Which HPD pipe do you have or are looking at? If I remember correctly they made a few different versions like one for torque or low end trail riding, one for top end trail riding and race pipe for modified engines.

The only real world data that I have with HPD pipes is on their race pipe with a few different versions of ported cylinders and one factory cylinder, a couple of different carb sizes and some other stuff like that.

I've only done testing with a factory cylinder with a couple of different brands of trail pipes with various bolt ons and if I knew how the diameter and length of the HPD pipe compared to them then I could probably give you an idea of where to start with a primary spring. The 2 things to look at with the primary spring are the stall speed and shiftout speed. If you get stall speed too low then the quad will feel lazy on the bottom end and mid range, kind of like a manual equipped quad taking off in the wrong gear. If the shiftout speed is wrong then you'll have the max RPM's either too high or too low.

 

I thinks it's the one for low end trail but I'm not for sure it came with a scrambler I bought but it does not have any markings at all