I've tuned a couple of bikes using both 2-1-2 and 2-2 systems. What I've seen is the 2-1-2 have an advantage to about 2500 rpm. At the lower rpms, the exhaust can use both pipes from the cross-over back, it's basically a more open exhaust. At the higher rpms where scavenging is important, the actual design and cam matter most. I don't know enough or have tested enough to make comments here.

If I had to guess, a 2 into 1 is the best of both worlds if sized right to the motor/cam. I happen to like the sound of 2-2 so that's what I run.

 

Two into one exhaust systems tend to increase low and midrange power. This system is perfect for street use. The best exhaust I had was a two into one on my Sportster. With turning you can bring the power thourgh the mid range into the high. The length of the megaphone and the outlet diameter also plays a part in exhaust systems. The right system can add horse power to the motor. I have thunder headers on the bike now. I love them, I think an all out race system could give me more power. Just my two cent.

 

A properly designed 2-1 pipe uses one cylinders exhaust to scavenge exhaust from the other and vice a versa.... or pushy pully effect as someone wrote. By properly designing this effect and adjustment of diameters and lengths of head pipes you can make a pipe that produces power over a broader range.... Street Riding/Road Racing. We have all seen road race cars and bikes that all use a "Collector System" to create this effect on any applicatin that needs a relatively broad range of power.
Technically a single exhaust, per cylinder, 2-1 will make the most "HORSEPOWER" as is seen on dragracing aplications be it auto or car.... however this is only done over a very narrow RPM range .... the tuning being the Diameter and the length of the pipe. I believe that Diameter has more to do with the displacement and flow capabilities and they use the length to fine tune the effective RPM range to move the power band up and down a bit.

I have a pair of Vance & Hines longshots and as someone mentioned they are a 2-1-2 but they are not a very good one as far as the "Scavenging of Exhausts" goes. This is because the junction is not designed to be smooth and efficient but to be not-noticeable so as to maintain the classic look of a 2-2 pipe. I'd think that most of it's performance is from uncorking the flow over stock but not from being as effecient as possible. It helps on toning down the sound a bit since you can't get much baffling in a straight pipe and now the exhaust does have twice the volume to flow through even if it is a convoluted and restrictive path. I like their looks but these pipes will be history as soon as I try and make any serious HP/TQ

 

Cross-overs, or "H pipes" have been used in rodding for decades. Don't forget tho, you can't just change one end of "the pipe" and have it do everything. A faster intake & exhaust stream gives you more low end to mid-range torque. . nobody rides at WOT all the time.. not even 1% of the time... unless you're a drag racer. So you tailor your system to 'your' riding style

 

Well, I've pretty much settled on 2-1, since I already have the headpipes and collector. I'd still like to find a source for tubing and "Y" pipes, so I can custom tailor something else.

 

What are you thinking of doing? Have a drawing of the concept?

 

Nah, just thinking of trying longer primary tubes to keep the velocity up, then into a smaller than usual exit tube. Gotta get home so I can pick up a few different sizes of exhaust tubing and figure out how to fit a baffle.

First, however, I'm gonna do something a lot like yours and jeepster's. I don't want to chop up a Rush muffler, but I do like the 2" baffles, so I want to find a pipe that they'll fit. If not, I'll probably weld a disc and ring on one of the baffles so I can slip it in and screw it into a slightly larger ID pipe.

 

If you make the baffle too small, which I think a 2" baffle might be, you would most likely end up with some high back pressure issues leading to a very poorly running setup. This is based on 2-1 pipes.
I have no hard data to back it up, but when I was building mine the general consensus of some buddies who build race motors was anything smaller than 2-1/2" with any kind of baffle would be too restrictive and make it difficult to tune properly.

 

Thanks. I've read a lot of your posts, and I most definitely respect your experiences with performance mods. I'll certainly keep that in mind. Might have to do the baffles like that other guy who did a 2-1 Tommy gun exhaust. He welded a couple of partial discs into his pipe. Cut slots, slid 'em in, and welded 'em up.

I like the RB Racing concept, where they have a "venturi effect" in the exit pipe. I have no idea, yet, how I'd accomplish such a thing, but perhaps a couple of reducers, big/little/big would do the trick. :shrug: (If that's not a smilie, it should be)

 

I will be looking forward to seeing what you build.