First of all I don't think any turbo shop is using 39mm flappers on internals anymore. They just aren't very good. Internal gates are not really something that bigger is better. What you want is a valve that is just big enough to get the job done and w/g spring that is as stiff as you can stand and still keep the boost down. Allow me to explain.
First lets look at keeping the boost low enough and not having it creep. For this, you need a larger w/g hole and a larger flapper to cover it. If this was all that life was about then bigger would be better. And you can use just about any w/g actuator spring and boost can be kept down to 10psi if you want. Unfortunatly, keeping the boost down is only 1/2 the job.
Now lets look at the other side, the side most people don't understand. Keeping boost high enough and not letting it fall off at top end. You would think this is easy because you just shut the valve and get more boost. Well it's not so easy because you have lot of exhaust pressure on the valve pushing it open. Let's say you have 30psi exhaust back pressure. Well the larger the valve the more force 30PSI will apply to it. Remember it's Pounds per Square Inch, so the more square inches of valve you have, the more force is applied. So the two things we can do to help keep the valve closed against these exhaust pressures is use a valve with less square inches of surface area, or we can use a stiffer w/g actuator spring.
So you see how these two factors don't like to live together, on one hand you want a huge w/g and on the other you want a little one. The compromise comes in when you can narrow the boost range you need the turbo to operate in. If you need you turbo to hold solid boost anywhere from 5psi to 35psi, you are dreaming. But if you can accept 15psi as a minimum boost level, then you can have a turbo that will hold high boost at the track and not drop off at top end. This is pretty much what we've fine tuned. You can run down to 15psi which should be fine for pump gas, and our adjustable reverse mount actuator allows you to hold high boost to redline on race gas.
Kevin
www.agpturbo.com
