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Discussion Starter · #1 ·
Google is cool! Found this on of all places Mustangforums.com. It seems to make sense. Is the headers on the Bonneville tuned? Also with debaffled or straight through exhaust/mufflers, is back pressure now so minimal that "Reversion" is no longer an issue? I sure don't want to do anything that would reduce HP or damage the engine. Here's what was written.

NOW - A couple of people ask about the Cross Over on a Motorcycle - They usually only have 2 Cylinders.

It is actually a little more technical but I’ll try to keep it simple.

In the case of 4 stroke motorcycle engines, the reason for a cross over is this...

After a cylinder fires and the piston cycles “up” - exhausts gas vents through the Exhaust valves, along with the remainder of the “explosion”, exiting the cylinder into the exhaust system.

This explosion creates 2 pulse waves… a Sonic Pulse - and a Thermal Pulse.

When the faster Sonic Pulse leaves the exhaust it has created a vacuum in the pipe behind it, which air pressure will enter the pipe to fill the vacuum. This air will collide with the Thermal Pulse and cause it to reverse. The timing of this event causes exhausted gases to be returned to the cylinder while the exhaust valve is still open therein not allowing all the spent gas to escape and also polluting the environment for your next stroke cycle with fresh air/fuel mix.

This “Event” is call Reversion and seriously degrades performance and lifetime of valves and cylinder heads.

The Cylinders fire opposite of each other – or nearly so on a Harley. So when One side is firing, the other side is idle. Having the cross over pipe allows the pulses to dissipate back and forth between the tubes instead of - or before exiting the exhaust and therein greatly diminishing the effects of reversion and so improve performance and engine life.

Yes - you see a lot of bikes with duels and no cross over. These are usually "Tuned Pipes".. meaning their length and size are tuned to work with the S/T Pulse in such a way that reversion can not return to the engine before the exhaust cycle has ended.
 

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Nice explanation Loxpump, but strictly speaking it's incorrect. Unless I misunderstand you, you are describing the "Kadenacy" hypothesis that espouses that a "slug" of gas blasted out of the exhaust port and down the exaust pipe creates a low pressure zone behind it, drawing additional gases from the cylinder, or in the case of your explanation, into the exhaust pipe from the atmosphere. What you describe sounds like an extension of this hypothesis, which was conclusively proven to be incorrect during the second World War (Griffen, 1940, "Rapid Discharge of a Gas to Atmosphere"), but is still believed by some engine "experts"; probably because it makes such good intuitive sense and is easy to grasp.

What we're really dealing with are finite-amplitude waves which work at much higher energies, and do not follow the simple laws of acoustic theory (Ernshew, 1860, "Mathematical Theory of Sound"). The pressures involved can be 10,000 times that of acoustic "sound" waves. In the most simple terms, when a "compression" wave leaving the exhaust port meets an obstruction, or a change in section (header junction, cross-over pipe, exhaust pipe outlet, header step, etc.) a reflected wave called an "expansion" wave travels back up the pipe in the opposite direction from the compression wave. In both cases, compression and expansion, while the pressure "fronts" travel in opposite directions the net gas particle direction is encouraged away from the compression wave source.

The trick in a "tuned" exhaust system is to have these reflected waves occur at just the right time to improve "scavenging" of the cylinder and to draw in a fresh charge. There are relative simple formula's which can help you get in the ball park for pipe lengths, but there's much more to it than that. You have to decide pipe diameter (determines exhaust flow velocity and the strength of waves), whether you are interested in primary waves (first reflection), secondary waves (second reflection), etc., and at what RPM you want to "tune" for.

Your cross over pipe is Triumph's way of 1) effectively doubling the size of your relatively restrictive exhaust system, and 2) generating additional waves to broaden the range of RPM's you exhaust system is "tuned" for. You might answer, I no longer have a restrictive exhaust system, but generating additional compression waves can still benefit you. If you've ever seen "stepped" headers, they are designed specifically for this purpose and are used successfully on lots of high performance motors.

