I'm pretty impressed with the 337, and I was running a very deeply ported Indy 440-2D MW on my 517 that I was already well satisfied with.

The big single is maybe a tick softer below 3500 but at about 4000-4200 (where my converter flashes) it pulls just as hard and then over 5500 I'd say it's a good bit stronger all the way to 6800. I never ran my 337 unported so I'm not sure how much is the manifold and how much was Larry's handiwork, but I'm guessing this manifold raised my peak torque RPM (or at least it doesn't fall off as rapidly) and I really feel it in the shift recovery to redline.

These heads flow ~335 at .500 and .360-ish at .600 on Dwayne's Bench so I think the 2D (as good as it is and out of the box can support up to 700) might have been leaving a little on the table up top. The rule of thumb for optimum power is your Manifold needs to outflow your port by at least 5-7% and the first thing you notice on the 337 is that despite the low carb height, the runners have amazing Taper (runners are larger cross sectionally at the plenum). its I think impossible to acheive taper in a dual plane configuration because the runners cant get that big to begin with.

To my understanding, port taper works kind of like a check valve, it slows the reflected pulse pressure in the runners, effectively allowing the open plenum to 'act' even Larger at WOT. Larry definitely increased the taper, i think he learned that from all the years doing NASCAR manifolds.

When I used to port Street Dominators back in the day I think that was a big part of why they improved, but it was hardly scienced-out, the SD has distribution issues between the 2-1 and 5-7 (consecutive firing) holes that are fairly easy to fix.