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700/900 Series General Forum for the Volvo 740, 760, 780, 940, 960 & S/V90 cars |
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940 turbo dies at junctionsViews : 941 Replies : 13Users Viewing This Thread : |
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Aug 15th, 2018, 13:32 | #11 | |
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Quote:
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.
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Gavin 1997 945 CD (B230FK) [RIP: 1991 945 Turbo (B230FT) 1992 945 SE turbo (B200FT)] |
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Aug 15th, 2018, 17:27 | #12 | |||||
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Last Online: Apr 9th, 2024 21:44
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Location: Holywood
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Quote:
Quote:
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). Quote:
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. Quote:
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. Quote:
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. |
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Aug 15th, 2018, 19:30 | #13 |
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Last Online: Oct 24th, 2022 09:28
Join Date: Sep 2014
Location: Peterborough
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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? |
Aug 15th, 2018, 20:51 | #14 |
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Last Online: Yesterday 20:42
Join Date: Jul 2015
Location: Birmingham
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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
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96 945 Classic 2.3 LPT with some mods... To do: Install upper brace, Catch can, install sound deadening (50% done), Fit electric leather seats, repaint! List is endless... 16 BMW 330e |
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