Slant Six Cylinder Head and the Revised Combustion Chamber by Doug Dutra.

http://slantsix.org/articles/combustion-chamber/sl6-combustion-chamber.htm

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The new GM Vortec 4200 has a similar bore and
stroke to the 225 slant six ~ 3.66 x 4.01" - both are undersquare stroker inline sixes. But, the new head design makes a big difference, 270 horsepower stock ( 275 ft pounds at 3,600 RPM with most of that available at 1,200 RPM ).
I believe it comes with a 7 quart oil capacity, it's designed to be a work horse for trucks and SUVs.

How does GM improve air flow to help top end, but get that kind of low-end torque from a motor about the same size as the slant six ?

I'd sure like to see a slant head like the new Vortec