Shopping: Real Estate |  Costumes  |  Guitars
This Issue Archived Articles Blog About Us Contact Us

The All-Electronic Blow-Off Valve!

Complete control with an atmosphere-venting electronic BOV

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Do-it-yourself fully electronic blow-off valve
  • Electronics available in kit form or fully pre-built
  • Uses electronically-controlled solenoid valve
  • Atmosphere venting without any idle problems
  • Huge flow capacity
  • Suits variety of budgets
  • Unique data-logged pressure traces show effectiveness
  • Part 2 of 2-part series
Email a friend     Print article

Last week we covered the use of the Delta Throttle Timer DIY electronic module to control the action of a factory or aftermarket blow-off valve (BOV). By using a solenoid and a one-way valve in the vacuum line, we could make sure that the BOV didn’t open at idle (and at other unwanted times) but that it still vented to the atmosphere with a satisfying Psshhht! each time the throttle was snapped shut. (Go to The $70 Electronic Blow-Off Valve for more on this approach.)

But this time we’re going to push the concept one step further and use a fully electronically-controlled BOV. That’s right – there’s no vacuum feed at all; instead the opening and closing of the BOV is done purely electronically.

And furthermore, an electric BOV is used as well!

Electronic BOVs

Click for larger image

A BOV is just a big capacity valve that opens when there’s a strong manifold vacuum present. Well, that’s how all standard ones work, anyway. In fact, what the valve is designed to do is vent the pressure build-up that otherwise occurs between the closing throttle and the still-spinning turbo. (For more, see ‘How BOV’s Work’ at the end of this story.)

That pressure build-up in front of the throttle body only occurs when the throttle is being closed. In fact, more accurately, it occurs only when the throttle is being closed fairly quickly. So if we monitor the output of the throttle position sensor, and if the voltage falls rapidly, we know that the throttle is being closed fast. And on a turbo car, that means a boost pressure build-up in front of the throttle.

The way that this voltage can be monitored is to use an electronic module that constantly watches throttle position and triggers a relay when the throttle position sensor output voltage falls fast. Normally, this would be very hard to do but thanks to Silicon Chip ( electronics magazine, a complete solution is at hand. They’ve come up with what’s called the Delta Throttle Timer and it’s a device that can be used to do all the hard control work.

Since you only ever want the BOV to open when you’re quickly lifting your foot off the throttle, it’s ideal in this application. Furthermore, the Delta Throttle Timer incorporates a timer circuit so that you can keep the BOV open for a preset time.

But how can an electrical relay trigger a BOV? Well, what we do is replace the BOV with a large electrically-controlled solenoid. Feed power to the solenoid and it opens, venting the excess pressure. Don’t feed power to the solenoid and it stays shut. Easy, huh?

The Electric BOV

The solenoid that you use as the BOV can vary from well-priced to extremely cheap. And from incredibly durable to shorter lived. Let’s start at the top end of the range.

  • Goyen Controls CA-series Valves

Click for larger image

The Goyen Controls CA valves are the pick of the bunch. Tested for an incredible 1 million cycles, they will last literally for the life of the car. They can also hold boost pressure without the slightest concern.

The CA valves are part of a series called the “T series reverse jet pulse dust collection” valves. They use a pressure-cast aluminium body, a nylon-reinforced Buna N elastomer diaphragm and are suitable for temps from -40 to 82 degrees C. The valves are available with different electrical coils – in this application you need a 12V coil. The valve shown here is a 1-inch design; they’re also available in sizes from ¾ inch up to an incredible 3 inches!

To shortcut your chase, the 1-inch valve is catalog number CA25T. Goyen valves are manufactured in Australia with outlets worldwide – do a web search for your nearest distributor.

The price is about AUD$115.

  • Garden Irrigation Valves

Click for larger image

The alloy Goyen valve is the top-line – but what if you want to keep things right down at the budget end? In that case head for your local garden irrigation shop and have a look at the plastic water irrigation solenoid valves that they have available. These valves are available in a variety of sizes, with the pictured ¾ inch size the smallest that you’d use in this application. (Note that despite the plumbing size being the same as the smallest of the Goyen jet pulse dust collection valves, the actual flow-put of these water valves is lower. However, they still vent plenty of air and so can be used.)

Click for larger image

Larger garden irrigation valves are also available. This one easily pulls apart (the screws are undone as part of this process) so that the internal spring can be replaced with a stiffer one if that’s required. Despite garden irrigation valves being normally used on 24 volts AC, they generally work fine on 12V DC. They’re also pretty good at holding pressure, but obviously their durability won’t be anything like the Goyen valve.

