Been a lot of discussion lately with pulley sizes and some people are still a bit confused on how the pulley ratios work.

HD's come stock with a 3.25" upper and L's come stock with a 2.93" upper. Both trucks come with the same size lower.

HD's stock are 6 lbs of boost and L's stock are 8 lbs of boost.

Aftermarket pullies are named for the boost increase they add on a Lightning, NOT a harley truck. A 6 lb'er adds 6 lbs to a Lightning, it does NOT add 6 lbs to a harley truck. The rough general rule of thumb has been 2 lbs less for a HD, but since its a ratio, this isn't fully accurate. From a generalized standpoint, boost is a linear function of blower RPM. More RPM = more boost. While up high this is not true because hot air is less dense therefore less boost, and low RPM its a bit different as well, the generalized ideal boost level vs RPM for the middle of the "s-curve" of boost vs RPM can be linearized.

http://www.nhtoc.com/vbforum/newthre...newthread&f=18
That thread was our sticky for a while, but with everything in text form for some its a bit harder to interpret. I took Ryan's approach and threw it into a chart and a table that should clear it up for everyone.

The first table is the calculations for pulley combos & net boost for L's vs. HD trucks

The second table is the pulley combos for least to most boost on any truck depending on the combo of uppers & lower's. So a 4 lower & L upper makes more boost than a HD upper and 6 lower, etc.

The chart is blower RPM vs net boost for the known L pulley combo's, and where the HD pulley combos fall into place at.

This chart and tables are up for discussion as I may have overlooked something, but PLEASE READ THE ENTIRE LIST OF ASSUMPTIONS & EXAMPLE ON THE RIGHT OF THE CHART BEFORE BRINGING UP DISCUSSION POINTS!!!



This will soon become a new sticky