940 turbo dies at junctions

[gpl1968]

Originally Posted by TonyS9

It won't cause any problems dumping metered air, the engine does not remember or know the air is pressurised. It just assumes the amount going into the engine is the amount going into the MAF.

In my understanding (which isn't perfect) I think the engine ignores the MAF during idle, but doesn't like it being disconnected. Therefore you could use a CBV (always open in idle) open to the atm rather than a temporary opening dump valve or connecting to pre-turbo pipe work. The valve will close when off idle and the air will be measured via the MAF as normal. I'm not sure though.

The MAF may take a little longer to react to the bigger step increase in ait flow when the valve closes for power.

The Air Mass Meter does as the name describes. It measures the mass of the air flowing over the wires. The pressure in the system is irrelevant, the mass stays the same, so if you dump air to atmosphere via a Blow Off Valve you will cause the effective air/fuel ratio to rise until the system stabilises.

The Compressor Bypass Valve serves two purposes:

1, It eliminates compressor surge.
Without the CBV as you accelerate the pressure builds downstream of the turbo. When you lift off, the pressure keeps building until the back pressure on the turbine causes the turbo to stall. This can cause excessive turbo lag after gearchanges.
With a closed loop CBV when you lift off, the CBV opens due to the vacuum signal from the inlet manifold, and creates a circular path for the air to flow around the turbo. This allows the turbo to keep spinning, reducing turbo lag during the next acceleration phase.
A BOV (open to atmosphere) serves the same purpose as a CBV in this respect as it reduces turbo lag between gearchanges.

2, It maintains the correct air/fuel ratio as, unlike a BOV, no air is lost to atmosphere so that the amount of metered air in the system is always known by the ECU.


When the car is idling or cruising at part throttle, the turbo isn't spinning, the air is free to move around the system as in a normally aspirated car so the action of the CBV is irrelevant.

[TonyS9]

Originally Posted by gpl1968

The Air Mass Meter does as the name describes. It measures the mass of the air flowing over the wires. The pressure in the system is irrelevant, the mass stays the same

Kind of agree, its the air mass that is measured, not the pressure. it can do that irrespective of the pressure, but its all done at near atm pressure.

Originally Posted by gpl1968

, so if you dump air to atmosphere via a Blow Off Valve you will cause the effective air/fuel ratio to rise until the system stabilises.

Strongly disagree. When you close the throttle the mgt assumes (from the throttle switch) the air is dumped and adopts the last setting mixture found at idle using the O2 feedback. It really doesn't care about measurement because it knows the measurement will have too much error, it just assumes the air is dumped then goes into closed loop O2 feedback mode (lean idle).

You are assuming that the engine counts the mass up and then averages out the fuel over a period of seconds. It doesn't do that, it measures mass flow RATE and adjust the fuel according to a map on a fraction of a second basis. The stalling is not because of a measurement problem, its because the system assumes the air is dumped and it only has to fuel normal idle air. Some fine adjustment comes from rpm feedback and O2 sensor, but this is very slow, an O2 sensor scan take a few seconds. (it varies the mixture and looks for the O2 sensor flip).

Originally Posted by gpl1968

The Compressor Bypass Valve serves two purposes:

1, It eliminates compressor surge.
Without the CBV as you accelerate the pressure builds downstream of the turbo. When you lift off, the pressure keeps building until the back pressure on the turbine causes the turbo to stall. This can cause excessive turbo lag after gearchanges.

If the system maintains pressure turbo lag is less relevant, I believe the problem is more related to damage and stress on the turbo, however modern race/rally systems are now storing pressurised air for lag elimination on throttle on/off.
But yes it allows the turbo to spin, while the pressure is lost and is better than some intermediate mix. However lag is eliminated if you have enough stored air, the problem is measuring the flow rate in the manifold.

Originally Posted by gpl1968

With a closed loop CBV when you lift off, the CBV opens due to the vacuum signal from the inlet manifold, and creates a circular path for the air to flow around the turbo. This allows the turbo to keep spinning, reducing turbo lag during the next acceleration phase.

Not quite as I see it. The bypass allows the pressurised air to escape via the air filter so its not blocked by the stalling turbo. Contrary to popular believe the engine does not use the extra air (molecules), the air has to leave the system. There is no significant recirculation.
I think (but def not sure) that the removing the CBV will reduce the lag between gear changes because the pressure is not lost and the engine has no time to stall, however its a dynamic environment and the slow response of the MAF may cause a weak mixture signal briefly perhaps. Although any MAF dependant fueling is usually significantly overfueled because of the slow response and unknown load (the main reason IMO this system has poor fuel economy).

A BOV is simply noisier than a CBV because the air filter is more noise insulated, but also has the adjustment disadvantages I explained earlier.

Originally Posted by gpl1968

2, It maintains the correct air/fuel ratio as, unlike a BOV, no air is lost to atmosphere so that the amount of metered air in the system is always known by the ECU.


When the car is idling or cruising at part throttle, the turbo isn't spinning, the air is free to move around the system as in a normally aspirated car so the action of the CBV is irrelevant.

As I've already stated I am fairly certain the fueling assumption is incorrect, There is no (or almost none) using of the MAF during idle. Air flow and load is assumed (this is why auto boxes need to signal the high load during idle from engaging the drive, and AC-On to the mgt).

Disclaimer - I am fairly certain on the stall reason, but less on the use of the MAF during idle, so not sure about predicting an open CBV function. When I've time I will try idle with the MAF out of the pipe work but connected. I think it should work, but mgt maybe paying it some attention.

I'd also mention that I am trying to fully reverse Engineer the mgt system but that its not that easy. I'm not trying to make anyone look bad or make myself look clever, I may indeed have some of it wrong. I do want to discover its operation and be able to predict the consequences of modifications.

[T5Sweep]

Interesting stuff, I think I'm going to try the cheaper option first and go with the Saab unit.
Out of curiosity, what are the advantages to having adjustable spring rates in the fancy ones?

[Chooch84]

Controls the moment when the "Whoosh" is recirculated. Have a read of the link below...

http://www.ardideas.com/Turbo-Upgrad...ents_b_18.html