All valves used in this application are likely to be directional – you should always go by the arrow marked on them and if there is no arrow, test them to see which port best holds pressure.

So that’s the solenoid valve sorted, now what about the control system?

The Delta Throttle Timer

Click for larger image

As mentioned, the Delta Throttle Timer was developed and designed by Silicon Chip electronics magazine. It is one of a number of projects that will be covered in a unique Silicon Chip publication - High Performance Electronic Projects for Cars - which will be available from newsagents in Australia and New Zealand, or online through the AutoSpeed shop.

The book will be an absolute must-have for DIY modifiers.

The electronics design and development of the Delta Throttle Timer were carried out by the skilled and modest electronics engineer John Clarke, while I came up with the concept and did all the on-car development. (During this period I wore a different hat to an AutoSpeed contributor, working for Silicon Chip Publications as a freelance contributor.) So while by no means should the Delta Throttle Timer be seen as an AutoSpeed-developed project, we’re very happy to endorse it.

(The Delta Throttle Timer is sometimes known as QuickBrake. This is because the module was first used as a quick brake light trigger in the March 2004 Silicon Chip magazine Increase your driving safety with Quick Brake .)

The Module

If you have assembled other electronic projects before, the Delta Throttle Timer (or QuickBrake) kit shouldn’t cause you too much trouble. There are 18 resistors, 13 capacitors, 13 semi-conductors, assorted terminals, the relay and two trim-pots. Solder and hook-up wiring is supplied. Follow the instructions carefully – in fact to gain the article in full colour (important when following a component overlay) we suggest that you subscribe to the on-line version of the article at Quick Brake .

However, if you’re not confident with component identification, component polarity and soldering, buy the fully built and tested version – then only a few simple connections to the car are required.

Neither version comes with a box, however the Delta Throttle Timer (we’ll call it DTT from now on!) fits straight into a 130 x 68 x 42mm plastic electronics ‘jiffy’ box. Alternatively, you can put it in any box that you want, making sure that the bottom of the printed circuit board can’t come into contact with anything metallic (which could cause shorts).

Click for larger image

When you have either built the kit or received the built-up module, have a good look at it. Orientate it so that the relay is on the right. Now you’ll have two sets of terminals on the left and a long strip of six terminals on the right. The top-left terminal connects to ignition-switched 12V – that is, a battery positive supply that is on when the ignition is on. The terminal right below connects to ground – in other words, to the car’s metal body. The lower left terminal has two inputs but as they’re connected together, either one can be used. This input is for the wire that connects to the throttle position sensor.

Click for larger image

Before you can connect the signal input to the throttle position sensor you need to find the right wire on the sensor. To do this you’ll need a multimeter. Set the multimeter to Volts DC and connect the black lead to the car’s body. Turn on the ignition. With the other multimeter input, back-probe the working throttle position sensor until you find a wire that has a voltage on it that varies with throttle position. Typically, this will be in the 1-4V range and the voltage will rise when the throttle is opened. This is the wire that you tap into for the DTT signal.

Connect up these wires to the DTT. (Note that the throttle position signal wire doesn’t need to be cut – the DTT just taps into it).


Now that you’ve made these connections you can do some testing.

Then turn Pot 1 (Sensitivity) anti-clockwise as far as it will go. (Note that these are multi-turn pots so you may not come up against a positive ‘stop’ when you get to the end of its rotation.) Turning the Sensitivity pot anti-clockwise increases sensitivity. Next turn Pot 2 (Time) clockwise to decrease the period that the timer will stay on. Finally, check that the moveable link is in its right-hand position, which causes the DTT to turn on with fast throttle lifts.

Switch on the ignition, wait for 10 seconds, push down and then quickly release the throttle. The LED should come on and the relay pull-in for a short time. (The 10 second delay after switch-on is needed because the DTT has a built-in pause to avoid false-alarming when power is first applied.) Then turn the Time pot anti-clockwise a little to extend the relay’s ‘on’ time. The range of adjustment is from 1/10th of a second to just under 2 minutes - in this application around a second is fine. Adjust VR1 clockwise until the DTT responds only when the throttle is being lifted moderately quickly.

The Plumbing

If your car already has a blow-off valve, you can use the standard fitting that connects to the intake system between the turbo and the throttle. If the car didn’t come with a BOV (and/or you’ve changed the intake plumbing) you’ll need to organise a new fitting. We suggest that it is placed as close to the throttle as possible – this keeps the air passing through the BOV cooler (as it’s after the intercooler) and also is nearer the beginning of the pressure wave that builds when the throttle is closed.

The Sound?

