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.