You are wrong. You are saying that a bleed valve or amal valve vent off anything above a set pressure. They don't, but they do work in the same way. If it bleed off above a set pressure, it would need to know the boost it was seeing, then bleed a specific amount off to reach the target or set level - hence closed loop - and it doesn't!

Lets go back to how an actuator boost control system works.

At low rpm, there isn't enough exhaust gas to spin the turbo, but suddenly. there is an exponential increase in gas, which generates, boost, which generates boost..... a highly unstable positive feedback system.

We use a wastegate, and a wastegate actuator to limit this boost. The wastegate is a simple valve, which directs exhaust gas around the turbo straight out of the exhaust pipe, altering the speed of the turbo.

THe actuator consisists of a pressurised canister, diaphram, spring, and rod. When u feed pressure into the actuator, the rod moves, which is attatched to the wastegate. Different actuators have different lift off pressure's. The std one is 4psi, the -31 is 7 psi, and the -34 about 14psi.

These figures are the LOWEST AMOUNT OF BOOST YOU CAN EVER RUN WITH THAT ACTUATOR. The lift off pressure is when the the actuator rod starts to move. So if u feed in 6psi to a -31, it will just sit there, but at 7psi it will move, and start to open the wastegate.

How do you adjust boost pressue with an actuator? By adjusting the rod. If you shorten the actuator rod, you are compressing the spring (this is what you are doing when u stretch the rod to slide onto the wastegate arm). Imagine you have shortened it by 4mm or so, and it's a -34 actuator. Upto 12psi, its not moving at all. Then when we hit 14psi, it starts to move internally - ie the spring . This never changes, no matter what the rod length is. But, although it's moving internally - the ROD isn't moving yet - the boost pressure is instead opposing the extra tension that you put on it when you shortened it. The result is more boost - AND it comes in faster and harder - because there is less wastegate creep.

Waste gate creep? At the std lenght, the -34 starts to move at say 13psi, and the wastegate starts to open at this point. Because it is opening, there is less gas to spool the turbo, and from this point to wherever the boost settles at, the boost rises slower. If you imagine the actuator with the smaller shorter rod - at 13psi, the wastegate is still firmly shut, so boost is still rising quiclky. This is why a std actuator gives sloppy boost response - it starts bleeding off at 4psi!

****The stiffer the actuator, the faster and harder the boost comes in, with less boost spikes, and less drop off at top end****

I keep saying "std rod lenghth" etc. What is this? Well, it's probably THE most important thing when setting one up. We know that we can lengthen the actuator to reduce boost. So why can a -34 run 12psi minumum? How come i have a -34 and i get 6psi?!! It's because there is not enough preload. Every actuator has a maximum rod length, which is associated with it's preload.

If the rod is too long, what happens is as soon as the car sees any boost whatsoever, it opens the wastegate. With the wastegate open, u will never see any proper boost. What usually happend is u might see 5 or 6 psi at the very top end of the revrange, and the car is very sluggish. What is the max rod lenght? It's known as "half a hole preload". What this means is that when u are putting the rod onto the wastegate, you need to stretch it at LEAST half the diameter of the hole in the arm which slides over the wastegate.

Amal valves and Bleed valve info to follow...