The valves covered here don’t make much of a noise when they vent to atmosphere – at least not on the guinea pig Maxima V6 Turbo. There is the rush of lots of air being released but it’s not the sort of sound that an aftermarket BOV makes. For me that’s perfect – I can hear the air being dumped without frightening dogs, people, birds and the guilty. But if you want the valve to make lots of noise, install a whistle in its outlet. Seriously. It works...

Setting Up

Click for larger image

If you’ve got the DTT working properly (LED lighting with throttle lifts and then staying on for about a second) you can now connect the output relay to the solenoid. The wiring connections are shown here.

Start the engine and make sure that it idles as well as it did before the modification. Wait until the initial start-up delay of the DTT has elapsed and then (if the engine is warm!) blip the throttle hard.

Depending on the size of the turbo, you’ll probably be able to hear the BOV open on each quick throttle release. Go for a drive, making sure that the engine behaves perfectly but the BOV is venting to air on each sharp throttle lift. If the engine wants to stall, you’ve probably got the DTT Timer set for too long an ‘on’ period, so adjust the Time pot to shorten this.

Venting Metered Air?

Some people are concerned that an atmosphere-venting BOV is getting rid of air that has been measured by the airflow meter – air that should have found its way through the engine. The worry is that this will change the mixtures. However, the amount of air going out through the BOV on throttle-lifts is very small in the overall scheme of things, and bad running is much more often caused by the BOV being open when it shouldn’t be. The system covered here overcomes that problem.


A fully electronic BOV – especially one using the big Goyen valve – has the ability to shift a huge quantity of air and so reduce the pressure build-up (and pressure waves – see below) to near nothing. It’s also easy to vent it to atmosphere without making the car run badly, has full electronic adjustment of when (and for how long) it functions, and can even be way cheaper than an off the shelf aftermarket BOV!

Sounds like a winner to us...


So how often have you see a BOV actually tested for its ability to reduce the pressure spike that occurs when you shut the throttle on a hard-blowing turbo? Not very often, right? And in fact, what does that pressure spike actually look like?

We decided to do some high-speed logging of the pressures that occur in the intake system between the turbo and the throttle blade when you snap the throttle shut. We used a Fluke 123 digital Scopemeter and a Fluke pressure transducer.

Click for larger image

This screen grab shows the pressures with the electronically controlled Goyen BOV working. Each vertical division is the equivalent of 20 kPa (about 3 psi) manifold pressure and each horizontal division is 200 milliseconds. So from far left, the car is holding 70 kPa, grading down slightly to 60 kPa (about 9 psi) before the throttle is suddenly closed. The pressure abruptly spikes by 10 kPa (about 1.5 psi) but the spike is very short-lived (about 50 milliseconds) before it rapidly and smoothly falls away. In fact, the pressure drops from the spike of 70 kPa down to 20 kPa in less than 100 milliseconds (ie one tenth of a second). The fall to less than 10 kPa takes about 650 milliseconds (ie 0.65 seconds) in total.

And what a different story there is without the BOV working!

Click for larger image

Again the throttle was abruptly closed at 60 kPa boost. The immediate pressure spike was about 15 kPa (only a little higher than without the BOV) but what follows from there is completely different. Rather than dying away quickly, the pressure is both much slower to fall off and is also accompanied by very rapid pressure waves, with these waves starting off at about 20 kPa peak-to-peak and then gradually dropping to about 10 kPa. The frequency of these varies from 15-20Hz. In other words, there is a pressure wave of up to 20 kPa racing up and down the intake system between the throttle and the turbo. It is very likely that this wave battering against the turbo compressor is potentially much more damaging that the initial pressure spike itself. Also note that the trapped pressure takes a lot longer to decrease – to drop to 20 kPa takes about 500 milliseconds (half a second) compared with one-fifth of that when a BOV is fitted.

Note that if even if you don’t have an elaborate digital logging system, a very good feel for what’s going on can be gained by plumbing an undamped pressure gauge to the intake system between the turbo and the throttle. (An undamped gauge is one that will respond very quickly – many turbo boost gauges are damped to smooth any pulsing that occurs in the intake. Instead use a commercial general purpose pressure gauge – cheap on eBay). In the case of this car, the pressure waves could be easily seen (as needle flickers) on the gauge.


With the Delta Throttle Timer set up to trigger a BOV you can very easily also use the module to perform a completely different second function. As covered in our story Quick Brake, in auto trans cars the Delta Throttle Timer works very well as an early illuminator of the brake lights – it triggers them much more quickly than the normal brake light switch. (Of course, when your foot actually gets to the brake pedal that switch takes over as normal – the Delta Throttle Timer just has the lights on earlier.)

