Quote: I would think the triangle is more likely to keep the body/sheetmetal square (in just the horizontal plane), than chassis/frame control.
A couple of things here....
There aren't many flat sections of sheet metal on the car, pretty much everything has a bend, curve, or brake to it. Flat sheet by itself, has little strengths in the one direction, put a 90* brake in it, it stiffens up quite a bit. Entire aircraft and race cars are constructed in what is called "monocoque" construction that way, basically the way a unibody works. A cowl section of a unibody car is probably the most substantial, and strongest part of the car for obvious reasons.
The engine brace does two things: We stitch weld up the inner fenders,and some of the torque of the frame rail is transmitted into the inner fender, which is tied to the cowl from the inner fender brace on the one side, and the engine brace cross bar helps keep them from flexing inward and also transmits some of the load back to the cowl with the other bars.
We can armchair engineer this all day long, but the fact is that every part of our chassis stiffening package was tested and bench marked, and delivered something like 9800# per degree of twist.
