I think I have a improved the constant for the #3250 pipe by using a average constant of the twin disk and flat disc constants (1650+1200)/2 = 1425. I am guessing that the cone is 32.5 cm down the pipe and the header length is 15 cm. Taking Ext for my 15cc as 155 and my target RPM as 10300 with a 14X8 APC :
(1425*155 ) / 10300 = 21.44 inch = 54.5 cm
I am getting the most power with about a 7 cm extension pipe. Ad 1 cm for gaps where the pipe is in silicon pipe.
32.5+8+15= 55.5 cm
But I am not getting 10300 yet, I am only getting 10000. So :
(1425*155) / 10000 = 22 inch =56 cm
That’s creepy close
Cheers Piet