Turbo Esprit Fact Repository

Installing a Blow Off Valve

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

If you can find a 3 way vacuum solenoid you can run one barb to the vacuum pump (maybe) and the other barb to boost. This way the BOV actually would see vacuum on the back side when you switch the solenoid.

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.

An optimal set up would allow the blow off valve to open with as little pressure as possible. To accomplish this, we need to equalize the pressure inside the blow off valve under boost and decrease the pressure on the back of the diaphragm only when we close the throttle body. Usually this would be done by running a vacuum line from behind the throttle body to the BOV. This way when the throttle closes, this line would see vacuum and not only would the excess boost on one side push the diaphragm open, the vacuum would help the process. Of course even with this setup there are pressure losses, and the two sides aren't 100% equal (under boost), but close enough that we don't need to install a spring as heavy as what would be required on a dump valve (so a car running 8psi boost only needs a spring set to maybe 5psi not 12psi.)

Alternative idea

You could build a mechanical BOV of sorts with another throttle body. It could either be activated by cable or with some kind of vacuum actuator (like on the HVAC controls.) The benefits (if tuned properly) could outweigh a traditional BOV.

Autospeed has an older article on an All-Electronic Blow-Off Valve.