Modern Vespa

Many have probably seen this.
Just thought it stood out for simplicity of explanation of both motor and exhaust.
Really good stuff.

If you DON'T know this guy - he is brilliant - and funny.
Ton's of other good stuff(ing) to be found on his site.

Definitely a lot to chew on there in terms of thinking about optimizing crank case volume to match the pipe along with what I agree is definitely the best explanation I've ever seen of how (and why) tuned pipes work the way they do.

Thanks for sharing, CM!

He talks too fast. I'm even more lost. Never mind. I know that it works somehow, so that's ok. My spark plug is kind of brown and it runs well.

Nice video. I especially enjoyed the part where he demonstrates an expansion chamber pressure wave by tilting the water bottle back and forth, and his deadpan: "I'm left with the feeling this didn't help much." Laughed out loud.

Couple chuckles on responses.
Nomadism - that's what playback speed setting in YouTube is for!
🙂

I thought it was interesting that he pointed out that a pipe is much like a piston, only far more effective at pumping: and that rotary isn't as sensitive to case pressure as reed.

I thought It was a solid visuals on how a motor works…

A lot of it only made sense if you already has a solid understanding of the fundamentals and were looking to understand more about how the different components interact with each other, e.g. how resonance timing and exhaust timing interact to optimize the pumping action of the pipe.

As a pre-requisite to that video, this other video from CM2's thread explains the fundamentals of the two-stroke as a pump really well, so maybe start with it:

I knew that larger crank case volume favored lower power bands and torque, but I'm getting a much better feel now for *why* that's the case and what the influencing factors are.

And his comment about reeds versus rotary made perfect sense to me--air pressure makes (by design) a big impact on a reed, but ain't nothin' to a piece of rotating steel.

Also, the relationship of reed stiffness to crank case volume was something I'd never considered, either.

So much good stuff in there.

Couple of thinking points. The 125 and 200 cases both have the same size crankcase. Smallframes (like 50 Special) are only 25% less volume. What does that mean in reality when choosing go faster parts.

When he (Alex) talks about rpm, mid for him is 10,000rpm, as most of his engines rev to 20,000rpm. Perspective needed. A stock scooter fully screaming is usually little over 6000rpm.

How all this information relates to us is the tricky part. And as always depends on what kind of performance you are aiming for. But generally for road scooters, anything that makes the crankcase volume bigger (bell crank, bigger reed block, longer conrod, vacuum pump line etc) will make more speed, while running crankcase reeds. On rotary inlets, increased crankcase volume only helps with the longer cylinder port timings. Piston ported reeds have more cylinder specific recommendations, big subject on its own.

You make a good point that the specific values need to be translated to match our specific motors, e.g. the RPM range.

All the fundamental concepts remain the same, though, and we have to take our information where we can get it since largeframe motors are definitely a bit of an oddity in the two-stroke world.