exhaust theory

[truckinduc]

Ok, So I have built a few custom exhaust before. I had a discussion going on TOB but all thats lost now.

Im looking for someone to give me Definitive proof why a 2 into 1 into 2 system is better than a 2 into 1.

Im not looking for answers like, "It just is better". I want this to get technical.

Thanks peeps.

[Jobu]

Because the lazy sandwich meat people decided that's what it needed.    And it looks better with two cans.

I don't know a lot about this, but maybe I can get some discussion started.  Seriously though, it is not necessarily better one way or the other.  Just look at MotoGP.  They can't even decide what is better.  The book "MotoGP Technology" has some interesting discussion in it about how the teams kept switching between different setups.  Of course, it matters more on those bikes because they are tuned so precisely and accurately.

On the 2V Ducs, I haven't really heard one way or another, but I do know that Termi and FBF started making their full race systems as 2 into 1 systems.  Seems they can get a little more HP out it with the 2 into 1 system.

[Norm]

I think Doug L. did some comparisons a while back, I don't have the site address but a seach should do it.
In order to be definative I think you'd have to try hundreds of different set ups & dyno each. I doubt if anyone has done all that work, I sure haven't. I've built a couple of dozen exhaust systems, 1 can, 2 cans, different kinds of cans, baffles facing rear, baffles facing forward, no baffles, different tube lengths. different tube diameters, step tubing, and while I've never dyno'd any of it, my seatofthepants dyno can't really tell any difference. All of this has been done on the 904 2v series motors, I don't know sh*t about the 4v motors

[A.duc.H.duc.]

Just as an additional point, if there isn't much HP difference between a 2 into 1 setup and a 2 can setup, the advantage goes to the 2 into 1 setup for weight savings alone.

That may be why a lot of race systems try to run just 1 can.

[hypurone]

The two main points as I see it for an exhaust system are flow and scavenging. A third would be exhaust gas speed but it is a relatively small part of the equation compared to the other two. It gets complicated by the number of cylinders you are tuning and the room you have to work with etc. I don't see any advantage to the 2-1-2 system if the flow and scavenging are done properly. IE; lengths are tuned , routes are smooth and diameters are adequate. Now if for some reason a 2-1 system is lacking in one or more areas and adding a second exit point can solve the equation, well maybe... But that would point to a poor design to start with...

Just my .02 worth from 20 years of auto tech work, but hey take it with a grain of salt, or you favorite shot... 

[Norm]

http://www.moto-one.com.au/performance/900monstercarbfinal.html

Here's a little more info.

[Norm]

Here's Doug's site:

http://www.visi.com/~moperfserv/

[clubhousemotorsports]

I think I found the problem. "Definitive proof why a 2 into 1 into 2 system is better than a 2 into 1" does not exist.

Each motor will be a bit different in the demands for an exhaust. In theory (that's what most of this is anyway) the engine would be built on paper (or PC) and then the exhaust system would be built. What characteristics are you looking for? Low-end, Mid-range or top end? lengths and diameters are settled on and then the pipe needs to fit the bike.
And then the designers get a hold of things and start messing with the lengths and dimensions just to make it look good.

As you are able to make exhausts you should try them both, you will need to tune them differently for each system and then dyno your results. In the end you will have a system that fits your Bike engine wise as well as to your own style.

I have a custom exhaust for my monster and to be quite honest I would not care if it lost me a little power as it is just what I wanted. At the far end of the spectrum you have the racers who NEED every last 1/2 HP and cannot afford to loose any. Oem exhausts work very well, they are just quiet and much too heavy.
have fun

[Popeye the Sailor]

Ok, So I have built a few custom exhaust before. I had a discussion going on TOB but all thats lost now.

Im looking for someone to give me Definitive proof why a 2 into 1 into 2 system is better than a 2 into 1.

Im not looking for answers like, "It just is better". I want this to get technical.

Thanks peeps.

The reason an X-Pipe crossover works better gives more effective use of exhaust gas velocity.

The idea behind the X style crossover is to unite the two cylinders for better exhaust gas scavenging. Instead of two separate cylinders doing their own work, the crossover uses the pulse created by a firing cylinder of one side to create a vacuum in the other cylinder because of surface tension. When the other cylinder is ready to fire, instead of the piston having to force the exhaust gas out of the cylinder, the vacuum that was created by the other cylinder bank helps suck the exhaust gas out of the cylinder, hence the term "scavenging."

Good enough? It better be. I refuse to do a flow analysis on this one.

[Norm]

In theory. I've never found a difference on these motors. I've even tried 2 different shapes of Xovers.........nothing. We're lucky, the 904 series, air cooled Ducs just don't seem to care. Years ago I worked with BB Chevies & Fords & got VERY significant changes with all of the changes above, just not with these Duc motors.

