its the heads, the shape of the chamber where the valves go is a heart shape as opposed to other engine heads
vortex head is on the right and other engine head on the left.... http://images.customclassictrucks.com/techarticles/0405cct_head06_z.jpg
http://en.wikipedia.org/wiki/Vortec .... go there, tells all about vortec engines, the numbers go by liters, so a 5.7 liter would be a 5700