Theory

Think about this, your turbo is spinning at ~80,000 RPM providing boost. When you take your foot off the gas to change gears all that boost that your turbo just created actually "backs up" and causes the turbo to slow.

What we want to do is not only get rid of any of that excess boost that isn't going to be used, but try to keep the turbo spinning as fast as possible (so when we hit the gas again, the turbo doesn't have to spin up again.)

Overview

The Esprit does not come with a true Blow Off Valve, it is equipped with a dump valve. The difference is that a BOV will stay open until the pressure on each side of the throttle is equal, the dump valve only opens when the boost is over a certain pressure. A Blow Off Valve gets rid of the boost (up to a point) when we close the throttle, a dump valve only gets rid of boost OVER a certain pressure.

The reason most turbo cars have a BOV is that when the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and (in the absence of a BOV) a pressure wave will travel back to the turbocharger.The result is that the compressor wheel will stall (a phenomenon known as “compressor surge”) and slow down very quickly.This is hard on the bearings and decreases the turbo’s lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened (than if the turbine was spinning freely).

With the BOV in place, the pressurized air in front of the throttle body is released, and the turbo can freewheel happily since it has little resistance. When the throttle opens again, the turbo will have a head start and will get up to speed faster, since it never stopped spinning in the first place.

The dump valve's major failure is that it closes once the pressure drops below it's limit. This means that as long as the compressor is turning (and creating boost,) there will be pressure between the compressor and the throttle (and this pressure acts as a brake on the compressor wheel.) So a dump valve saves our engine, but it certainly doesn't provide any kind of a performance increase.

Tuning

A BOV shouldn't leak under maximum boost, but stay open under as little pressure as possible. This is a balancing act that really needs to be tuned for maximum benefit. With an adjustable BOV you need to set the spring at the weakest setting that allows the diaphragm to stay closed under maximum boost.

Also, you want to keep the vacuum lines as short as possible to the back of the BOV.

Additional performance increase

Venting to atmosphere is cool and makes a fun sound, but you're also missing another performance trick. Even with a BOV venting to atmosphere there is still a pressure differential between the front and back side of the compressor. This pressure differential acts like a brake on the compressor, slowing it down (although not nearly as bad as our earlier set up.)

By plumbing your BOV vent back into your intake you can come closer to equal pressure on both sides of the compressor.