulk of the waste gas itself. Assuming a straight pipe, the wave of positive pressure would travel to the end of the tube on its own, seeking a balance with the atmosphere. In an exhaust system, it will also have more to push against than just the closed end of the tube, since the gases will have begun to be pushed from the combustion chamber by the upstroke of the piston, adding force to it and maintaining the pressure at an elevated level. When the pressure wave reaches the end of the pipe, a pressure reversion occurs. The pressure wave expands into the open air with such force that it creates an isolated zone of low (negative) pressure at the end of the pipe. This negative pressure then moves back up the pipe as a wave, again at or about the speed of sound. If the pipe is made to a length such that this wave of low pressure arrives at the exhaust valve during the overlap period (while the intake is opening and the exhaust has not yet closed) it can do two things: extract the last remaining waste gas from the current engine cycle, and create a lower pressure within the combustion chamber, aiding the entrance of the incoming fuel charge of the following cycle. The point in the engine's power range at which this wave resonance harmonizes with the overlap phase is controlled by the length of the pipe. As the pressure waves will always travel within the same narrow range of speed, longer pipes work at lower rpm, and shorter ones at higher speeds.

That is how an exhaust scavenges. The diameter of the pipe is chosen to keep the gas velocity high enough to provide a strong pressure wave at the end of the pipe without unnecessarily impeding the flow of gas in the process.

As to the physics of gas flow, any time you force gas under pressure into a tube, you will get a flow rate that depends on the rate at which the gas enters the tube, and the diameter of the tube. These two factors will determine the pressure that is the motive force for the gas. At any point along the tube where the cross section increases, there will be a drop in pressure, and an associated reduction in velocity.

Mufflers throw a wrinkle into the entire equation of tuned exhausts because they have the effect of absorbing and damping the very pressure waves used to tune the exhaust. Because of this, we need to "fool" the exhaust system into seeing a sudden increase in cross section as an exit to the atmosphere. The multiple steps in the systems of today are an alternative approach which attempts to create smaller, serial pressure reversions to accelerate the exit of the gases by reducing the pressure in front of the wave.

With regard to the thermodynamics involved, it is important, as with all other aspects of performance tuning an engine, to understand that what appears to be a continuous flow of power from the engine, it is instead a repeated series of the same four separate and brief events in rapid succession. At 8000 rpm, there are 67 individual exhaust events (or intake, power, etc.) each second. Each of these is completed to the extent that the pressure within a good performance exhaust has dropped to at least atmospheric by the time the next one occurs, which leaves them with a total life span of 14 milliseconds before they get in the way of the next one. Since the pumping action of the piston would only move the gas out at 4000 fpm or so, this is why the action of pressure waves traveling at 1000 fps is so important to speed things up.

Even if it worked, increasing the velocity within the pipe by elevating its heat would do so be increasing the pressure within the pipe. You could accomplish exactly the same thing by reducing the pipe diameter. Would you? I thought not.

The gases leaving the combustion chamber do continue to expand, but only for the first 6 inches or so of travel, after which they begin to contract. Consider the actual BTU value of any single exhaust pulse, and compare that with the heat needed to raise the temperature of just the header pipe one full degree. Then consider the rate at which the pipe is capable of absorbing heat versus how long the the exhaust pulse even exists. What you will eventually realize is that the pipe gets hot as a cumulative result of thousands of individual exhaust events, and that the heat transferred to the pipe by any one of them is relatively minor. And that's the truth of the matter, poopy soup or not.

You are, however, free to believe whatever you might wish to. But the work of the likes of Gordon Jenkins will be at odds with you.

Even if it worked, increasing the velocity within the pipe by elevating its heat would do so be increasing the pressure within the pipe. You could accomplish exactly the same thing by reducing the pipe diameter. Would you? I thought not.

mototune usa doesnt seem to agree with you on this, nor does the stepped header design. Both start with a smaller than normal exhaust passage.

well this is interesting, even jet hot believes that old myth.

Nevertheless, JET-HOT Sterling will normally boost power when applied to headers for two reasons. First, the coating promotes denser, more potent fuel/air charges by insulating the engine bay from exhaust heat. At the same time, it accelerates the pulsed-vacuum effect on “tuned” headers, resulting in more effective scavenging of cylinders. The increased velocity of exhaust gases produced by higher exit inertia not only clears each cylinder more quickly; it also draws in the next fuel/air charge more efficiently.

Can it be? I was right? Why yes, I do believe it's so. :thumbsup:

http://www.jet-hot.com/Pages/tech1.html

Dyno tests done by a commercial interest in support of their product? Credible? Like when a pipe manufacturer puts up charts showing a 6 hp gain over stock, but no one else can duplicate the results? Right.

BTW, nominal exhaust pipe size is that which is equal to the size of the exhaust valve, or in the case of multiple valves, a circle with an area equal to the two valves. Not really a very large pipe, in most cases.

I was right? Why yes, I do believe it's so.
I knew you would.
Dyno tests done by a commercial interest in support of their product? Credible? Like when a pipe manufacturer puts up charts showing a 6 hp gain over stock, but no one else can duplicate the results? Right.

BTW, nominal exhaust pipe size is that which is equal to the size of the exhaust valve, or in the case of multiple valves, a circle with an area equal to the two valves. Not really a very large pipe, in most cases.

I knew you would.

As I knew you would attempt to drown out anything that you didnt agree with by overpowering the discussion. You big bully. :thumbsup:

Hey man lighten up, it's just a forum and should allow for differing opinions. Your next beer is on me.

