It looks like this little Chinese pit bike took a page straight out of David Vizard’s exhaust design playbook.  

You’ll often see these “expansion chambers” on 2-strokes, but the concept is spreading to 4-strokes.  There are some high-dollar Harley aftermarket exhaust systems incorporating it and talking like it’s the greatest thing since sliced bread.  (Finally waking up to the phenomenon of scavenging I guess.)  If you read the fine print which is written rather deceptively, it adds 5% HP.  But read it carefully…it adds 5% to the claimed +5% of the complete exhaust system.  So it really only adds 0.25% overall.

Dave wrote on 06/24/25 at 06:38:08:


You’re right, Dave. Bad example.  He’s a better example for a known 4-stroke (Kawasaki Vulcan S 650).  I’d share the HD exhaust system I referred to, but it’s not popping up in my FB newsfeed at the moment.

What I’m really trying to educate myself on about exhaust tuning is:

1) with a bike like a stock LS650 that has no valve overlap, isn’t scavenging a moot point?

2) somewhat off topic, but related - larger diameter headers improve flow and help with top end performance.  They also get blamed for lost low end power due to slower velocity that impacts scavenging.  So, if there is no valve overlap and scavenging (stock LS650), would there be any low end power loss with larger diameter headers?  It seems like only upside with larger diameter pipes and a stock cam.  I know DBM proved “too big” can hurt low midrange with the DR650 cam that has some valve overlap and potential for scavenging.

Why am I bringing all this up?  I’m experimenting with an aftermarket exhaust system on my Vulcan S 650 so I’m trying to educate myself on exhaust system design and tuning.  And SurvivingPhilly with his muffler question on this forum made me thinking of putting my thoughts out here on The Cafe board.  I’d love to hear from people smarter than myself.  I must be running out of stuff to read on the internet and watch on YouTube.  lol.

zevenenergie wrote on Today at 03:56:00:


Thanks.  My lengthy post on this topic is on a Vulcan forum.  Completely agree that the stock exhaust on my bike is an engineering work of art and magician to jump through all the regulatory requirements around the world.

I’m making good careful progress with the aftermarket exhaust I’ve installed as well as the ECU flash.  I don’t think anyone on this forum really cares.  Note: the exhaust system I shared above is not the exhaust I installed on my bike.  Just an example of how it loosely (or should I say closely) follows David Vizard’s concept in a motorcycle application with limited space.  I think it’s pretty well conceived and other high dollar motorcycle exhaust system sellers are designing similar setups.

I shared the general topic here as fodder.  DBM has done the real work on the Savage with testing various exhausts and how they accomodate his engine with a more aggressive cam profile that can actually benefit from valve overlap and scavenging.

The video at 11:11 illustrates valve overlap.

The process of removing exhaust gases during valve overlap on the intake stroke is called scavenging. During this phase, both the intake and exhaust valves are open, allowing the exiting exhaust gases to help draw in a fresh air-fuel mixture. This process improves engine efficiency by ensuring a cleaner combustion chamber for the next cycle.

Stock Savage Camshaft = no valve overlap = no possible benefit from scavenging.

What I am learning is that there is an optimal primary pipe diameter for scavenging to occur on engines with valve overlap.  Smaller primary pipes do this better than larger diameter primary pipes.  Smaller meaning “appropriately sized and optimal” (not too big).  Moving through the system - the best overall design setup having pipes (and ultimately muffler) that get progressively larger to the exit.

Scavenging and low back pressure can coexist.  But can you make it quiet too?  Vizard provides a thoughtful approach to doing it.