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!
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
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
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
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.
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.)
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
As mentioned, the Delta Throttle Timer was developed and designed by Silicon
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
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
(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
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).
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
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
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.
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 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
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
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
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.
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!
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
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
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.
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 all-electronic approach covered here uses a completely different