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I had a 23mm I.D. header made and I just tried it out as a replacement for the 20mm stock one. (For my 55cc reed valve engine reving to 8500rpm with torque pipe). It gave better low end power and 6% more speed which means a broad range effect which may be attributed to better delivery ratio (% of cylinder space filled with fuel mixture). I think the too small header (for my higher rpm) causes a stronger suction after the exhaust pulse leaves which sucks out needed fuel mixture. The residue pattern on top of my piston backed up that belief. Some suction is desirable to help the mixture transfer up into the cylinder, but not too much. I think the calculator for header size on sheet 2 of my expansion chamber calculating Excel file is pretty right on. Now I need to make a few more runs and then look at the piston top to see how the pattern of residue has changed.
By my calculator the stock 20mm header is right for these specs:
engine
48cc - 7900rpm
55cc - 6900rpm
66cc - 5700rpm
So for the stock top rpm it looks like it's too big for the 48cc and just right for the 66cc.
By my calculator the stock 20mm header is right for these specs:
engine
48cc - 7900rpm
55cc - 6900rpm
66cc - 5700rpm
So for the stock top rpm it looks like it's too big for the 48cc and just right for the 66cc.
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