Types of motorcycle exhausts?

[vybrano]

Usually motorcycles have exhaust in a form of a pipe - for example I have 5 motorcycles and 1 ATV and all of them have exhaust as a pipe with muffler at the end. But there are some motorcycles, that have "fat" exhausts (pic). Very common at moto-cross bikes I think?

To me, it looks like a exhaust, that you would find on a 2 stroke engine. But these bikes always look to me as a 4 stroke.

Why are some motorcycles having it? Why majority of bikes have just a pipe and these few have "fat" one (if they're not 2 stroke?)?

Also is there a way how tell 2/4 stroke bikes apart from mile away? I thought that exhaust could be telling thing, but it doesn't seem so... .

 

[JTSpeedDemon]

The exhaust ports on a 2 stroke are near the center of the cylinder(that bike in the pic is a 2 stroke).
Most likely those are 2 stroke bikes, and those are called expansion pipes.
Some 2 stroke race kart classes have subdivisions, a "can" class with a muffler, and a pipe class with expansion pipes. Sometimes they can make the difference between 10 HP in the can class and 20 in the pipe class.
It would be odd to have an expansion pipe on a 4 stroke.
The science behind an expansion pipe has something to do with the "shockwave" of the engine. I don't know much beyond that, so I'll defer to 'sid.

 

[vybrano]

Expanding part will suck smoke out; and then narrowing will send wave back to block new mixture escaping to exhaust?

Sure it's 2 stroke? I try to dig up some identifiable bike with this type of exhaust and see if it really is.

---------- Post added at 10:35 AM ---------- Previous post was at 10:30 AM ----------

Wikipedia:

Honda Nova Sonic RS
The Sonic is fitted with the European CBR125R and Indonesian CS1 4-stroke engine.

 

[itsid]

expansion chamber exhausts are 100% a two stroke thing!
JT is correct there.

And that appears to be an NSR engine maybe an NS engine, not a CBR125

And while there are pre-muffler 4strokes with a somewhat bulged out pipe..
neither the Honda nor the above Suzuki is one of those..

'sid