The $70 Electronic Blow-Off Valve

Vent your factory or aftermarket BOV without any problems!

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Electronic control of your factory blow-off valve
  • Vent your factory BOV to the atmosphere without idle problems
  • Electronics available in kit form or pre-built
  • Fully adjustable control over when BOV opens
  • Part 1 of 2-part series

Tried venting your factory blow-off valve (BOV) to the air? Engine didn’t like it, did it? You might have got those Pssshts! that you were after but you also got a bad idle and black smoke. The reason for those unwanteds is that the BOV was open at idle – not just when you were quickly backing-off the throttle.

But now we’ve got a solution for you. It’s an instant intelligence upgrade for your BOV! Instead of just relying on manifold vacuum to trigger it, this version adds electronic control to your existing BOV. You don’t need to buy a new BOV, you get a perfect idle, and you get an atmosphere-venting BOV!

Oh yeah, and from as cheap as about seventy bucks....

How It’s Done

So what’s the secret? Instead of the BOV always seeing manifold pressure, what we do is put a solenoid (an electrically-controlled on/off valve) in the hose. Now the valve sees manifold vacuum only when we want it to – that is, when we feed power to the solenoid and it opens. You don’t want the BOV open at idle? Easy – just keep the solenoid shut! (Click on any of these diagrams to give a bigger view.)

The other tricky part is opening that solenoid just when we want the BOV to trigger. 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.

The Delta Throttle Timer (DTT) constantly monitors throttle movements and when it sees the throttle being opened fast, it trips a relay. It knows what the throttle position is by monitoring the output of the throttle position sensor. Since you only ever want the BOV to open when you’re quickly lifting the throttle, it’s ideal in this application. Furthermore, the DTT incorporates a timer circuit so that you can keep the solenoid (and so if there’s enough engine vacuum, the BOV) open for a preset time – eg a second.

There’s only one other trick. Because the boost pressure acting on the BOV helps keep it shut against leakage, we still want boost to reach the factory BOV even when the solenoid’s shut. And this is easily accomplished by putting a one-way valve around the solenoid, so that boost can act on the BOV but vacuum only triggers the BOV when we want it to.

All starting to sound too hard and expensive? Not really. If you can wield a soldering iron and recognise electronic components, the Delta Throttle Timer control module will costs you only AUD$24.95 as a kit.

The one-way valve that you need can be salvaged from the brake booster of a heap of cars at wreckers (this whole handful cost AUD$2). If you are sourcing these valves look for cars that place the valve in-line in the manifold > booster hose rather than building them into the booster itself.

And then all you’ll need in addition are two or three ¼-inch brass T-pieces (about AUD$4 each from a pneumatics or hydraulics supplier) and some hose. Oh yes, and some hose clamps.

But let’s take it all step by step.

The Delta Throttle Timer

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, 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 Increase your driving safety with 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).

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.

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.

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

Any solenoid valve that can hold boost pressure and be worked by 12V can be used. This includes industrial 12V quarter-inch solenoids like those made by Goyen (they’re often used as LPG shut-off valves in LPG cars), boost control solenoids out of turbo cars, or even the valves often used in the vacuum hose plumbing of cars. The latter won’t be suitable for mega boost but in practice they’ll work fine in most street applications.

The solenoid and one-way valve are plumbed into place as shown here. (Click on the diagram to enlarge it.) You may need to use some rubber hoses of different diameters to adapt the one-way valve to suit the rest of the plumbing. As mentioned, the cheapest source of one-way valves is to raid the wreckers and find some brake booster valves, but another approach is to use one of the valves shown in this story - Do-It-Yourself Peak Hold. Make sure that you get the flow direction of the one-way valve the right way around!

Here’s what the assembled solenoid and one-way valve bypass look like.

Extensive testing showed that one additional part needed to be added to the system. This is a vent between the solenoid and the BOV. Without it, there’s nothing to stop vacuum being retained when the solenoid shuts – so keeping the BOV open all the time! A small vent stops this vacuum being retained while having no affect on the opening of the BOV. One approach to making the vent is to solder up one arm of a brass T-piece, then drill a small hole through the solder plug. Even easier is to insert a piece of hard plastic hose in the plumbing and make some small holes in it with a hot pin.

Don’t forget to disconnect the side of the factory BOV that normally connects back to the intake system ahead of the turbo so that the BOV now vents to the atmosphere.

Setting Up

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.

Before connecting the solenoid to the DTT, test that the solenoid valve opens when power is applied and shuts when power isn’t. If you find that the valve works the other way around (ie is normally open and shuts when power is applied) then use the ‘common’ and ‘normally closed’ connections on the relay of the DTT.

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. If there are other problems check the orientation of the one-way valve and that the vent in the plumbing between the solenoid and the BOV is working.

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.


By using the DTT and a few bits and pieces of plumbing it’s possible to revolutionise the way in which the factory BOV works. And of course this approach isn’t confined to factory BOVs – if you have an aftermarket one that produces the sound you want but causes the engine to run badly, just the same approach can be taken with that valve too.

Next week: installing a completely electronic BOV system – there’s no vacuum feed needed!


With the Delta Throttle Timer set up to trigger a BOV you can very easily use the module to also perform a completely different second function. As covered in our Quick Brake! story, on 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 Link Position, Sensitivity and Time settings for triggering a BOV are the same (or 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 Common and Normally Open contacts of the relay. For more details, see our QuickBrake! story.

And yes, a blow-off valve IS useful on an auto trans turbo car...

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.

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.

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.

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 approach covered in this story allows you to keep the BOV closed at idle and yet still open it very quickly when the throttle is being closed.

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