Why dual pulley On single belt setups?
I imagine the reasoning behind that, if it was used, is that it was already a part on hand and no other expense was needed to use it. It works. I kind of get the vibe that the race hemi and Max Wedge programs were on a bit of a budget. If so, that would be a no cost added part.
Also, the high-rpm motors had a tendency to toss belts, so the deep groove pulleys prevented that. I have always believed that the deep-groove pulleys used on the MW and Race Hemi were actually truck parts that were intended for use with 7/16" or 1/2" V-belts...this made the 3/8" V-belt sit deeper in the groove and they just happened to be dual-groove.
Pic shows a race Hemi with the belt installed in the rear groove.
