The 5-7 pairing also enriches mixture from the carburetor - a staggered sequence has proven to be stronger at the nozzle than more equal intervals. The carburetor intervals with conventional SBC V8 1 8 4 3 6 5 7 2 firing order:
1-3 (left front pair) 270° - 450° - 270°
2-4 (right front pair) 270° - 540° - 270°
5-7 (left rear pair) 90° - 630° - 90°
6-8 (right rear pair) 450° - 270° - 450°
{breathe}
A picture helps me to visualize this:
In comparison a dual plane (180 degree) manifold typically feeds 2 - 8 - 3 - 5 from passenger side plane and although 3 and 5 are adjacent in the layout they are at opposite ends of the upper plenum:
This is a great thought exercise around intake design principles and the role of firing order and intake dynamics.
But now lets pivot back to the I6 intake in question:
with the Chevy firing order:
Which helps to visualize the issue/difference with the center carb tuning:
Typically the center carburetor feeding cylinders 3 & 4 needs slightly richer jetting due to the difference in firing intervals between its 360-360 and the end pairs 240-480 intervals (as Jaguar detected 60 years ago).
