Venting Boost, Part 1

All factory – and very nearly all aftermarket – turbo cars use the same basic approach to controlling turbo boost. A wastegate is fitted that allows exhaust gases to bypass the turbine. The wastegate is variable in opening; when it is fully open, more gas bypasses the turbo and so turbo speed is slower, resulting in less boost. When it is fully closed, the entire engine’s exhaust gas is fed through the turbine and so turbo speed is higher, resulting in a greater level of boost. By varying the opening of the wastegate, the boost level can be controlled so that it doesn’t exceed a certain value.

However, there’s another way of controlling boost – by venting it from the intake system. Some factory turbo cars use this approach as a fail-safe to prevent boost rising too high if the exhaust wastegate jams shut or a hose falls off the wastegate actuator. These factory cars either use a plenum chamber over-boost valve (like in mid-Eighties Nissan engines found in the Pulsar EXA, VL Commodore Turbo, etc) or alternatively, the factory blow-off valve (BOV) is designed to leak if the boost rises too high.

Very few aftermarket tuned cars use intake venting as a means of controlling boost – one of the few workshops that we know to use this technique is Nizpro. However, especially if used in conjunction with a conventional wastegate system, venting intake pressure can be useful in a number of applications.

Pluses and Minuses

Like all things, there are good points and bad points about taking this approach to controlling boost. Firstly, the negatives.

If intake venting is the only means of controlling boost, the turbo always has the full amount of engine exhaust gases going through its turbine. In other words, there is no control over turbo speed. That’s OK if the turbo is fairly large for the application, but if it isn’t, the turbo may be spun so fast that it is destroyed.

Another potential disadvantage is in airflow metered cars, where the air that is bled off to control boost must be returned to the intake after the airflow meter but before the turbo. If it isn’t – instead the air is vented to the atmosphere – the air/fuel ratio will be incorrect, because the meter will be measuring lots of air that doesn’t end up going into the engine.

But a positive of the approach is when intake venting is used in conjunction with a wastegate, the boost level can be dropped to below the level held by the wastegate. For example, if the minimum boost achievable with the wastegate diaphragm connected straight to the intake manifold is 7 psi, you cannot normally drop boost below this value. But if you’re forced to buy a batch of low octane fuel, you might want to run as little boost as possible. By venting the intake it’s possible to run just 1 psi of boost – or even zero boost!

And you can drop boost below wastegate level on the run, too. For example, if the intercooler is a bit small for the power and boost levels being used, it’s easy to use a temperature switch (either electronic or mechanical) to sense intake air temp and if it rises to horrific levels, automatically drop boost back to nearly nothing – that is, below the minimum wastegate value.

On a more prosaic level, you might also want to decrease boost to zero if you give the car to someone to drive who is inexperienced – a valet mode, if you like.

Finally, if the wastegate works fine except at the very top end of the engine load range - where boost always rises that bit too much - you can use intake venting to knock off those few psi of boost that otherwise always intrude.

So venting the intake to control boost is very useful, especially if combined with a normal wastegate system.

But isn’t it complex and expensive – you’ll need to add lots of extra valves and control systems? The answer to that depends on how far you want to go and what you want to achieve. At its simplest it can be quite cheap and use your existing BOV to do most of the work.

Doing it with the BOV

The easiest way of achieving intake venting control over boost is via a blow-off valve.

As this diagram shows (click on it to enlarge it), when it is open, a recirculating BOV takes air from the boost side of the turbo back to the intake side. The BOV is opened via a vacuum signal picked-up from after the throttle butterfly. When the throttle is abruptly closed, a strong vacuum signal passes down the vacuum/boost hose leading to the BOV, and so the valve snaps open. This relieves the pressure build-up that would otherwise occur in the plumbing between the turbo compressor and the closed throttle blade.

But when you’ve got your foot down, there's a positive pressure being developed everywhere in the intake - including in the vacuum/boost hose that goes to the BOV. 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.

But what happens if you don’t send the boost pressure signal to the BOV? In that case it’s very likely to open, bleeding off boost and so limiting how high boost goes. It’s just like those factory BOVs that are designed to leak at high boost.

Let’s look at how to achieve this outcome with an aftermarket, recirculating BOV. We’ll use the GFB "plumback" BOV which can be easily disassembled to have its internal preload spring changed.

The GFB valve contains a piston which is pushed into the ‘valve closed’ position by an internal spring. A small amount of spring preload adjustment can be made by turning the knurled fitting on the end of the valve, which also doubles as the vacuum hose connection. The internal piston uses a tapered acetal piston seal and low-friction polyester piston ring.

The valve is plumbed-in so that boost pressure acts on the base of the piston. In other words, boost is always trying to open the valve. This force is resisted two ways. Firstly, there’s the spring helping to keep the piston closed, and there’s also the boost pressure feed coming down the small vacuum/boost feed hose. In effect, the boost pressure on one side of the piston is balanced by the boost pressure on the other side of the piston, and together with the helper spring, the valve stays shut and so holds boost.

