I've never really shopped around much on those. I just pick one I think will get the job done and go. They don't work at all a high percentage of the time, since you're under load. Vacuum drops off then. All that's really important is that they do hold some advance at part throttle. Yeah, you can argue it, but I just don't get "that" specific about it. I do at least get an adjustable can though so I can tailor it a little. It's just not that critical. That's my opinion and I stick to it.