Exhausts

I have those pipes on my monster, I imagine the main reason for going 2 into one into 2 is to keep the routing compact and only require one exhaust flapper valve, it’s also got more to do with fashion than anything else.
Expansion chambers are not required on 4 strokes
Would there be an advantage in having a 2 into 2 exhaust?, probably not for road use , it would likely be bulkier/heavier and most modern exhaust systems are well designed to achieve the required flow and back pressure, of course small gains can be made by using systems that are performance orientated but usually there’s a pay off, more noise, less torque, flat spots that sort of thing, I tend to think that most bikes now have so much power that trying to get another few bhp is a bit pointless unless your racing, especially when you consider the cost, the above silencers are the best part of 1k, I bought the bike with them already fitted, they’ve probably saved a bit of weight over the standard set up but have they added 1k’s worth of performance, I don’t think so!.

 

There’s a lot more to exhaust design than routing and making them look good. On a 4 stroke, individual pipes rarely work well. Header length and size affect performance quite a bit if you’re getting down to the nitty gritty. On multiple cylinders, joint the headers and where you join them can play a huge roll in getting more out. If you get a few hours over the dark nights, look up scavenging and gas flows.....that’s before you get to silencing.

 

In a nutshell.. yes..

Fundamentally exhaust design is all about evacuating that exhaust gas from the combustion chamber, but its a little more In-depth than that.

In its simplest form you want a pipe to allow the exhaust gas to go aout of the engine - a chimney if you will.

Think of an exhaust in terms not of gasses but as pressure and shock waves.
On the two stroke what you see is the expansion chamber which is tuned to manage the shockwave in a particular way, and reflect it in a certain way.

I four strokes you find the terms like tapered headers, and stepped headers, they're not just packaging constraints to get it to fit, but also assist in pulling the gasses out and controlling the shockwave in a manner which can assist you.

Get it right, and you actually have a situation where the pressure wave is assisting in pulling spent gasses out of the engine, creating a vacuum effect and actually assisting the intake circuit by maximising the combustion cycle.

Those tapers, steps and bends are all doing a job in the best systems, and a hell of a lot of flow modelling goes into them. Igor Akrapovic and Yoshimura did stacks of work on those seemingly inconsequential elements and their placements - yoshimura duplex systems being a case in point with their tins on the back of header pairs, again to manage and redirect pressure waves for advantage.

Formula 1 did it with the stepped headers which were designed to reflect an exhaust pressure wave back down the pipe as it got close to the head, and in so doing actually pulled the outgoing exhaust gas with it.... rinse and repeat.

It's safe to say that by the time the charge gets to the silencer, 95% if not more of the work is already done. The silencers are there really to give a manufacturer a chance to put their own stamp on the product and tune the noise and aesthetic elements and also to conform with ever increasing legislative requirements.

In the case of the japanese manufacturers with their inline four engines the 4 into 1 is the most efficient, but will give a harsh sound, whereas the 4-2-1 will give a different engine sound, and of course lets not forget the aesthetic consideration of symmetry across the bike. a pipe on each side is a lovely sight compared to a monosided pipe.

In addition to all this we have length to consider. Lets take something like a bordi era ducati or a GSX-R IL4, who's route is quite simple and linear. The bore on the pipe can be quite small because the distance travelled is quite short. the smaller bore helps with the acceleration of the exhaust gasses whereas a bigger bore pipe will slow them down., and compromise the back pressure as a result.

When the fashion changed to the stumpy can in MotoGP the associated header pipes became a lot smaller in diameter as a result of reduced tract length.

On underseat exhausts i.e. long, practically double tract lengths you end up opening the bores right up to keep that pressure wave moving and exiting at the right point before firing back down the way it came, so in essence your 54mm bore on that underseat exhaust 916 is doing the same as the 45mm bore on the 888, but what you've done is moved the 888's exhaust 1m further back and added a handful more bends in the system - and bends create pressure drops and flow restrictions.

Back to your question though - you can have a pipe from each cylinder of course, the desmosedici had them independently from each bank in their GP4 system, but that was still matching pairs rather than four individual pipes, and it was probably done in order to make each tract as short as possible, with the front back only having one bend to get under the engine before hitting the silencer.

In order to optimise your system for dual pipe though you'd probably end up with asymmetric bores as the rear header would need to twist and turn more than the forward header unless you have it launching straight backwards D16 GP4 style. You'd get the assymetry because ideally you want it to behave and perform no differently to the front header, with the timing and pressure of the pulses being a perfect match the the other cylinder.

 

When the v4 came out, much of Wolf’s FB posts was trying different pipes and mapping to see which worked best before they raced them. It was the same with the V2 as well. I suspect that part of the current horrible system they have is best trade off for performance and noise. That can only happen with trial & error and lots of cash!