Why the need for boost controller for ext. wastegate DP?

[tta1401]

I'm looking at getting a new DP and I've noticed that some descriptions say when ordering any external wastegate down pipe, a boost controller is need to allow it to function properly and avoid any damage. I thought that the external wastegate WAS the boost controller? I'm confused?? Can someone please explain?

*EDIT*
After looking at the pictures on the various vendor's websites, I think they picture a boost controller already installed on the DP. I think that is why I am confused.

 

[tta1401]

After researching, most of them have set springs so that is why.

 

[greeneyegi]

i added one to my ext wg because i had an issue with boost creep,solved my problem.combination of spring weights was opening a little bit at 24# but not all the way till 26#, added rjc controller too ,it fixed problem after a lot of adjusting.

 

[TurboDave]

The external wastegate comes with "a" spring of some value. That's your base boost, or lowest value available. Without a controller you could never get above that value.

 

[williamsGN]

http://cottonsperformance.com/proddetail.asp?prod=10402-25&cat=95

Looks like you got your answer.. here is what I run on my external.