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Discussion Starter #1
Hello, new here but not really new to 2 strokes. Dirt bikes before the kids arrived lets say 3 decades ago. Hydroplane racing just over a decade ago, now messing with light aircraft, and the common factor, all 2 strokes!

So, the aeroplane is going to need a new exhaust soon which, one way or another has to be built. However, now I'm looking at the original, I'm starting to question its design.

The engine is 2 cylinder inline, 580cc. The exhaust ports are side by side and feed to a Y manifold before exiting to an expansion pipe of sorts.

Whilst I understand the theory behind reflected waves and pipe lengths, this has thrown me by having a Y manifold. Doesn't the outgoing pulse from cylinder 1 interfere with cylinder2, or does the pulse from one cylinder create the back pressure for the other, by halving the length of the pipe?

Any thoughts on this?

Cheers
Smiffy
 

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If the exhaust port open duration is 180 (or close) then at top RPM the return pressure wave from one cylinder comes back to the the second cylinder as its exhaust port is opening which does no harm but actually adds to the sound wave from the first cylinder to have stronger return waves. But at some RPM one or both of the return waves comes back at an inopportune time and decreases power. Complicating things even more is that the return waves only partially are expended each time they come back to a cylinder (with open exhaust port) and so just bounce back towards the pipe again. This happens like 7 times. So imagine trying to formulate everything that is happening in a pipe calculator with return wave simulator. I'm smart as a whip and I tried and tried and tried and could never figure it out on my spreadsheet pipe calculator. No one else has figured it out either. I'm hoping within 10 years to have access to AI and just say "Alexa-10, please redo this program to include all 7 generations of return waves." problem solved www.dragonfly75.com/motorbike/ECtheory.html
 

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Discussion Starter #3
Ah, sounds a doddle then!

Exhaust port timing is about 180 degrees, although haven't physically confirmed it yet though.

So, I put some numbers into a simple pipe calculator, I treated it as half the cc's (single cylinder) running at twice the rpm. The end result gave me a tuned length close to what I've already got, but then I started to wonder if the belly diameter calculated will be correct.

I'm trying to achieve a fairly wide powerband, idle at around 2000 rpm. Peak power at 6000 and redline 6800.

35600


35601
 
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