On an OEM exhaust system, just before the Y-piece, there's a metal box that the 4 pipes(Headers) go into.
What does this do?
Is it some sort of restriction?

The O2 sensor goes in there too.

What will happen if I cut that box out and extend the headers and connect them to a new Y-piece?
I will also reconnect the O2 Sensor.

You DON'T want to do that -- next step would be buying a new header.

That section you're referring to is the collector. The bike is tuned for its effect (which is the opposite of being a restrictor). It is there to generate power and taking it out to modify it unless you're an expert in building headers would have the bike running like shit, at best.

Here's an overview of how it works:

Two exhaust pipes pair together first, from two cylinders that do not fire at the same time (360 degrees apart here ??). As the exhaust pulse enters the collector, it exerts a negative pressure wave (suction) on the other. That negative pressure wave travels back up the other pipe, and at the time when that cylinder's exhaust valve** and intake valve are both open on the intake stroke (during what is called "overlap"), it has the effect of sucking more air-fuel mixture into that cylinder through its open intake valve while its exhaust valve is still open (but closing), thus getting more charge into the cylinder to be compressed and burned than it would if the header wasn't exerting this effect. The lengths of the pipe to the collector are designed to time this effect. The pairing of the pipes from the other 2 cylinders does the same. Typically, this effect has max effect in certain parts of the RPM range, and other things such as ignition and valve timing, fuel mixture (jetting or mapping), etc. must be tuned for optimum results. If it's only a 4-1 collector (all 4 down tubes mate simultaneously in one collector), the peak effect in the target part of the RPM range might be optimized, but usually at the expense of bigger flat spots elsewhere in the RPM range. To avoid bigger flatspots (typically in mid range RPMs), in a 4-2-1 arrangement after the first two pairings meet in a collector, you have a second collector that mates those two. That's why it's called a 4-2-1 header (instead of a 4-1), and that second collector exerts the same kind of negative pressure wave (suction) on the pipes upstream, usually resulting in less peak power than a 4-1, but also with less pronounced flat spots in the RPM range. Look closely at your Bird's header, and you'll see that the part of it you've described really has 2 collectors: the first two cylinder pairings, and then the pairing of that result. When it splits back to 2 for sound muffling through 2 cans, it is referred to as a 4-2-1-2 system.



** actually, there are 2 intake and 2 exhaust valves in each cylinder on the Bird, but each pair opens and closes at the same time, so I've used the singular "valve" to illustrate the process.

I have actually thought about something similar to what 'flipper is contemplating with the exception of cutting the collector behind the merge but just in front of the Y to the mufflers. It doesn't look as if it would be too hard to fabricate a transition pipe and build from their to a single outlet exhaust using a can of adequate capacity of course.

Then I looked some more, thought about the penalty in noise and the PITA exhaust fabrication is when you don't have a TIG welder handy and just decided that the stock arrangement is fine for me. I bought this bike for touring after all and probably don't see redline in more than second or third and that only once or twice a week. Screw it, with 60 odd pounds of luggage distributed through 2 Givi 360's on the sides and a 460 on the top rack, I ain't beatin' anybody anyway.

Why, I'll bet I can't even stay with a stock '02 with a pipe.

The injected factory header is tuned for more low end torque. The carbed factory header is tuned for more mid-range and top end power. It's in the cylinder pairings. The injected header pairs 1-3 and 2-4 for the first collector then merges the two collectors together. The carbed header pairs 1-2 and 3-4 for the first collector then merges the two collectors together.

I think I'm correct with the primary header collector numbers, but might be off. I know that they're paired differently and the carbed has a more definate "hit" at 7K compared with the injected header.

That's why the 97-98 are faster.

Dont forget the silver '02

Close -- you're right about the carbed pairings, but the FI pairs the 1 - 4 and the 2 - 3.

But this illustrates why you have to be an engineer who has the right tools and lots of design knowledge. I haven't checked to know for sure, but I'd guess that the different pairings are still pairings of cylinders firing the same number of degrees apart, even though they are different cylinders paired together. How that makes the change in performance parameters is beyond my understanding.

BTW, Micron 4-2-1 pipes use the carbed pairings no matter what model (carbed or FI) bike you have. I believe Akropovic mimics Honda's distinction. Don't know about other pipes. I have a Micron on my FI '03 with the PCIII mapped for an Erion 4-2-1 as recommended. It rips.

The cylinder firing remains the same for both carb & EFI versions, so that would mean the carb version pairs cylinders firing 90 degrees apart at the crankshaft, and the EFI pairs cylinders firing 180 crankshaft degrees apart.

Paired cylinders firing 180 degrees apart is the best for mid range torque, albeit at a minor cost to top end hp. The EFI bikes made up for this by adding ram air.

AFAIK, both versions of the XX have the same top speed & hp output, but the EFI makes more torque in the middle.

The Akropovic on my 05 Bird uses the carb version pairings. The OEM header pairs 1 & 4 and 2 & 3.

The Akra is designed for the FI model as it has the o2 sensor hole.

The mid range is still good and the top end is awesome.

That is after a o2 eliminator and a Power Commander.

It was crap without that.

JohnS

The square box on your header is also a catalytic converter. There are two more after the O2 sensor in the stock slip-ons.

Are you sure? I am (almost) positive that there is no catalytic converter in the junction box, only in the pipes.

The cats on my bike are in the muffler link pipes. Thats why they are fatter.

The collector boxes are just boxes.

I have seen this mentioned elsewhere and think that a lot of people are confused about the exact location of the cats.

If the collector had a cat the O2 sensor would be downstream of the cat and that would defeat its purpose.

JohnS

See. Told ya!

I have cut the collector open and there is no cats in there. Didn't see any dogs either.

I'm pretty sure that US 49 state bikes have no cats or O2 sensors in the entire exhaust system.

The California and Euro models might have all kinds of smog stuff, not sure.

Well, I haven't cut open my California header, so I'm not sure where I got that idea.

I know they definitely are in the stock exhaust cans.