Similar to an old Tri-y, Bob. It pairs up two cylinders on each head into 1 collector, then merges both of these collectors into the large 4" collector. The logic behind this, as it was explained to me, is to better scavenge the cylinder in essence making the engine think it has a higher velocity on the exhaust gasses???? I'm really not much of an engine guy, but on a 408 stroker motor I had in a Maverick we played a bit on the dyno with conventional headers, step headers, and big tube headers with the merge collectors and found the big tubes with merge collectors developed another 18 foot pound of torque above what the conventional header developed. As I've said, my main goal for this engine will be to build lots of torque, and get in in at the lowest possible RPM. Hope this explains it a little better.