That photo looks like a 2 stroke to me....the carb inlet is at the bottom of the crankcase.

The exhaust system of david that you mean has a chamber that fools the exhaust gases as if they are coming into the outside air. And after that comes the silencer. That chamber seems a bit big for a bike.

Modern engines have to meet high emission requirements, so it looks like you can make power improvements yourself with a self-built or aftermarket exhaust.
I have serious doubts about that.
With the current injection systems, labda sensors, air flow meters, throttle position, variable ignition times, and on-board computer, the exhausts have also long since stopped being pipes.
you really have to come from a good house if you want to design an exhaust that performs better than the standard exhaust over the entire speed range.
The chance that you disrupt something is not unthinkable.
So you should consult the forums or someone who knows this bike very well. In other words, it's not a piece of cake

Before you know it, your "piece of cake" is locked in the incineration room.

But don't let my insights put you down.
I hope you'll create a topic where we can follow the developments. I'll do my utmost to support you with advice.

The knowledge you are looking for can be found in four-stroke tuning programs.
By working with these, you will gain a great deal of insight. The better packages include power simulations and flow simulations.
What you do not understand immediately, you can easily look up or learn via YouTube videos.

https://www.youtube.com/watch?v=J-KdITP_d4U

At 11:11 you see that the intake stroke already starts and that fresh mixture is already flowing into the cylinder while the exhaust valve is still open and the exhaust is sucking. That is something different than scavenging of the combustion chamber.

What I'm trying to convey is that there's more to exhaust design than simply removing burnt gases from the combustion chamber.

Because the intake and exhaust valves are briefly open at the same time, a phenomenon known as valve overlap, the exhaust flow can create a vacuum effect (or scavenging), which helps pull more fresh air-fuel mixture into the cylinder. This significantly enhances volumetric efficiency, especially at higher RPMs.

Beyond camshaft timing, the exhaust system plays a crucial role in this process. It's largely a game of pressure wave dynamics: the length, diameter, and shape of the exhaust pipes determine how these waves behave. Specifically, the primary pipe length (often called the pitch length) must be tuned to reflect pressure waves back toward the cylinder at just the right moment, ideally during the valve overlap period.

In a Vulcan S 650, which is a parallel-twin engine, both cylinders share a common exhaust system. This means the exhaust pulses from each cylinder can interact and influence each other, creating complex pressure wave patterns. As a result, the optimal pipe thickness and length can't be calculated the same way as for a single-cylinder engine. The presence of the second cylinder changes the timing and amplitude of pressure waves, which must be considered in the exhaust design.


David’s exhaust concept is a clever solution, managing to reduce noise while retaining some of the performance benefits of open pipes. It’s simple yet ingenious. But it only works effectively if it's integrated into a system where valve timing and pressure wave behavior are properly calculated and understood.



The process should go like this:

 Understand the physics, how valve overlap, exhaust pulse timing, and wave reflections influence scavenging and backpressure.

Define your goals, more torque at low RPM? Better top-end power?

Calculate the system, tune pipe lengths, diameters, and merge points based on pressure wave timing and engine RPM targets.

Then, and only then, do you move on to adjusting the engine management system (ECU) to take full advantage of the physical setup, fuel mapping, ignition timing.

Without that foundation of understanding, even the most nicely made exhaust won’t perform as intended.

It seems like you are thinking:
applying david's idea is the same as mounting open pipes but with less noise, so just the right thickness of pipe, and you are done.
But it is not that simple.

And even if you start working with software you have to enter somewere that you use David's principle and how do you do that? Enter that you use open pipe,s?David's idea must also be calculated to function optimally.

Tuning engines seems simple but it is complex.
If you mistake a wrongly designed 2-stroke exhaust for a David Visar version for a 4-stroke, then I would just imitate someone else's setup.
And everyone will praise you for your wisdom.

Thanks for your thoughts, Zevenenergie.  We’re on the same page about valve overlap, scavenging, and how it works on a parallel twin with a highly engineered 2 into 1 stock exhaust.  And your comments about the single cylinder Savage are duly noted as well.

Long story, but my goals changed along the way so I’ve had to take 2 steps back to go 3 steps forward.  I’m very close to where I want to be performance-wise while also keeping excessive noise in check.

Diagnostic dyno scheduled for 7/12. Go from there.