There are a few that you can use. I have about 6 different types which will potentially work well, I'm just waiting for some time to dyno test, I used a vauxhall valve on a car a while ago which looked like it would work well but at high boost it seemed to jam causing over boost or under boost no matter how I plumbed it in which was very annoying as it would have been a very cheap option for people.

I will be re testing the vauxhall valve in the future with a few other changes to see if it can be made to work properly at high boost.

Rob,