Reviving an old thread because I have actual data to share.
If you occasionally peruse the autocross subforum, you'll probably know I'm about the only guy in the country autocrossing my Grand National right now. I've been knocking out weaknesses in the car one at a time for the past 18 months or so, and I've made immense progress.
So much so that I finally got to the point where I thought I needed a blow off valve. Throttle control in autocross is extremely important. Stuff happens really quickly, and having a laggy motor costs you badly. It was always difficult to correct the car when the back stepped out, and the lag in slaloms when I had to lift was annoying.
So, I put a Tial 50mm recirculating valve on the car.
Below is a data log from the Wilmington Champ tour a few weeks ago. This was pre-BOV. You can where the arrows are pointing there was lag between throttle input and the boost coming up after a throttle-close event.
Here's today, an event with the BOV installed that happened to have a couple offsets and slaloms that had me coming all the way out of the throttle and going right back in.
It's pretty clear, the lag is gone. The car is super responsive now. In fact, I over drove a corner today and ended up in a four wheel drift that I was able to modulate and get out of without looping the car. I've never had enough throttle sensitivity in this car to control a drift. It always spun or just got really out of shape. I have the kind of control to handle something like that now.
So there it is. A recirculating blow off valve works with these cars.
If you're running a speed density system, you can get one that vents to atmosphere. If you're still running a MAF, use a recirculating one.