The more you think about turbo boost control, the more implications there are for any given system.

Let’s just refer to a traditional wastegate system.

(That’s where there’s a bypass passage – the wastegate – around the turbine. Open the wastegate and exhaust can bypass the turbine, slowing turbo speed and so dropping boost pressure. Remember – the less open the wastegate, the higher the boost pressure.)

In this discussion it doesn’t matter if it’s an electronically controlled system or a simple pneumatic system.

Let’s say that boost pressure is sensed from the compressor outlet of the turbo. If the maximum desired boost is 10 psi, the maximum outlet pressure of the compressor will also be 10 psi.

But the situation changes if the boost pressure is sensed from the intake manifold. If 10 psi is again desired, the boost pressure at the manifold will be 10 psi, but the boost pressure being developed by the turbo will need to be higher.

Why?  To take into account the pressure drop through the boost plumbing and intercooler!

So 10 psi at the intake manifold might need to be 12 or 13 psi at the compressor.

That all sounds fine, but there’s a very important point to think about. Most cars spend nearly all their time at part throttle – and the above scenario assumes full throttle.

With a well-matched turbo, boost will be developed at part-throttle. So the throttle might be half-closed, but the turbo is boosting – that is, flowing more air than is being accepted by the engine.

If the boost is being sensed from the compressor outlet, at 50 per cent throttle, the wastegate might need to be (say) 75 per cent closed to spin the turbo fast enough to develop the 10 psi outlet pressure.

But if boost is being sensed from the intake manifold (ie after the 50 per cent open throttle), the wastegate might well need to be completely shut to allow the turbo to develop this boost pressure – that’s because a lot of the boost is being dropped across the throttle. In this situation, the turbo is trying to develop boost that, because the throttle is half closed, cannot be felt in the intake manifold!

So? Well, imagine what happens in cruise. You’re on the highway at 100 km/h and the throttle is open only a small amount. In this condition there’s enough exhaust gas flow to spin the turbo and, since the boost control is always aiming to develop peak boost pressure, the wastage is not fully open.

If boost is being sensed from the intake manifold, the wastegate will probably be closed – all the exhaust gas is being channelled through the turbo, even though boost in the intake manifold isn’t being developed or used.

If the boost is being sensed from the compressor outlet, the wastage will still be closed sufficiently to develop boost, but it won’t be as closed as the above scenario.

So, in either case, the turbo is working when it isn’t really needed – and so the turbine is developing back pressure on the engine.

This harms fuel economy.

It would therefore seem logical to sense when cruise is occurring, and to then open the wastegate and so reduce back-pressure, so improving fuel economy.

And some cars do just that.

But this approach has a downside. If the driver is cruising along but then suddenly slams the throttle open, boost will be delayed as the turbo will take time to come up to full speed.

On the other hand, if the turbo is already spinning fast, a sudden need for boost will be accomplished more speedily. In fact, because some boost will already be present on the turbo side of the throttle body, response can be very quick indeed.

So, how to balance the requirement for low backpressure cruise fuel economy (no turbo spinning wanted – exhaust flow fully bypassing turbine) with good throttle response (turbo already spinning, so developing boost upstream of the throttle).

One way to do it is to sense driving style. This can be achieved in a few ways: measuring the rate of change of throttle opening, measuring the frequency with which large throttle openings are used, or directly measuring car acceleration (eg longitudinal and lateral).

Depending on driver behaviour, the boost control system can then be automatically configured to be in ‘economy mode’ or ‘performance mode’.

Again, some cars – eg the Porsche Cayenne Turbo – do just that.

Simplest for the DIY’er would be a dashboard switch – eg one that for cruise conditions swapped the boost pressure measuring source from the intake manifold to the turbo compressor outlet.