[Popeye the Sailor]

Yeah basically. Personally I wouldn't bother. Not worth the gains.

[bigtime]

The reason an X-Pipe crossover works better gives more effective use of exhaust gas velocity.

The idea behind the X style crossover is to unite the two cylinders for better exhaust gas scavenging. Instead of two separate cylinders doing their own work, the crossover uses the pulse created by a firing cylinder of one side to create a vacuum in the other cylinder because of surface tension. When the other cylinder is ready to fire, instead of the piston having to force the exhaust gas out of the cylinder, the vacuum that was created by the other cylinder bank helps suck the exhaust gas out of the cylinder, hence the term "scavenging."

Good enough? It better be. I refuse to do a flow analysis on this one.

Your answer is correct, but two into one systems also scavenge.  Something that people don't consider is that the crossover reduces noise without loss and this is considered a benefit in any application.  I read an article in a car mag a while ago that referenced an extensive series of tests on crossover systems and that 60% had better power gain, 40% had little or no (edit) change (no gain or loss) and 100% had lower noise so there was no negitive.  Of course these are properly designed systems, just welding a crossover in a system can have negitive impact if not designed properly. 

[OT]

There is a decent writeup on exhaust theory and practical experience in Kevin Cameron's Sportbike Performance Handbook.  It focuses largely on the role the exhaust geometry plays in creating pressure differences inside the piping (so-called negative or backward flowing waves) and the effects those waves have on the engine's ability to charge and exhaust the cylinders.

For your scenario on any one bike, the main things that change are resistance to flow by the muffler(s) and the position (forward or backward) of the crossover in the exhaust system (because the overall length of the exhaust system is fixed).

A simple test could be to dyno a S2R1K and a M1000Sie and see what effect the radically different 2-1-2 exhaust systems have on the same engine, as well as a S2R1k with and without a cat.  It would be interesting to see if the torque/hp curves are markedly different (within normal variability as evidenced by tests Brad has posted on his moto-one website).  FWIW, Akrapovic had a racing exhaust for the M1000 that (I don't believe) has any crossover and it dynos (on their old website) as weaker below 4500 RPM and stronger above that with a max Hp gain in the high rpm range.  They also have dyno curves for their replacement for the S4R that show hp gains down low and up top, with no effect in the midrange.

I think it's possible to test the effect for a single bike using a few assumptions.  It would seem you have three scenarios (for the same sized pipe) to play with:

* Pipes with no junctures/crossovers (Akrapovic).
* Same pipes with 'crossover' right before the muffler (2-into-1 system).
* Same pipes with 'crossover' somewhere upstream of the muffler(s) (2-1-2 system).

Your original question, then, could be "what is the effect of moving the 'crossover' to different locations between the muffler(s) and the heads"; that is, what's the effect of length on the hp curve....

Presuming the single, larger muffler's resistance has the same effect on the pressure wave as the dual mufflers have -- one test idea could be to dyno a 2-1 system, then cut off a few inches of the header/exhaust pipes (so the overall length between the exhaust valve and the 'crossover' is shorter), reinstall the 2-1 muffler, and dyno again.  Repeat until you can't cut anymore of the exhaust pipes off.  This would (hopefully) measure the effect of header length on the shape of the torque/hp curve and might be extrapolated to predict the effect of a 2-1-2 system, where the distance from the exhaust valve is shorter than it is in a 2-1 system....

Obviously, one larger muffler may offer different resistance to flow than a pair of smaller mufflers and resonance in the pipes could be different for the 2-1 crossover vs the 2-1-2 crossover but, as Norm said, those are other experiments...

[evoasis]

I wanna put a dirt bike (white bros silencer) on my bike using stock headers and oem left under tail exhaust mount... Anyone have any pictures of a custom single sided set up and appx cost estimate?

Christopher

[Popeye the Sailor]

Presuming the single, larger muffler's resistance has the same effect on the pressure wave as the dual mufflers have -- one test idea could be to dyno a 2-1 system, then cut off a few inches of the header/exhaust pipes (so the overall length between the exhaust valve and the 'crossover' is shorter), reinstall the 2-1 muffler, and dyno again.  Repeat until you can't cut anymore of the exhaust pipes off.  This would (hopefully) measure the effect of header length on the shape of the torque/hp curve and might be extrapolated to predict the effect of a 2-1-2 system, where the distance from the exhaust valve is shorter than it is in a 2-1 system....

That is a HUGE amount of time, material, and dyno resources to get info any third year engineering student could figure out with some basic fluid mechanics. Doing such a test nowadays would be silly.