Boost pulley combo question

[cayman150]

I am trying to figure out the same type of thing, Here is what I found somewhere that may help you decide.

I've also attached a .zip file of the Excel spreadsheet that you can use to plug in the upper and lower pulley values to show you what the final boost numbers will be with various pulley combinations.

Here are the forumlas. L and HD have a 7.4" lower pulley. L has a 2.93" upper, Harley 3.25.

2# is 7.8" 3#:8.17 4#:8.5" 5#:8.75" 6#:9" 8#:10"

Lower/Upper=ratio ratio times 5250=blower rpms at shift point of 5250. You can plug the values into a spreadsheet and see what I am talking about...

Here is the formula to calculate maximum potential boost

a = lower pulley diameter.
b = upper pulley diameter.

1.84 x ( a / b )
------------------ x 14.7 - 14.7 = BOOST
2.7

14.7 is PSI at sea level, 1.84 is eaton blower displacement in litres, and 2.7 is the engine displacement divided by 2.

Plug in the values for an L with a 4lb and a HD with a 6lb and see what you get. Hint: they are not the same.