I can see why some builders feel RPM is tied to duration, Sometimes on some of our stock cylinder engines we are forced to raise exhaust ports to 198-200 @ 76% of bore to reach target BMEP at tuned RPM. Criteria for our pipe is calculated from projected heat and velocity from bmep,calculated kh ,intermediate,belly, stinger factors, blowdown PSI and blowdown duration. All angles are rated bmep and work in conjunction with each other to control expansion and velocity and not lose amplitude @ 15 degrees before the exhaust port closes. Trying not to be too technical here. We will offer a alcohol version because the exhaust gases are cooler and velocity is different .