The only reason I can envision for removing your crossover pipe is cosmetic or because you want to generate a strong primary wave to help your engine develop maximum horsepower within a very narrow rpm range and you don't care what happens outside of this range. In this case you'll likely be using a straight pipe of a tuned length and have no muffler at all. You will know when you're "on the pipe" by the howl the pipes make at the RPM's tuned for. I did just this for a high performance BMW R100S but the power peak was so narrow I had to cut a 45-degree angle at the end of the straight pipe to broaden the reflected secondary wave at the expense of some peak power. The Harley guys would look at my 38mm straight pipe and always comment that it was a tiny pipe, and wouldn't it be better if it were 2" or more in diameter. The answer was no.
 

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One last thought on cross over pipes. While aftermarket manufacturers may give what sound like good reasons for why they don't include them on their exhaust systems, there's no escaping the fact that to make them correctly is quite expensive in comparison to simply bending a tube. Triumph (and Harley, and BMW, and Ducati, et al) would have saved a bunch of money had they simply left it off.
 

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After having my exhaust on/off the bike a few times, I have had the balance tube removed and blanks welded on the pipes. Making a little over 80 horse's I'm not too concerned about losing a little power here and there.


...I'm just happy not to have to deal with that dam tube anymore.
 

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Discussion Starter · #8 ·
Jimbonnie,
I can't take credit for my incorrect explanation, all I did was copy and paste what I found on the mustangform site. I'm not smart enough to even come up with an incorrect explanation myself. My reason for removing it is purely cosmetic and ease of removal and replacing the headers. I don't think the seat of my pants will notice a few ponies missing if that does happen, and if Triumph did it just to increase the size of my restrictive exhaust, I no longer have that restrictive exhaust. If it's working ok for Sweat it should work ok for me. I'm sinking farther into the dark side, there is no returning.
 

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Intake and exhaust tuning is a bit of science and a bit of voodoo. Most folks can't feel 5 hp more or less, and I'm probably one of them, and so cosmetics and ease of maintenance play a part in my decisions also. I'm not criticizing your post Loxpump, just adding to the general knowledge.
 

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This has all the elements of another "wide is better" tyre debate! Fantastic! I tried to work this out myself and even attempted the maths. A Graham Bell's book Four Stroke Performance Tuning is a good reference source. It's all really complex and frankly I never got an answer. My reason was that I wanted to fit Triton type swept back headers. Obviously there's no place for a cross-over pipe. We fabricated a test set in mild steel with a considerably shorter overall length and baffled mega employing the science that we could. Really it could be best explained as informed guess work. And a little knowledge is a dangerous thing. The result was 3 bhp more at the wheel but way too much noise! can't honestly say what elements contributed to the gain. The project's on the back burner. I'd still like swept back headers.
 

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On 2007-02-23 20:59, Loxpump wrote:
Google is cool! Found this on of all places Mustangforums.com. It seems to make sense.
The bad thing about the internet is that anybody can post any sort of nonsense. If it sounds right, who's to know the difference? That's why you need to look at stuff with a critical eye.

This explosion creates 2 pulse waves… a Sonic Pulse - and a Thermal Pulse.
Hmmm... gases do leave the exhaust port at the speed of sound. And I suppose that some heat leaves as well, but it travels down the pipe at the speed of... uhhh... heat?
When the faster Sonic Pulse leaves the exhaust it has created a vacuum in the pipe behind it, which air pressure will enter the pipe to fill the vacuum. This air will collide with the Thermal Pulse and cause it to reverse. The timing of this event causes exhausted gases to be returned to the cylinder while the exhaust valve is still open therein not allowing all the spent gas to escape and also polluting the environment for your next stroke cycle with fresh air/fuel mix.
This actually is true, except for the thermal pulse part. Exhaust gases leaving the end of the pipe do create a reversion wave that travels back up the exahust system at the speed of sound. And if it arrives at the exhaust port at the time it opens, it can retard the escape of exhaust gases from the exahust port. Only thing is that this is mostly effective in TWO stroke engines, and can actually be put to use to prevent overscavenging (i.e. it prevents the escaping exahust gases from leaving too energetically and dragging incoming air/fuel mixture along with it).
So when One side is firing, the other side is idle. Having the cross over pipe allows the pulses to dissipate back and forth between the tubes instead of - or before exiting the exhaust and therein greatly diminishing the effects of reversion and so improve performance and engine life.
While a crossover tube may indeed diminish the effects of reversion, I have always been taught that its primary purpose was to effectively double the muffler volume that each exahust pulse "sees". And if you think about it, what part of the exhaust cycle (very high pressure exhaust gases escaping versus relatively low pressure reversion) are likely to be more important, or be more affected by the presence or absence of a crossover tube? Hint: It's all about dealing with that big, loud, sonic-speed shock wave that escapes each time an exhaust valve opens.