With regard to the thermodynamics involved, it is important, as with all other aspects of performance tuning an engine, to understand that what appears to be a continuous flow of power from the engine, it is instead a repeated series of the same four separate and brief events in rapid succession.

Hey, grayracer513, as much as I enjoyed your technical discourse, you probably recognize that thermdynamics is a quantiative science, not a qualitative one. Thus the value of your arguments are somewhat dimisned. There's a saying in engineering, if you can't describe it numerically, then you don't understand it. Using this definition, few understand the thermodynamics deeply, and not to the depth that you demonstrated.

:thumbsup:

The task of accurately modeling the fluid dynamics of a modern 4T engine is often left to CFD (Computational Fluid Dyamics) applications and CAD/CAM programs, and yet the results are still debatable. In my experience, pipe and sliencer optimization is still done the old fashioned way, on the dyno, cut and retry. Any pipe manufacturer that developes good analytical tools would be wise to keep them to themselves.

Years ago I remember reading a book on the design of two stroke motorcycle exhaust systems, and have yet to come across anyting for modern four stroke bike. Should you know of a reference, please let me know about it.

Hey, grayracer513, as much as I enjoyed your technical discourse, you probably recognize that thermdynamics is a quantiative science, not a qualitative one. Thus the value of your arguments are somewhat dimisned. There's a saying in engineering, if you can't describe it numerically, then you don't understand it. Using this definition, few understand the thermodynamics deeply, and not to the depth that you demonstrated.

:thumbsup:

Years ago I remember reading a book on the design of two stroke motorcycle exhaust systems, and have yet to come across anyting for modern four stroke bike. Should you know of a reference, please let me know about it.

Thermodynamics, like any real science, is indeed quantitative. I just did not have the numbers necessary to put the discussion in that context. And you're quite correct, "if you can't put it into numbers, it ain't science." This was what we were trying to accomplish with the turbocharged Camaro in the thread I linked to; repeatable results from a controlled experiment.

But the two ideas at the core of my argument are both provable. Most people do not understand the dynamics of an operating engine as a serial event; the result of a stream of repeated actions, but mistakenly see it as a continuous flow of air, fuel and exhaust. This hampers their understanding of the processes involved to a very large degree.

Also, one could fairly easily quantify the thermal energy contained in any single exhaust event, and then simply compare that to the energy needed to raise the temperature of the exhaust by any certain amount, factoring in the rate at which the gas can deliver heat to anything versus the rate at which the pipe material will absorb heat versus the rate at which it will radiate heat to the surrounding air, versus the time available for the gases to transfer heat before their complete exit. This would lead you eventually to know how much cooling of the gas actually took place. Obviously some, as the gas stream starts out a good deal hotter than it is when it reaches the end of the pipe. Then, you could extrapolate what effect this has on gas velocity within the system. But, one needs also to bear in mind that the majority of that cooling occurs as a direct result of the pressure drop that happens when the gas is released from the combustion chamber in the first place, and also, there is the whole question of what, if anything, a minor change in the gas velocity does to the resonance in the system that actually extracts the exhaust.

You're also right about the value of modern CFD and engine modeling software. There's almost no way to express how much this development has advanced the state of the art. But even then, the accuracy of the model is only as good as the information put in, and how many and how well the numerous variables are dealt with. "Cut and retry" still ends up being a common tool.

I read some time ago a very well written series of two articles by Gordon Jenkins (who developed a very accurate formula for the construction of two-stroke expansion chambers, perhaps the one you read), on tuning both the intake and the exhausts of 4-stroke engines. He went into great detail in covering the subject, published formulas, and included a thorough discussion of collector exhausts and the effect of firing order and cylinder separation. I've been looking for it for a couple of years, and haven't been able to turn it up. The principals are still applicable in open exhausts, but, as I said, the use of silencers alters things somewhat. There is, that I've seen, very little published work on more modern systems.

ex-inwb.jpg

Thanks for the photo of the text, I'm thinking of buying it. Amazon has it.

Look at the published date(s). This is most likely the same info that was published in the articles I read, as that's about the period I recall reading it. Good worthwhile reading, I'm sure, but still not as current as the newest stuff out.

You should offer up something more contemporary if this is outdated.

Not saying it's outdated in the least as to its relevance to open exhaust systems, just that it very likely doesn't cover stepped pipe theory, or other aspects of dealing with high performance muffled systems.

We are saying the same thing. I dont know what it has in it, I just happened to come across it last night. By the way stepped headers were around in the 50's at least on planes they were. The old c118's I worked on in the military had them.

this is a pretty interesting site.

http://www.headerdesign.com/extras/engine.asp#Stepped-Tube_Headers

The C118's were an open exhaust, too, right? In one sense, a single step in an open exhaust behaves a little like a megaphone (diverging cone). But the write-up on the stepped tubes reiterates what I've said about them being the initializing point for scavenging waves (vacuum waves reflected when a pressure wave encounters a sudden increase in cross section):

Stepped-tube headers are used on some engines to reduce pumping losses on the exhaust stroke, or to intensify and advance the scavenging wave. The primary header pipe is stepped-up in size at a location about one-half to two-thirds the way down its length from the exhaust port. This causes a small scavenging wave to be reflected back to the valve from the location of the step. This increases the pressure differential across the valve during the exhaust stroke, which effectively increases the exhaust port flow potential. The step also causes the scavenging wave, reflected back from the collector, to be stronger but narrower.

The book you posted is likely a very good resource.

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