How is a wastegate spring's PSI determined?
 

When a waste gate spring is listed at X psi, how is X measured or determined? Is it simply the pressure where the gate cracks open, the gate is fully open, or is it an objective measurement like how many pounds it takes to compress the spring a certain amount?

I'm asking because I would think that pre-turbo exhaust back pressure would play a big role in determining when a gate actually opens, and when picking a spring size I would think back pressure needs to be taken into account. Does it?

Yes. I have a 18psi wastegate spring but due to exhaust back pressure (when running off only the wastegate spring for boost control) it cracks open around 14 psi boost and the boost holds around 14 briefly until the rpms keep going up and boost pressure also goes up with the rpms, I think I end up getting close to 17-18 psi boost in high rpms

I don't think there's a standard way of determining it across manufacturers and applications. The spring psi is just some nominal value.

It's going to depend on those backpressure effects, the way the spring has been coiled up (think of all the various suspension spring designs), temperature. Then there's the fact that the valve position vs exhaust flow relationship is non-linear.

I think it boils down to educated guess and check as to what spring rate to go with, which costs time and money.