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Earth's slightly elliptical orbit means that our distance from the sun changes by a little over 3% in the course of a year. At aphelion today, Earth will be 94,502,714 miles (152,087,376 km) from our star, about 3 million miles (5 million km) farther away than we will be 6 months from now. Read about aphelion, and join EarthSky's Deborah Byrd at 12:15 CT (17:1

Guess who's coming for dinner...

Yesterday, Filipp Romanov used a remote telescope in Chile to capture this photo of an object suspected to have come from outside our solar system. And now it's official! Having further studied the space rock's orbit, the Minor Planet Center designated the object 3I/ATLAS yesterday evening, with '3I' meaning it's the 3rd interstellar object scientists have ever identified. And early signs suggest it's a comet. Find out more.

I enjoy getting this in my email,, Enjoy!

Dave wrote on Today at 09:14:54:


looks like it's in the oring

Quote:

digitaltrucker wrote on Yesterday at 15:43:29:

To which of those "problems that don't exist" are you referring?

I don't have any of these issues on my 1997 machine...nor do I remember reading on these forums that this is a wide-spread problem. How about just a replacement OEM cap? And if you've tried different OEM caps...then the problem is the fuel filler neck...not the cap.

Where is the vent?

Can water get in the vent while washing or in the rain?

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.

Following…. The Natchez Trace route is intriguing.  The 10 day ride “just cuz” is starting to resonate.  Someplace cooler and less swampy really sounds good.

MM - the description of the group ride guys made me laugh.  Between me and my 2 main riding buddies, we’ve got your list covered!  Our 3 giant meals a day guy gets out-voted.  We’re not doing $30 steak dinners for lunch!

Is you “Zen Bike” the souped up RE350?

Route 315 is where the Cherohala Mountain Trails Campground is located.  If you take 315 south it is a wonderful ride down to Route 74, which is a road they have tried to develop into a tourist trade area for the Ocoee River.  If you take 74 West you can then take 68 back north to Tellico Plains - it is a nice loop.

Or - when you are on 74 you can take68 south to Copperhill/McCayesville and go on down to Suches and Dahlonega.  You can then get on 19 and go up Blood Mountain on your way to Blairsville.  From there you can continue up 19 north and get on 74 west for a few miles until you find 123 north and go by the Field of the Woods.  From there you can continue north to 68 north and return to Tellico Plains.

And....there is always the Cherohala Skyway, 129 north to the Tail of the Dragon, 411 west, 360 loop back to the Tellico Plains.

Just be aware that you might have to repeat something when we get there!

But to stay with your original questions:
If an engine has no valve overlap, is scavenging pointless?

If there is no overlap, the intake and exhaust valves are never open at the same time, so no negative pressure from the exhaust pulse can affect the intake manifold. Scavenging in that classical sense does not play a role. Other effects, such as back pressure, resonance and gas flow velocity, remain relevant.

Do you lose power at low speeds with larger headers if there is no scavenging?

Large diameter headers allow the exhaust gases to flow more slowly at low RPMs (less speed, less pulse energy). This creates less kinetic 'pull' of the gases from the cylinder during the exhaust stroke. This can lead to less efficient exhaust gas discharge, so less good filling in the next cycle, despite the lack of overlap.
Also thermodynamic: slower flowing gases cool down faster, less expansion force in the exhaust, less exhaust efficiency at low RPMs.

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.

andrewmanzano wrote on Yesterday at 17:33:12:


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