Triumphrat

Loxpump · 02-23-2007, 10:59 PM

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.

Normandy · 02-24-2007, 02:01 PM

Thanks, very interesting.

sweatmachine · 02-24-2007, 02:04 PM

I just got rid of mine, kinda. I got a set of Bubs, no crossover pipe.

Jimbonnie · 02-24-2007, 04:14 PM

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.

Jimbonnie · 02-24-2007, 04:29 PM

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.

SCCTrim · 02-24-2007, 05:19 PM

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.

Jimbonnie · 02-24-2007, 05:33 PM

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

And that's another reason...

Loxpump · 02-24-2007, 08:58 PM

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.

357Bob · 02-25-2007, 12:28 AM

UUHH!!

Jimbonnie · 02-25-2007, 01:03 AM

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.