Reading the above books, it's important to keep in mind that both authors' porting generalizations refer to comparisons among fully-developed old-school racing engines, and shouldn't be taken out of context for modification purposes. I've spent the last few days running engmod2t simulations again, and I can't help but notice that low-end power seems to be about trapping efficiency, while top-end power seems much more about flow per unit time. Here are links to the Jennings and Bell books: I have not read Jennings book but it is always on my Christmas wish list so I cannot compare the 2 but I don't believe you are getting everything that Bell is trying to say. The short circuiting that you speak of I believe is talking of fresh air /fuel mix being swept/pushed out the exhaust and not getting trapped back in the cylinder.Bell believes that although this maybe a problem the proper blow down phase and staggered transfer timing purging instead of trapping residual exhaust in the cylinder is well worth the trade off. I tend to believe he feels that way because of the limiting factors of port shape within a given cylinder makes calculating the port time area a long and tedious math equation that may not work in that particular scenerio. he said although it s arbitrary to pick a rpm and modify the exhaust height he feels it is best/easiest that way. Reread Bell's book again he states that a certain exhaust port height will tune an engines rpm(lack of better words) then you would use exhaust port shape, pipe,transfers and including primary compression tailoring the hit or spread of power. He goes on to say at high RPMS a large time-area for transfer ports creates short circuiting( I assume short circuiting means that fuel gets rammed back into the transfer ports from the strong positive exhaust wave). Jennings states that a small time-area exhausts ports combined with large time-area transfer ports is good combination for broad range/ low RPM power while the opposite is true for narrow range/high RPM power. However, Bell says that higher RPM engines require longer transfer port durations then low RPM engines. He says that the longer the transfer ports are open, the more time is allowed for the negative wave from the exhaust to pull fuel through them (as does Jennings). His port time equations don't even include RPM, You input the dimensions of the engine and it spits out how long the port should be open. Graham Bell starts off by saying that he doesn't believe port time area and its all about the duration the ports are open. Jennings book talks about port time area and porting the cylinder to have peak efficiency at a certain RPM.