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I'm back. The book is called "Four Stroke Performance Tuning" if anyone wants to get it. And you should. It's awesome. 
Anyway, the feeling I'm getting from it is simple. Like has already been said, equal length runners with as straight a shot to the intake ports as possible. Also when an isolated runner mani (like a dual carb side draft setup) cannot (or will not) be used a log style is better than a curvy racy looking manifold (or the stock one in our case, weirdly). That's exactly what the name implies. A cylinder shaped plenum with straight runners to the intake ports. You even want the plenum to extend a tiny bit past the outer most ports to give the mixture a "cushion" before it changes direction to enter the port so the fuel doesn't get thrown out of suspension as much. Also whenever possible divide the mixture before it changes direction to get a more even distribution to all cylinders. Finally a nice feature to have built in is dual runners. One smaller diameter set fed by the primary throat and another fatter set fed by the secondary throat. This was explained to aid in every job the manifold does, but obviously would add complication to the design.
I was going to scan images, but it focuses mostly on V8s, and the only inline manifold it shows is the log style. See below.
This, from what I'm understanding, would be the ideal design barring the dual runner feature, which is probably just too much to really do in a practical way if you're not casting it, but IDK.
This isn't my design. It is a repro of the one in the book for inline engines. Carb in the center, log plenum, and equal length straight runners.
Also note that the carb should be spaced as far from the plenum floor as possible given whatever hood clearance you have to work with. Same concept as the plenum ends going beyond the outer most runners. Cushion the charge and gives the fuel as much time as possible to atomize before it changes direction.
Anyway, the feeling I'm getting from it is simple. Like has already been said, equal length runners with as straight a shot to the intake ports as possible. Also when an isolated runner mani (like a dual carb side draft setup) cannot (or will not) be used a log style is better than a curvy racy looking manifold (or the stock one in our case, weirdly). That's exactly what the name implies. A cylinder shaped plenum with straight runners to the intake ports. You even want the plenum to extend a tiny bit past the outer most ports to give the mixture a "cushion" before it changes direction to enter the port so the fuel doesn't get thrown out of suspension as much. Also whenever possible divide the mixture before it changes direction to get a more even distribution to all cylinders. Finally a nice feature to have built in is dual runners. One smaller diameter set fed by the primary throat and another fatter set fed by the secondary throat. This was explained to aid in every job the manifold does, but obviously would add complication to the design.
I was going to scan images, but it focuses mostly on V8s, and the only inline manifold it shows is the log style. See below.
This, from what I'm understanding, would be the ideal design barring the dual runner feature, which is probably just too much to really do in a practical way if you're not casting it, but IDK.
This isn't my design. It is a repro of the one in the book for inline engines. Carb in the center, log plenum, and equal length straight runners.
Also note that the carb should be spaced as far from the plenum floor as possible given whatever hood clearance you have to work with. Same concept as the plenum ends going beyond the outer most runners. Cushion the charge and gives the fuel as much time as possible to atomize before it changes direction.
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