Beware of the sweet-sounding bull$#!+
 

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So, for us simple folk...crossover essential/not essential???

My service dept acted as if I were crazy for considering it. They made sound as if it could actually dmage th eengine in some way. They stated that the aftermarket headers are actually designed to wokr without one, but the stock ones are not.
 

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While our modern/retro bike's werent designed for ulitimate power, you can bet alot of engineering went into the exhaust system on our bikes. I've made a ton of change's to mine, while at the dealership yesterday speaking with the owner he wondered outloud if Triumph was upset at so many on this side of the pond changing all sort's of thing's on our bikes.


I laughed at this, but at some level it's probably true. I'm anxious to ride my bike once it's back together to see how it perform's since removing the tube. I dont believe I'll see a big change, I'm sure I could feel a 5 horse difference, 1 or 2 though would be more difficult.


I've had my bike fired twice since mounting the headpipes and 1 thing I'm certain of is that the exhaust pulses are stronger since removing the tube..kinda make's sense.
 

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Discussion Starter · #17 ·
If Triumph didn't want us to modify our Bonnies, why did they make to so easy and so much fun to do?
They should have put one of those tags that mattresses have on them, " Do not modify by penality of law".

[ This message was edited by: Loxpump on 2007-02-25 08:16 ]
 

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I wouldn't pretend to out-think or out-brake the computer controlled integral braking system on my BMW when I had it.

I do however mess around with mufflers not meant for a specific application..... KNOWING all the while that I don't know a danm thing about what I'm doing.
----- I draw the line at headers; I ain't gonna mess around with something that a great deal of science, math, theory, and practical experience has already sorted out.

ZARD' exhaust systems could have earned my $750.00 for a system IF they produced results via a Dyno graph. Oh I don't doubt that it's a good exhaust system! The guy who designed it is well known in the racing circuit..... but the street is a far cry from the race track. Hell, I still haven't heard or read anything about ANY of the various systems they offered up for bid a few months ago......

GOOD exhaust systems are designed with a purpose in mind. And never has that been better demonstrated then by my experience with my D&D's vs my experiments with Big City Thunder vs my Predators. The differences between these cans is nothing short of dramatic!

Back in the day, the hot set up was either a "tuned" two into one (or four into one) exhaust system. I remember when factory exhaust systems started employing cross-over tube technology. It was said that the cross-over tube replicated the effect of a collector on a 2-1 or 4-1 exhaust. Suddenly a great deal of technology and research went into designing factory exhaust systems; and performance was the goal. In addition to performance, sound deadening systems soon became a priority.

Bottom line, I put stock into the research that has resulted in the header systems DESIGNED for our bikes. I may still mess with different cans, but I will NOT pretend to be able to out think the tech' geeks who designed the factory cross-over tube. Nor will I try to out-guess the tuned headers (length, diameter, etc.) of say.... the Bub exhaust 2-2 system.
 

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When I had my 2into1 lowpipe system on,I kept the x-over and basically added another for the can.My bike had a noticable increase in torque,where even a hint of throttle would make the bike lunge forward.The exhaust pulse from the can would push your hand away from the opening and could be felt 10' behind the bike.The shorter high version had no x-over,just the collector,had great bottom and mid range and a strong pulse.I think there is something to be gained by merging the pipes,one way or another. :???:
 

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Crossovers are nothing new...my 1960 BMW R60 had one and it's about as low tech a machine as I've owned. My belief has always been that it's just a good way to relieve back pressue. In other words, each cylinder gets to use both mufflers. However, the crossover probably diffuses the combustion shock wave...reducing any scavenging effect. That's why well engineered collector systems work well with our bikes: proper scavenging. The same might apply with Sweatmachine's Bub system. It's my understanding that the Bub mufflers offer very little back pressure. That and a well-engineered headpipe length may explain why they work so well...better scavenging and little or no back pressure.
 
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