But if the boost pressure in the vacuum/boost feed hose is lowered in level to atmospheric, it’s just the spring that stops the valve from opening with the boost pressure. And the spring (especially if we shorten it or change its preload!) isn’t designed to do this work alone and so the valve opens a little, venting boosted air back to the turbo intake and so controlling boost.

OK, let’s summarise. If we use an aftermarket BOV like the GFB plumback design, we can easily control how much it bleeds off boost by:

- Adjusting the internal spring preload and/or length so that it will leak when not aided by boost pressure coming down the vacuum/boost feed hose

- Then controlling the boost pressure it sees in its vacuum/boost feed hose

If we want the intake venting control of boost to happen in only certain situations (eg after buying a tank of bad fuel), the BOV can be configured to operate normally almost all the time. It will receive its usual vacuum signal and open on throttle lifts, and it will receive its normal boost signal and stay closed when the hammer’s down. But when you want to control boost by intake venting, the BOV no longer sees boost on its vacuum/boost feed and so opens early, controlling boost pressure.

So how do we control boost pressure in the BOV feed hose in this way?

Controlling Boost

The following assumes that you’re using a recirculating BOV that has either preload adjustment of its spring or is easily disassembled to allow changes in internal spring length or stiffness. The BOV must also be plumbed-in so that manifold boost pressure acts on the base of the piston, trying to force it open.

The first step is to disconnect the vacuum/boost feed hose and see what the maximum boost has dropped to. For example, you might normally run 15 psi boost and with the BOV vacuum/boost feed hose disconnected, this might drop to 10 psi. If that’s enough of a drop, great! But if you want a greater decrease than you get on this first test, reduce the BOV spring preload, or shorten or soften the internal spring. Test again and see what the max boost has now dropped to. If the boost still hasn’t dropped by enough, you might want to replace the internal spring with a very soft one and see just how low the system will let you go. (In big turbo and high boost cars you’ll probably need to open both the wastegate and the BOV to drop boost right back to zero. You may also need two BOVs.)

Assuming that you get the boost decrease that you want by having the vacuum/boost feed hose disconnected, the next step is to work out how you’re going to drop boost in that hose as needed.

One simple approach is a small vent used in conjunction with a solenoid valve. This diagram shows how the system is arranged. A T-piece is placed in the vacuum/boost feed hose and a solenoid plumbed into place on the boost side of the T. When the solenoid is open, boost pressure reaches the BOV, holding it shut. When the solenoid closes, boost can no longer reach the BOV and so the BOV is no longer pushed shut by the boost. The vent allows the air trapped between the closed solenoid and the BOV to bleed away. The restriction in the vent line prevents too much boost being bled off when the solenoid is open. The restriction can be as simple as soldering-up one arm of the brass T-piece and then drilling a small (eg smaller than 1mm) hole through the solder plug.

To decrease boost to the level that was gained in the test above (the one where the vacuum/boost feed hose was disconnected), all that needs to be done is to electrically close the solenoid valve. The solenoid can be any small industrial or automotive solenoid that will hold boost and vacuum.

If the plumbing is organised as shown in the diagram above, there will be a small vacuum leak through the vent when the car is off-boost – especially at idle. This is easily prevented if a one-way valve is placed in this vent line, orientated so that air can flow out of this hose. A suitable in-line valve of the sort shown here can be salvaged from the brake boosters of many cars. If you want, you can return this vent hose to the intake before the turbo but after the airflow meter – that way, the air/fuel ratio will remain exactly as standard. However, in practice, very little air flows out of this vent hose.

The solenoid, which in normal driving is held open, can be closed by a manual switch on the dash, or a temperature switch can close it when intake air temp rises too high. You can also use a Simple Voltage Switch kit (see The Simple Voltage Switch) working off the airflow meter output to drop boost if it rises too high at the very top end of the engine’s output. (If the car uses a frequency outputting airflow meter, use the Frequency Switch kit - see AutoSpeed Shop).

Alternatively, if you use a factory boost control solenoid that can be pulsed, the Independent Electronic Boost Control kit (see The Independent Electronic Boost Control, Part 1 can be used to pulse the solenoid so that you can get a gradual and smooth change in boost that relates to engine load.

Still a BOV? If you use the one-way valve in the vent line and don’t need to change the spring characteristics (length, rate and preload) to get the boost drop that you want, in addition to its new boost-venting function, the BOV will still work as a conventional blow-off valve.

Conclusion

Venting the intake is not the primary approach to boost control that we’d take (we like a wastegate performing the main boost control function), but venting can be very useful in fine tuning boost levels when you want to either go below the wastegate setting or want extra control at the top end of the engine’s power band.

Next week: we fit a boost venting system to a car.

The GFB blow-off valve described in this article was supplied courtesy of the manufacturer.

Huge Boost Drop? Removing boost pressure from the vacuum/boost sensing line of the BOV will usually drop boost sufficiently. But what if you want absolutely zero boost – and the BOV simply isn’t big enough to blow sufficient air to achieve this? The answer is to fit an extra, large solenoid valve. See The All-Electronic Blow-Off Valve! for more on suitable valves.