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Installing a Blow Off Valve

Why a Blow Off Valve

Although Turbocharging wasn't “new” when Lotus added a turbo to the Esprit, it certainly wasn't commonplace. Lotus' implementation leaves some things to be improved, one of which is the use of a dump valve instead of a blow off valve.

The problem with a dump value is that it's a simplistic device that only opens to prevent overpressurizing the intake. This has the side effect of stalling the turbo which can limit the life of the turbo, but more importantly it means that the turbo must spin back up when it is needed. A blow off valve should allow the turbo the free spin when the throttle plates close and the extra boost is bled off.

The perfect approach

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.) On an EFI car there is a single butterfly and running a vacuum line from behind it is easy. On a carbureted engine we have four butterflies, and the air and fuel have already been mixed after the butterflies. There are two options (at least) to plumb a BOV. The first is to use this mixed air to provide vacuum under throttle lift, the second is to control the BOV electronically. The issue with using the air/fuel mixture is that fuel can degrade the internals of some Blow Off Valves.

Building an Electronic Controller

By running a second vacuum line from our turbo to the back side of the BOV (equalizing the pressure inside the BOV,) and installing an electric solenoid that vents this second line to atmosphere (or vacuum)

The low-tech approach is to control the solenoid with a throttle switch that energizes when the throttle is closed. AutoSpeed magazine has two articles that you need to read if you're interested in getting hightech. The first is The $70 Electronic Blow-Off Valve the second is The All-Electronic 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.

other/bov.txt · Last modified: 2024/02/22 17:14 by 47.128.60.229