And it just so happens that the Sensitivity and Time settings (and Link position) for triggering a BOV are very similar to how you have them for QuickBrake...

Making it even easier is the fact that a Double Pole Double Throw (DPDT) relay is used on the module, allowing the simultaneous switching of a second circuit. All that you have to do to trigger the brake lights early is to wire the brake light switch in parallel to the second adjoining Common and Normally Open contacts of the relay.

For more details, see our QuickBrake! story.

How Blow-Off Valves Work

So how do blow-off valves actually work? Let’s first take a look at factory blow-off valves then check the aftermarket.

Click for larger image

In this diagram (click on it to enlarge), the car is on boost - the throttle is fully open and the turbo compressor's spinning hard. There's a positive pressure being developed everywhere in the intake, including in the vacuum/boost hose that goes to the factory blow off valve. This boost signal keeps the blow-off valve shut, meaning that all of the air being pushed by the turbo compressor must go into the engine.

Click for larger image

Now something's changed - the throttle is being closed and so a vacuum (like minus 0.8 Bar) is being created in the intake system after the throttle body. A strong vacuum signal passes down the vacuum/boost hose leading to the blow-off valve, and so the valve snaps open. The open valve connects the intake after the turbo to the intake before the turbo, relieving the pressure build-up that would otherwise occur in the plumbing between the turbo compressor and the closed throttle blade. At idle, the blow-off valve in most factory systems stays open - the minus 0.5 Bar or so that's present is enough to trigger it.

But aftermarket valves are different.

Click for larger image

Most aftermarket blow-off valves vent straight to the atmosphere, rather than returning the air to the intake system in front of the turbo. That's the reason that they make satisfying whooses - the air is dumped straight out. However, this often causes problems – two are most likely.

Firstly, if the turbo is spinning at idle, it will be pushing out a bit of air. This air will find its way out of the blow-off valve, spilling into the engine bay. The airflow meter will be measuring this air (measured before it's gone into the turbo, of course) and will be expecting all of the metered air to make its way right into the engine cylinders. When it doesn't, the engine will run rich - it won't be getting as much air as it should have got to match the fuel being injected. That's one scenario - here's the other.

If the turbo isn't spinning, air will be drawn into the open blow-off valve. This is because when the throttle butterfly is shut at idle, air still needs to be made available to the engine if it's to run. This air is provided by a throttle body idle bypass, so there is still a route into the engine. The air that gets sucked through the blow-off valve (an easier path than through the filter and airflow meter in many cars) is then both unmetered and unfiltered, so it will cause the engine to run poorly (lean this time) and may also cause dirt to be drawn in.

‘Pumb-back’ aftermarket valves return their exhaust air to the intake and ‘semi-plumb-back’ valves have a bet both ways. Adjustable valves can have their spring pre-load altered so that they’re not open at idle – they open only when the vacuum is even greater during a throttle lift-off. However, this reduces their opening time – ie they’re slower to open on the throttle lift.

The all-electronic approach covered here uses a completely different operational approach.

Did you enjoy this article?

Please consider supporting AutoSpeed with a small contribution. More Info...

Share this Article: 

More of our most popular articles.
Electronic module that can sound high or low temperature alarms or warnings

DIY Tech Features - 30 June, 2008

The eLabtronics Pulser, Part 2

Country driving skills have almost disappeared - here are some

Special Features - 4 October, 2011

How Not to Die this Week

If you love modifying cars, be sad

Special Features - 12 June, 2012

The beginning of the end of a 60-year era?

Reducing fuel use through electronic tweaks

DIY Tech Features - 27 September, 2006

Electronic Ways to Improve Your Fuel Economy

DIY timer module adds delays or extended 'on' periods to any electric car function

DIY Tech Features - 1 September, 2008

The eLabtronics Timer

How tyres really work on the road

Technical Features - 9 August, 2007

Tyres, Grip and All That

Examining the natural frequencies of suspension pitch and roll

Technical Features - 21 February, 2012

More than just bounce...

All the detail on how direct petrol injection systems work

Technical Features - 3 January, 2007

Direct Petrol Injection

A groundbreaking car...

Special Features - 27 October, 2009

Fifty Years of the Mini

Why if you're interested in economy or power, you must know about water injection

Technical Features - 15 April, 2008

The H2O Way, Part 1

Copyright © 1996-2020 Web Publications Pty Limited. All Rights ReservedRSS|Privacy policy|Advertise
Consulting Services: Magento Experts|Technologies : Magento Extensions|ReadytoShip