When this occurs, the wastegate is opened, bypassing exhaust gases around the turbo and so controlling turbo speed and thus boost pressure. An ECU-controlled solenoid valve regulates the boost pressure that is fed to the wastegate's lower chamber.
Standard BMW wastegate does not have the additional boost control connector in the upper chamber (as shown in the picture). That is something that many people have added to the wastegate (possible to benefit of it only with the Motronic Chip Upgrade). Many modern wastegates have that kind of additional boost control as standard.

In order to tune a 745i engine to higher power levels one has to understand the basic operation of 1) The Wastegate 2) The Control mechanism and 3) The Protection mechanism.

1. Wastegate
There are four different forces effecting the opening and closing of the wastegate valve:

1. Air pressure in the lower chamber
The more pressurized air is fed to this chamber the faster it forces the diaphragm and the valve to go up. The limit is approx. 0.3 bar.

2. Exhaust gas pressure in the exhaust manifold
Exhaust gas pressure at approx. 0.7-0.8 bar turbo boost will be high enough to open the wastegate valve alone (assuming no pressure in lower chamber) at full throttle acceleration.

Counter forces to those are:
3. Spring stiffness
The standard stiffness is 65 Nm, which equals to approx. 0.3 bar pressure below the diaphragm. Some tuners have added mechanical adjustment to the spring assembly in order to increase the spring tension and/or to limit the range of the diaphragm. Note that the wastegate spring in the earlier 3.2 L L-Jetronic 745i engine was harder with approx. 0.5 bar limit.
4. Air pressure in the upper chamber
If you add a connector for boost pressure to the upper chamber you can further extend the total range of available boost. You can get the thread drilled for the connector quite easily at your local metal shop. Here are some pictures
of the modified wastegate: picture1 and picture2

Summary of available boost ranges with standard 745i wastegate equipped with added connector in the upper chamber:
1. Full boost pressure in lower chamber. No boost pressure in upper chamber:
Available boost: 0.3 bar
2. No boost pressure in lower and upper chambers:
Available boost: 0.7-0.8 bar at full throttle. Approx. 0.8-1.0 bar at middle throttle acceleration. You get higher boost at middle throttle acceleration because there is not enough exhaust gas pressure to open the valve. It is simply a question of gas volume compared to full throttle situation.
3. Full boost pressure in upper chamber. No boost pressure in lower chamber
Available boost: Very high, Over 1.5 bar
In this configuration you have the combined forces of the spring and the boost pressing down the diaphragm. On the other side there is only the exhaust gas pressure.

These are the practical limits without any adjustments covering power range from 250 hp to over 500 hp, which to most of us is quite sufficient. Later on in this document we will give you examples of different ways to adjust the power ranges either from the cockpit or from the engine compartment.

The wastegate is controlled by the Engine Management System, which includes Motronic Unit and Knock Sensor Unit. Those send signals to the magnetic valve, which in turn controls the amount of pressurized air fed to the lower chamber of the wastegate. Click the picture on the right hand side to see the whole system.

The magnetic valve is quite "busy" all the time:
* During acceleration the magnetic valve is toggled on and off in order to get the boost on a bit earlier (at approx. 1800 RPM).

* Otherwise it is constantly fine-tuning the air flow to keep the boost at the same level.
* It allows a bit higher boost in the "S" mode.

Note that the standard Motronic program will cut off fuel supply at approx. 0.7 bar (even in fast peaks), so you should always install the Chip Upgrade program before changing the boost levels.

3. Engine Protection Mechanism
The 745i engine has two knock sensors, which are connected to the Knock Sensor Unit. When knock occurs the system starts to do two things at the same time:
1. Change the ignition timing to later in order to reduce power.
2. Open the magnetic valve in order to reduce the boost to the minimum immediately.

This function is important to understand when making changes to the boost levels and to the ways the boost levels are controlled. In some tuning configurations you will lose the automatic boost reduction function totally or partially and then you have to decide how to add alternative protection (like fuel/air mixture monitor) to your engine.

Now you know how the system works so next step is to select suitable way to increase power. Here we will show you few examples how to do it starting from the simplest way.

