So. With my pipe the distance from piston to expansion chamber is approx 435mm so I'll try to take out 35-40mm and see how much difference that makes. I tried putting all the data in your calculator with header, diffuser, belly and baffle dimensions from your design but I get some odd numbers. I set target rmp 7500. I get that E6 is .39 and F19 1.43 and if I try to set the header length so that E6 = F19 then header length is 115mm. Am I doing something wrong and what could it be?