Duncan pipe is hands down the best pipe available for the V-Force.

 

When looking at spring rates pay close attention to what you are reading,for instance...
HPD does not use the common 2.5" and 1.25 " loads, they use 2.35 and 1.29".,if you compress the spring to different lenghts it will obviously change the reading on the scale.
The stock v spring is not long enough to produce a 2.5"load,and it has a measured 1.25 load of 136lbs.
Do not be mistaken to beleive that length always means engagement speed.Look at AAen yellow snowmobile spring(short in snowmobile terms,but very high engagement).
Number of coils,pitch, corrected stress,wire size,distance to solid height,free length,etc are all part of spring characteristics. Also look for published numbers and specific heights,not graph lines.
Kevin M is good enough to put up some numbers,and it is good input for all who are tuning for specific applications. I'm just giving input that may help.