As power is simply a result of available fuel and air you should do following basic checkups and installations in all cases:
* Buy new air filter. Preferably a High-Flow version.
* If you have not cleaned your intercooler before. Do it now! You will be surprised how much dirt gets there. That work alone will increase the power already in your present setup.
* Make sure that the injectors are in good condition and deliver fuel to the spec.
* Install the Motronic Chip Upgrade. Note that the 745i Motronic ECU has a 8 position selector switch, which adapts fuel and ignition timing for the quality of fuel available in different countries. This switch is not meant to increase performance! Bosh service documents state "Unnecessary adjustment to the switch though, leads to poor driveability, and particularly to "search" during overrun or to increased fuel consumption, in some cases even to engine damage"!
* Install turbo gauge
* If you start right away with high power setups you should install immediately also a Rising Rate Fuel Regulator and a Fuel/Air Mixture Indicator.

In this setup you reduce the airflow between the wastegate and the magnetic valve in one of the three following ways:
A. Install a "bleeder" type of valve. This will reduce the amount of air to the wastegate in proportion to the air from the magnetic valve.
B. Install an adjustable pressure relief valve between wastegate and magnetic valve. It prevents boost pressure from opening the turbo's wastegate until the preset boost level is reached. Once the turbo spools up to the desired boost, the valve opens, quickly opening the wastegate.

In both above cases you can install the valve in either the cockpit or in engine compartment. Cockpit installation will cause some turbo lag, but from our experience that is not so bad compared to the convenience of real-time adjustment. One good alternative is electric motor valve in engine room with just the control potentiometer in the cockpit.

C. The third way is a warning example! Some people punch holes into the hoses in order to reduce amount of air reaching the wastegate. Do not do it! You lose all control of the power levels and you can not reverse it without buying new hoses. Also you may mix up the computer when in vacuum conditions the air is going into the hose.

With these setups you can control power levels between 0.3 bar and 0.8 bar. Mid range boost will be close to 1 bar giving excellent passing acceleration.

The possibility to control air pressure in the upper chamber gives a number of possibilities to control the power and power range. Typical setup today is the following:
The magnetic valve is connected to the lower chamber as in the standard BMW setup - Fig. 2 - .The pressure levels in there are therefore fully controlled by engine management system as described earlier. The air pressure to the upper chamber is fed directly from the turbo through an adjustable valve, which is usually installed into the cockpit as shown in the picture. This setup will not give maximum possible boost, but it is very nicely balanced due to changing pressures in lower and upper chambers. In theory the max. boost is not much more than in the first alternative because in best boost case the upper and lower pressures are the same offsetting each other. However, this setup seems to give good performance throughout the boost range.

The automatic boost reduction function in knock situations will only work in low power setup. For full 100% efficiency you need to install another magnetic valve - Fig. 3 - (in series with the manual valve) which is controlled by a inverted, separate signal from the knock sensor electronics. Knock will shut down the pressure to the upper chamber. Note that the knock engine protection with ignition timing is always at present.

As in this setup the Motronic and Knock Sensor control to the magnetic valve does not have much effect one could remove it completely - Fig. 4 - and apply constant boost to the lower chamber (it is needed in order to reach the 0.3 bar low boost level). Then this magnetic valve could be used in the upper chamber hose for knock shut down with a inverted, separate signal from the knock sensor electronics. That signal could also be used for a warning light installed e.g. in the instrument cluster.

* Adjustable valve for upper chamber section (for high boost limit between 0.8 and +1.6 bar)
* Adjustable valve for lower chamber section (for low boost limit between 0.3 and 0.8 bar)
* Magnetic valve (in series with upper chamber adjustable valve) for 100 % knock control

Note that if you just use the adjustable valve for the upper chamber section and leave the lower chamber to atmosphere level you will have a boost range from 0.7-0.8 bar to +1.6 bar. The adjustable valve for lower chamber is needed to get the minimum boost down to 0.3 bar ( if you need it!).

Ideally, the two separate adjustable valves could be replaced by one splitting valve (if available) - Fig. 6 -. Like said earlier, electric motor valves can be used also for easy cockpit control. Also fully electronic boost control units are available from several companies.